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Thread: MOV, Gas Discharge tube, or TVS Diode protection for output transformer??

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    Senior Member SoulFetish's Avatar
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    MOV, Gas Discharge tube, or TVS Diode protection for output transformer??

    I was hoping to get some help choosing the right components to protect against transient over-voltages across my output transformer primaries. I was thinking about using MOVs, but I'm unsure as to which are the important parameters I need to consider. What the benefits are of one devise vs another etc. I know there are those of you who have a lot of experience looking at transients in OT windings when connected to a reactive load, so I will gladly listen to what ever info you can share.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Quote Originally Posted by SoulFetish View Post
    I was hoping to get some help choosing the right components to protect against transient over-voltages across my output transformer primaries. I was thinking about using MOVs, but I'm unsure as to which are the important parameters I need to consider. What the benefits are of one devise vs another etc. I know there are those of you who have a lot of experience looking at transients in OT windings when connected to a reactive load, so I will gladly listen to what ever info you can share.
    Does your planned level of protection include the case of an open secondary (no speaker plugged in)? In this case you would have to allow for dumping energy stored in the full transformer inductance rather than just the leakage inductance. Another thing to allow for is that you have to protect against repeating outrages, at the audio rate, rather than just a single transient, such as induced by a lightning strike.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Does your planned level of protection include the case of an open secondary (no speaker plugged in)? In this case you would have to allow for dumping energy stored in the full transformer inductance rather than just the leakage inductance. Another thing to allow for is that you have to protect against repeating outrages, at the audio rate, rather than just a single transient, such as induced by a lightning strike.
    You first question is a good one! I wasnt considering no load protection but even with a switching jack there is that potential in event of a cable plugged in only at one end. So i guess it should be considered. But initially I was really just considering repeat events during signal conditions. Wasnt worried about lightning going through my output transformer, unless of course im playing Howlin Wolf's Smokestack Lightning.
    So, lets talk about figuring for the full inductance. Ugh, of course, I had to go over the top and spec the primary for -1dB at 20Hz-20kHz at 10k plate to plate
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    "Thermionic Apocalypse" -JT nickb's Avatar
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    You might find this thread of interest Output transformer protection, flyback diodes, MOVs, 2KV capacitors,

    The the bottom line, as I understood it, is MOVs will only you offer so much protection for so long. It's just a matter of choosing how long is enough for you. Personally I think a better approach would be to use small MOVs together with a means of detecting the abnormal voltage condition and shutting the drive off.
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    Old Timer J M Fahey's Avatar
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    Flyback diodes are very popular because they send spikes back to the power supply itself, which by definition can handle itīs own power.

    They donīt dissipate anything , just resend voltage spikes back to the supply capacitors where that energy came from in the first place..
    Juan Manuel Fahey

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by nickb View Post
    You might find this thread of interest Output transformer protection, flyback diodes, MOVs, 2KV capacitors,

    The the bottom line, as I understood it, is MOVs will only you offer so much protection for so long. It's just a matter of choosing how long is enough for you. Personally I think a better approach would be to use small MOVs together with a means of detecting the abnormal voltage condition and shutting the drive off.
    YES! I find it of great interest in fact. Some nice light reading i see (Thiele-Small, you say?). but there are some good figures to see how you calculated power equations from inductances. I'll likely have a question or 2.

    As for movs, it appears that the devices which have higher power ratings and seem better suited for this application, have high capacitances. would this be problematic across the primaries of the OT? Should I use these capacitance is in design an RC conjunctive filter? I would imagine if the performance of a mob degrades with each overvoltage , then capacitance values would not be stable either but I don't know
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    Senior Member Wombaticus's Avatar
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    Here's a relevant discussion about gas discharge tubes and MOVs over at AX84: AX84.com - The Cooperative Tube Guitar Amp Project
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    Quote Originally Posted by J M Fahey View Post
    Flyback diodes are very popular because they send spikes back to the power supply itself, which by definition can handle itīs own power.

    They donīt dissipate anything , just resend voltage spikes back to the supply capacitors where that energy came from in the first place..

    Juan, How do you connect it to do that? This is the situation that I see:

    1.You have current flowing through the transformer and through the tube to ground. It points from the power supply to the plate.

    2. The tube turns off, becoming a very high impedance.

    3. The effective inductance of the transformer primary wants the current to keep flowing, so the voltage at the plate connection becomes very high, higher than the power supply voltage by a lot.

    4. To prevent arcing, you put a diode across the transformer, pointing from the plate to the power supply.

    5. This causes curent to flow around the loop made by the transformer primary and diode.

    6. Power is dissipated in both the diode and the transformer, but in this case none makes it back to the supply

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    Quote Originally Posted by SoulFetish View Post
    I was hoping to get some help choosing the right components to protect against transient over-voltages across my output transformer primaries. I was thinking about using MOVs, but I'm unsure as to which are the important parameters I need to consider. What the benefits are of one devise vs another etc. I know there are those of you who have a lot of experience looking at transients in OT windings when connected to a reactive load, so I will gladly listen to what ever info you can share.
    As you've found by now, there's a lot of smoke around this fire. I used MOVs in a low production amplifier, and they worked and sounded fine.

    But that's not a representative sample, by any means.

    What you're after is suppressing voltage spikes that would puncture magnet wire insulation. How much you need depends on how well the magnet wire is insulated in the first place. In general, you don't know that, so you have nothing to base "how much voltage" on. ACK!

    However, normal operation for a push-pull amp involves having B+ minus maybe 50V across the active side, and that means that transformer action puts that much above B+ on the other side. So the transformer wires had better take two times B+ even when hot and 50 years tired. You need safety margin on top of that. Just to ballpark some numbers, call it 1000V for normal(ish) operation. So your protection devices had better NOT conduct on voltages in that range, and must conduct before the wire insulation gives up. Gotta guess a number. I guessed 650 volts per side on a 400V B+. Seems to have worked.

    I've had several discussions about what specs are needed, MOV capacitance, energy dissipation, short term dissipation and long term conduction. MOVs have more capacitance, but do not seem to affect the sound. I could not hear any difference nor measure any change in response when I clipped them out or soldered them into the prototype. But I've been told that TVS devices are just as capable at eating heat, but lower capacitance. Time for a listening test, I suppose.

    Does your planned level of protection include the case of an open secondary (no speaker plugged in)? In this case you would have to allow for dumping energy stored in the full transformer inductance rather than just the leakage inductance. Another thing to allow for is that you have to protect against repeating outrages, at the audio rate, rather than just a single transient, such as induced by a lightning strike.
    Separating the energy in the leakage and primary is good thing to worry about, but complicated. You have to take note of the difference between magnetizing current and transformed current. Magnetizing current is all that's ever in the core. In a push-pull transformer, it's quite small. Not so with SE. But in a PP, the magnetizing current is a few percent of the peak primary currents. The rest is transformed current - the stuff that whizzes through the fields into the secondary wires. Transformed current does not change the magnetic energy stored in the iron core.

    But it very much does get stored in the leakage inductance. So the leakage inductance may be much smaller, but it has much larger stored energy current to work with. Which one is biggest needs some hard-core pencil and paper work.

    MOVs are actually pretty good at getting rid of high amounts of heat. What matters is how long and how much heat goes into transient suppressor. I have seen MOVs melt their own solder joints and fall out, but be otherwise undamaged.
    I wasnt considering no load protection but even with a switching jack there is that potential in event of a cable plugged in only at one end. So i guess it should be considered. But initially I was really just considering repeat events during signal conditions. ...
    So, lets talk about figuring for the full inductance. Ugh, of course, I had to go over the top and spec the primary for -1dB at 20Hz-20kHz at 10k plate to plate
    As I said, more introspection is needed. The full inductance doesn't get nearly as much E = 1/2 * L * I^2.

    Tube amps are unimpressed by being shorted. The biggest nasty event is someone pulling out a speaker cable when it's going full tilt, and even then you tend to get current arcing on the plug being pulled.

    A more important nasty event is when the amp is left to sit with an open secondary. I ... um, accidentally... left the secondary open for some long periods on the Workhorse protos. Never had any damage at all, and could not figure out why it wasn't a smoking ruin. Turns out the real villain here is oscillation in amps with feedback on the power amp that go into ultrasonic oscillation with no load. That will kill things, OK. The Workhorse didn't use feedback, deliberately so as to hear more of the tubes' oddities. So an open speaker cable is not sure and certain amp-death. Maybe, but not certain.

    The the bottom line, as I understood it, is MOVs will only you offer so much protection for so long. It's just a matter of choosing how long is enough for you. Personally I think a better approach would be to use small MOVs together with a means of detecting the abnormal voltage condition and shutting the drive off.
    You have to note the time scale. The trip voltage on a MOV decreases an infinitesimal bit each time it conducts. Do it often enough and the voltage drifts down into normal operation and the MOV then has a big heat problem. This is very, very unlikely in amp use as we're discussing, especially when we can just pick a higher voltage MOV, given the uncertainty above about what the transient voltages will be. It's easy for it to be long enough.
    Flyback diodes are very popular because they send spikes back to the power supply itself, which by definition can handle itīs own power.
    They donīt dissipate anything , just resend voltage spikes back to the supply capacitors where that energy came from in the first place..
    That's a solid state amp consideration, JM. Tube amps have other issues, and no good way to dump spikes back into the power supply.

    As for movs, it appears that the devices which have higher power ratings and seem better suited for this application, have high capacitances. would this be problematic across the primaries of the OT? Should I use these capacitance is in design an RC conjunctive filter? I would imagine if the performance of a mob degrades with each overvoltage , then capacitance values would not be stable either but I don't know
    I think a quick and dirty test would tell you more than ten hours on the internet. Get yourself some 600-800V MOVs and solder them across the B+ and plate leads of an OT. Can you hear the difference? If you can, maybe you should go for TVS devices, which are reputed to be lower capacitance. If you can hear it, but like the difference, you're done.

    Gas discharge tubes are a different animal, and IMHO not suitable for OT protection.
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    Quote Originally Posted by R.G. View Post
    Separating the energy in the leakage and primary is good thing to worry about, but complicated. You have to take note of the difference between magnetizing current and transformed current. Magnetizing current is all that's ever in the core. In a push-pull transformer, it's quite small. Not so with SE. But in a PP, the magnetizing current is a few percent of the peak primary currents. The rest is transformed current - the stuff that whizzes through the fields into the secondary wires. Transformed current does not change the magnetic energy stored in the iron core.
    Well, I agree that it is complicated, but if you pull out the load, you lose that transformed current. You then have an inductor. Well, not quite. You have coupling to the other tube, the one that is turning on while the first is turning off. Since a pentode is not a perfect current source (has somewhat sloping characteristics) changing the voltage across it changes the current through it some, and maybe allows the absorption of some energy. Maybe this is why amps do not always blow up. But this looks hard to analyze!

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    Old Timer J M Fahey's Avatar
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    Sorry Mike, I was thinking about a trick I use in my own OTs where I wind an extra half-turns winding which does exactly that, clamp excess voltage (and current) not to ground but to supply, but that was not the point in discussion but instead what I wanted to emphasize is that a MOV, being resistive, *always* dissipates (most of the) power inside itself, degrading over time, while a diode is (almost) transparent, having a very low voltage drop.

    But letīs start again, focusing on the main point and referring to a conventional OT and standard clamp diode wiring : reverse biased from plate to ground vs. MOVS which are in parallel with some winding.

    To avoid drawing everything from zero, enter a section of Peavey Butcher:
    mov-diode.jpg
    I show both options:
    1) MOVs in parallel with plate to +V winding, might also be wired from plate to plate, same basic idea.
    I think of MOVs basically as plain resistors in series with back to back Zeners meaning said resistors are basically out of the circuit until some "zener" threshold is reached.
    Difference is that the conduction threshold is loose, less clearly defined but the advantage being that said resistor is way more robust, can take lots more abuse than a standard semiconductor junction.
    So you choose MOVs so the "zener" voltage is, say, 20% to 50% higher than V+ if plate to V+ or 2 x V+ if plate to plate or , say, 50% above expected peak voltage across load and you "should" be safe; 50% voltage overload "should" be well within transformer insulation (or it "should" be designed and built with that in mind )

    Sorry for all the conditionals involved, but I mention the way foolproof transformers should be designed, not so sure about how many do that.

    I bet many think: "hey, I have 450V +V , Iīll be generous and insulate for twice that, 900V or 1000V" ,while truth is when amp just reaches clipping and is loaded, one plate (almost) reaches ground and like in a see saw, the other plate (almost) reaches 2x +V (relative to ground).

    *With* MOV protection itīs limited to, say, 50% more, so a hair rising 3x +V if unloaded or very inductive speaker ..... and without protection and unloaded the sky is the limite ..... or to be more prosaic, transformer insulation puncture voltage.

    Problem with MOVs is that said power dissipation degrades them; I do not know what is the actual mechanism, not sure whether they short, fail open or wildly change threshold voltage, which makes them useless, guess must make some extra studying.

    2) clamping diodes (wired as shown here) , being reverse biased, are out of the circuit most of the time, even if amp clips but is loaded .
    Now if load is very inductive or plain absent , so transformr inductance takes over, seesaw action means that when one plate tries to go beyond 2x +V, the other one tries to go below ground, what forward biases the diode and safely discharges excess voltage and remaining current to ground.
    The diode itself dissipates little, thatīs what I wanted to emphasize compared to MOVs, and in heory lasts forever.

    They do fail now and then anyway, but many suspect itīs through a different mechanism: very high peak plate voltages appear anyway (although much narrower) because of parasitic inductance, which prevents the seesaw mechanism to be perfect, that extra voltage "zeners" protection diodes by force, like it or not.
    There must be something to this, because in theory 1200 or 1500 PIV diodes should be more than enough for most standard amplifiers, yet Manufacturers often use 3x 1N4007 in series or 3kV diodes .
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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Sorry Mike, I was thinking about a trick I use in my own OTs where I wind an extra half-turns winding which does exactly that, clamp excess voltage (and current) not to ground but to supply, ...
    I like that! But I think that for protecting an existing amp MOV devices (or something that behave in a very similar way) are the right way to go. I just do not feel competent to decide which of the available specific devices listed in a catalog is the right one.

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    Quote Originally Posted by Mike Sulzer View Post
    Well, I agree that it is complicated, but if you pull out the load, you lose that transformed current. You then have an inductor. Well, not quite. You have coupling to the other tube, the one that is turning on while the first is turning off. Since a pentode is not a perfect current source (has somewhat sloping characteristics) changing the voltage across it changes the current through it some, and maybe allows the absorption of some energy. Maybe this is why amps do not always blow up. But this looks hard to analyze!
    You're right. It gets complicated fast.

    My response to "complicated" was drummed into me by my early mentors: if you can't predict it, find a way to make the wobbles insignificant in the greater scheme of things. I'll usually fall back on estimates of boundary conditions, like "well, how big can the [energy/voltage/current] really be, anyway?" I figure that if I can handle that, I have the subtleties covered.

    In this case, the energy in the transformer core is the unbalanced current in the primary. A push-pull core with no signal has a residual magnetic field of zero. The bias currents cancel out, leaving only the degree to which the tube currents don't match. That changes with signal.

    For a voltage signal, the current in the core is the integral of Vdt/L from zero up to the voltage. Translated into human words, the current ramps up at a rate of V/L until stopped by something. The something that stops it is the output tubes. One significant case is with one tube dead. The core is then conducting the full bias current through the primary inductance. A worse condition is if the amp is motorboating. If this is happening, then it's swinging subsonically and there may be large currents in the primary. But that doesn't happen much in real amps.

    I suspect that if you could catch it on an instrumented amplifier, transformer death would tend to be around an event like the amplifier oscillating at high frequency and overheating an output tube till it opens. (a shorted output tube would clamp its half-winding and stop any transients) When this happens, the current in the leakage inductances is unclamped and makes a voltage that punctures the insulation on the magnet wire in one place. Later events work on that same place until an arc forms that welds the wires there. Death ensues.


    JM says:
    2) clamping diodes (wired as shown here) , being reverse biased, are out of the circuit most of the time, even if amp clips but is loaded .
    Now if load is very inductive or plain absent , so transformr inductance takes over, seesaw action means that when one plate tries to go beyond 2x +V, the other one tries to go below ground, what forward biases the diode and safely discharges excess voltage and remaining current to ground.
    The diode itself dissipates little, thatīs what I wanted to emphasize compared to MOVs, and in heory lasts forever.

    They do fail now and then anyway, but many suspect itīs through a different mechanism: very high peak plate voltages appear anyway (although much narrower) because of parasitic inductance, which prevents the seesaw mechanism to be perfect, that extra voltage "zeners" protection diodes by force, like it or not.
    There must be something to this, because in theory 1200 or 1500 PIV diodes should be more than enough for most standard amplifiers, yet Manufacturers often use 3x 1N4007 in series or 3kV diodes .
    Yes, and I think that's the idea behind the clamp diodes. The diode that helps you is the one that conducts from ground in the forward direction. No doubt some situations are prevented by just this. The problem that lurks here is the leakage inductance. The leakage cannot, by definition, be clamped by transformer action, so it acts like an inductor in series with each transformer lead. This much smaller inductance is charged up to the same current as the lead and plays V=L * di/dt when the current changes.

    I think that the clever guys who figured out protection by transformer action with diodes was dismayed to find out that some spikes leaked through, and used the easiest to find cheap diodes to catch the remaining spikes by avalanche/zener action. Then they added enough of them in series to not break over in normal operation.

    You're right that a diode going into conduction is not terribly dissipative. But the zenering action is. So in zenering mode, the clamp diodes do heat. Whether they die or not eventually is problematic, but they will last many more cycles than a MOV. On the other hand, MOVs degrade quite slowly. In AC power circuits, they only die when they drift down into the normal AC line voltages. This is years of constant use for 130V MOVs on a 120V line, and much longer than the equipment life for a 150V MOV on the 120V line. So a MOV with an additional 100% of B+ as a breakover in a tube amp is going to be essentially immortal; well, no more mortal than the resistors and caps.

    In an amp with a 500V B+, one might put 1000V breakover MOVs direcly across the transformer leads from CT to ends. That means that the MOVs would clamp the entire primary at about 2kV, still less than the 3KV and more that three 1N4007s try to catch, but that they would take a long, long, .... long ... time to degrade down to being only 1*B+, which is where they'd start burning out. So MOV drift can be made a non-issue.

    TVS devices apparently don't have the drift-down effect, and also have low capacitance. These have been developed after MOVs, and recently (...um, last decade? I'm getting old) have been scaled up to higher energies for clamping. So a TVS may be like a MOV, but better. I haven't tried them.
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    Quote Originally Posted by R.G. View Post
    However, normal operation for a push-pull amp involves having B+ minus maybe 50V across the active side, and that means that transformer action puts that much above B+ on the other side.
    For whatever reason I was confused about this statement earlier, but I just realized you are talking about the anode voltage swing during full drive and that potential difference across that side of the winding.

    Quote Originally Posted by R.G. View Post

    Separating the energy in the leakage and primary is good thing to worry about, but complicated. You have to take note of the difference between magnetizing current and transformed current. Magnetizing current is all that's ever in the core. In a push-pull transformer, it's quite small. Not so with SE. But in a PP, the magnetizing current is a few percent of the peak primary currents. The rest is transformed current - the stuff that whizzes through the fields into the secondary wires. Transformed current does not change the magnetic energy stored in the iron core.

    But it very much does get stored in the leakage inductance. So the leakage inductance may be much smaller, but it has much larger stored energy current to work with. Which one is biggest needs some hard-core pencil and paper work.
    I would really like to get a deeper understanding of the magnetizing current, leakage inductance, and the relationship of energy to the electromagnetic field in a transformer. I apologize for sounding so incredibly generic, but I don't know how else to say "There is more for me to know with these things".

    Quote Originally Posted by R.G. View Post
    As I said, more introspection is needed.
    Well I just went through all the threads and links to other threads and at this point, it's probably a full blown existential crisis.

    Quote Originally Posted by R.G. View Post
    The biggest nasty event is someone pulling out a speaker cable when it's going full tilt, and even then you tend to get current arcing on the plug being pulled.
    I've never seen the effect of current arcing in this way. What is the effect and why would it happen in this case?

    Quote Originally Posted by R.G. View Post
    I think a quick and dirty test would tell you more than ten hours on the internet. Get yourself some 600-800V MOVs and solder them across the B+ and plate leads of an OT. Can you hear the difference? If you can, maybe you should go for TVS devices, which are reputed to be lower capacitance. If you can hear it, but like the difference, you're done.
    Ah! I can't wait to get to this part! I'm not there yet. My chassis is drilled and ready; I'm drilling my turret board tonight and am placing what is hopefully my last order of bits and pieces I need for parts.
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    Quote Originally Posted by J M Fahey View Post
    Sorry Mike, I was thinking about a trick I use in my own OTs where I wind an extra half-turns winding which does exactly that, clamp excess voltage (and current) not to ground but to supply, but that was not the point in discussion......
    Whoa, whoa, whoa... Not so fast, Fahey. You just got mike all excited about this (probably, because he already knows what this would do), but I kinda' want to know what all the fuss is about this. I like to learn to tricks. How does this one work?
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    Not such a big deal, once you look at it from the right end of the telescope/microscope

    In fact I did not think it to protect tubes or tube OTs but my own transformer output *transistor* amps

    Nowadays donīt bother about this any more because DC to DC swiching converters are easily available, ferrite cores can be found anywhere at low cost or pulled from dead PC supplies, etc. , but I have been making loud (>15/20W and up to 100W) battery powered amps for ages (think 30 or more years ago).

    Tried old style switching converters but with iron core transformers and they were very inefficient (lots of copper loss) or way overheated (magnetic loss).

    If I worked at 50/60 Hz they were as large as any conventional PT and copper DC resistance was a deal killer.

    Adding weight to an already heavy portable combo which included a 12V7A lead acid battery and its charger was not exactly popular with the street performers which were my customers, and the pesky buzzy 50/60 Hz squarewave got *everywhere* , specially single coil pickups or cassette recorders (for the noobs: they were a kind of MP3 player ).

    Rising frequency to, say, 400 Hz made for smaller transformers but they overheated (I would need unavailable airplane grade paper thin laminations for that) and the whine became unbearable.

    So Plan C was to turn the calendar back and design an OT equipped SS power amp which could provide lots of power straight from 12V.

    Normal limit is between 15 and 20W into 4 ohms even using a bridged output, unless you can get 1 ohm speakers or lower (which I also tried).

    Would not save much weight, although I could design it for, say, 100Hz lowest frequency (no big loss since portable cabinets are small anyway), while converter transformers had to run at 50/60 Hz , but shaving a couple pounds was better than nothing and there was no whine, so I took that path.
    Only problem is that (bipolar) power transistors started to die more than I expected.

    Musicians were not plugging speakers, plus transistors are way less robust than tubes, relative to excess voltage, reversed voltage, second breakdown, etc. so even if loaded, overdriving was not healthy , so I started adding flyback diodes and made them more reliable.

    So far so good but Iīm TOC afflicted (like lots of people in this profession) so I started thinking about ways to recover that power which was usually shunted to ground with flyback diodes.

    That is already done on standard SS OTL amps, look at this classic "RCA70W/WEM100W" power amp:


    D9 and D10 not only work as clamps, but when an inductive load makes peaks beyond the +V and -V rails, those are safely dumped, not to ground (waste) but back to main filter caps, so that energy is not lost.

    I had been doing that with OTL amps since forever, and when started making (and killing) transformer loadd SS amps , after the initial success thanks to regular clamping diodes, TOC kicked in and started thinking about doing the equivalent.

    Of course, as-is I did not have the proper voltages anywhere: the saturating collector was near ground (or below it and being clamped) and the opposite was at twice +V ... useless, too far away from +V, remember that at idle they re already at +V and swing around that value.

    But the tiny clockwork wheels inside my head never stop turning and I had this nagging feeling that something could be done ... until it clicked: what I needed was that *some* OT winding stayed "always" under +V but reached exactly +V when clipping.
    The idea looks stupid simple *after* it appears, but not a second earlier.

    I added an extra winding, center tapped and referenced to ground and made out of thin wire (it does not carry continuous power but brief spikes *if* neded) , with a diode on each end pointing to +V main filter cap.

    Worked like a charm and of course did not use the conventional "plate to ground" diodes.

    Sounds like a lot of extra effort , specially compared to just adding standard clamping diodes, but was no big deal for me, because:
    1) I wind my own transformers anyway
    2) I wound the protective winding "bifilar" , together with the "real" one, where the main one uses the expected (thick) wire and the auxiliary one uses any random thin wire leftovers I have on old wire spools so it does not cost extra neither in time nor in money (leftovers are sold for peanuts, "by the pound" , to the copper recycler guy, so I consider them free)
    3) transistor OTs are much faster to wind (less turns of thicker wire) than tube OTs.

    OK, thatīs it, in a nutshell .
    Or considering how much I wrote, in a coconut

    I extrapolated that to Tube OTs, because the mechanism is the same.

    Anyway now I skip all that, can buy ferrite cores by the 100/200 unit box, for very low cost, and dedicated SMPS ICs make my life simple.

    In the old 50/60 Hz converter days, I used discrete multivibrators and then "upgraded" to 555 and CD4047 oscillators to drive the switchers, now all that is stone age technology.

    3AM now and very sleepy, but tomorrow will search a couple pictures of the portable amps I mentioned ... although Iīve already shown a couple of them here.

    That said, many of my 12V 60W RMS OT equipped amps are still running, one of them not 60 meters from my home, used by a famous Tango singer, and still using this "protected wasteless" OT.
    Juan Manuel Fahey

  17. #17
    "Thermionic Apocalypse" -JT nickb's Avatar
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    I did a bit of desktop experimenting on the idea of shutting down the drive automatically.

    1) spike detector
    When I mentioned it, what I had in mind was something very simple. Essentially you divide down & peak detect the output to a level that will trigger a small 600V TO92 type SCR on spikes. The SCR is connected via two diodes to each plate of the PI (via an "speaker open" warning LED if you wish). The few mA through the plate resistors (beefed up ones) is enough to hold the SCR. This cheap and simple circuit does work BUT I learned that the voltage spikes are present under normal use meaning it triggers when you don't want it to and vice versa. Turns out looking for spikes isn't reliable. In fact I was able to get bigger spikes with a speaker connected by choosing the drive frequency and level right. Reject idea # 1 as false triggering a no no.

    2) Load detection (a)
    Next idea was a bit more complicated. The basic idea was to measure the voltage across the load and compare it to the current through it. If there is no load then the voltage will be high compared to the current. Since the load is actually rather reactive and the are voltage spike present it required quite a bit filtering to get it to work reliably. The downside was that the the filtering slowed the response to around 200mS. Too slow.

    3) Load detection (b)
    Since the reactive load was problematic I decided to try DC. You a put a small voltage source in series with the output and detect the 50mA (not enough to upset the transformer) or so across small resistor. This worked well but again was slow to trigger.

    4) Output level detection
    I reasoned that without a load the output average level will be maximized. Trouble was I could get false triggers, especially with a hard low frequency drive. Reject.

    The big takeaway here is that damaging spikes are produced during normal operation too, just not quite so often.

    With a lot more thought and especially complexity I'm sure something could be made to work but on balance do has has been said - shove in a couple of 600-800V MOVs and forget it. It's just not worth the extra effort plus you are protecting the components from spikes caused during normal operation
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  18. #18
    Old Timer J M Fahey's Avatar
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    Yes, thatīs the point, big problem is that even in normal operation you still have *some* very high peaks; *definitely* at resonance because impedance goes through the roof.

    I think it was Loudthud who posted some scope screen videos showing overdriven tube amps into real speakers, he was showing complex impedance so curves were "oval" instead of "straight" because of phase shift but it could also be seen that clipping point was easily visible as 2 "bright bars" yet peaks went WAY beyond them.
    Dear Loudthud, can you repost them?
    Unless I read them wrong, they showed voltage relative to current and you were playing some overdriven chords.

    As a side note: Ultimate Attenuator is basically an SS amp "current stage" driven straight from amp speaker out, which is loaded by a power resistor.

    For the 8 and 16 ohms version, they use a 30 ohm power resistor (2 x 15 ohms in series) as tube amp load.
    When challenged about the mismatch designer answered that it was fine and in real use that proved true, I guess he counts on an 8 ohms speaker going *well* beyond 30 ohms at resonance with no ill effects so, why bother?.


    Juan Manuel Fahey

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    Quote Originally Posted by SoulFetish View Post
    For whatever reason I was confused about this statement earlier, but I just realized you are talking about the anode voltage swing during full drive and that potential difference across that side of the winding.
    Sorry - I was not being clear. Yes, it was the voltage across the winding. The active side is the side of the primary being pulled toward ground by the "on" tube. The tube can't "saturate" to less than about 50V, so the CT of the OT is held at B+ and the active tube pulls one end to at most 50V above ground, so that half-primary has B+ minus 50V across it. The other half-primary has to have the same voltage, by transformer action, but in its case it's going above B+, so its plate-end swings up to two times B+ minus 50V.

    I would really like to get a deeper understanding of the magnetizing current, leakage inductance, and the relationship of energy to the electromagnetic field in a transformer. I apologize for sounding so incredibly generic, but I don't know how else to say "There is more for me to know with these things".
    Be prepared for it to take a while to feel comfortable and natural inside your head. My first job as an EE involved designing both low and high frequency transformers, and it took a while for me to understand what the devil was going on inside them.

    That's probably because we can't see the magnetic fields that make this stuff happen. The short version is that exciting any winding (which becomes the primary for this purpose) lets current flow in the wire. This current causes a magnetic field around the wires, and in a coil that field is concentrated along the axis of the coil and cancelled between the wires. So it makes an M-field down the axis of the coil and out around the outside of the coil. The field is concentrated into the space inside the coil, but spreads out into space around the outside, getting much more diffuse as it spreads out.

    If we arrange the wires to fill the space inside and outside the coil of wires with iron, ferrite, or a few other materials, these materials have much less "resistance" to "conducting" a magnetic field than air or a vacuum does. So the wires' current can make a much bigger M-field inside the iron than it can in ordinary space. The parameter "mu" is the relative "conductivity" of the magnetic core material for M-fields. For vacuum, mu is defined as one. Mu is actually the multiplier of the magnetic "permitivity", which is one of those ungodly numbers in physics that everyone just looks up when they have to calculate things. The mu value for transformer iron is usually in the range of 10,000 to 20,000.

    The field in that coil of wire we talked about would rather flow in the "easy" iron path, not in the difficult path outside the iron in free space, and by a factor of 10K to 20K or so.

    But the factor is only 10K to 20K. Some of the field leaks out and flows in free space anyway. It's a similar case to electrical current flowing in conductors. It takes a lot of voltage (ie. electrical "push") to force electricity to flow in free space, but if you put a material in the path with a higher electrical conductivity, the electricity flows down that path. How much electricity flows in the path versus out in free space depends on the "resistance" of the path versus the resistance of free space. We recognize this as leakage for current flow too. High voltage stuff gets leakage paths in air. However, conductors and resistors are incredibly better conductors of electricity than free space. Metals like copper are better at conducting electricity than free space by factors of billions and trillions, not 10K or so. The point of this is to say that our transformer irons are good M-field conductors, but not nearly as good as electrical conductors are of electricity and that ... THERE IS ALWAYS SOME M-FIELD LEAKAGE ... no matter what you do.

    In an inductor, we "charge up" the iron core with AC. This causes an alternating M-field in the core. A bit of magic you're just going to have to trust me on is that the resulting M-field in the core actually pushes back on the wire and causes an opposing voltage in the wire that keeps current from flowing freely in the wire. This "back EMF" is what makes an inductor keep the current limited in a coil. This turns out to be a big deal to how transformers work. The incoming ***AC*** voltage/current causes the core to be pumped up to a certain alternating M-field level, and this balances with the incoming voltage/current to keep the M-field about constant at some level inside the iron.

    Here's the secret that makes transformers hard to understand. A second winding on a "charged up" core also sees the same back emf per turn as the primary that is charging it up. If you let current flow in the secondary coil, that energy has to come out of the M-field. That drop in energy in the M-field decreases the primary back EMF, and lets more current in the primary to balance out the lower back emf and keep the M-field in the core constant. In effect, the energy going out the secondary passed through the core, but did not affect the size of the M-field in the core. The M-field in the core kind of sits to one side of the action (in the very special case of AC to AC transformers) and keeps its M-field constant, and energy flows from the primary to M-field and to the secondary without changing the charge level of the M-field in the core. This realization took me years to get to, even working with transformers daily.

    Current in a secondary drains some of the energy from the M-field, reducing some of the "resistance" of the M-field to the incoming AC, and this lets in more AC to bring the M-field back up.

    After that, it's all about leakage and transient conditions. Since iron is not as good a conductor of magnetic fields as copper is of electrical flow, some of the field created by the wires always leaks out of the iron into space, and field that leaks out may not cut across the space filled with other windings and wires. This leakage field acts like an air cored inductor all its own. You can think of the leakage inductance as an invisible inductor that is in series with every wire lead coming out of a transformer. You can't see it or do anything about it, but it is there and does V=Ldi/dt and E = 1/2*L*I^2 on its own for currents in the wire. Keeps current and voltages into/out of the effective part of the coil around the iron core from being "perfect".

    I've never seen the effect of current arcing in this way. What is the effect and why would it happen in this case?
    Both the speaker and the speaker cables are also inductors. They store energy as E = 1/2 * L* I^2. When you try to open the conductor and stop current flow instantly, the inductance generates a kick back voltage of V = L * di/dt. That di/dt is the rate of change of current flow with time. If you were able to stop it truly instantly, the voltage would be infinite. What actually happens is that any inductor causes a kick back voltage that rises until **something** conducts to keep the current flowing. There's a lot of current flowing in the speakers.
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  20. #20
    Senior Member Wombaticus's Avatar
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    Why not gas discharge?

    Quote Originally Posted by R.G. View Post

    Gas discharge tubes are a different animal, and IMHO not suitable for OT protection.
    R.G., please elaborate. What properties, in particular, of gas discharge tubes do you believe disqualify them as being suitable for OT protection?

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    Quote Originally Posted by Wombaticus View Post
    R.G., please elaborate. What properties, in particular, of gas discharge tubes do you believe disqualify them as being suitable for OT protection?
    Gas discharge tubes break over at a certain voltage. This ionizes the gas inside, which then switches to being a heavily conducting plasma. The resistance drops dramatically, and smaller voltages and currents then keep the gas ionized and conducting. If a gas discharge tube breaks over at, for instance, 500V, it will conduct until the current drops to zero, or until the voltage across it drops below the value needed to keep current flowing. This dropout voltage is much less than the breakover voltage. So a 500V GDT might have a sustaining voltage of 100V.

    In this way, it's like a crude SCR. It fires, and stays conducting til something outside it shuts it down. That's GREAT for lighting strikes, but not so good if your power supply wants to keep 500V across B+ all the time. Something gets damaged or dies. You need a lot of design work to make sure that what dies is what was intended to die.
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  22. #22
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by J M Fahey View Post
    I think it was Loudthud who posted some scope screen videos showing overdriven tube amps into real speakers, he was showing complex impedance so curves were "oval" instead of "straight" because of phase shift but it could also be seen that clipping point was easily visible as 2 "bright bars" yet peaks went WAY beyond them.
    Dear Loudthud, can you repost them?
    Unless I read them wrong, they showed voltage relative to current and you were playing some overdriven chords.
    I think what JM is referring to is post 37 in this thread: How do the diodes protect the OT?
    Note: The only things I know of that will play these video files is Window's Media Player and RealPlayer.

    The file MVC-531W.MPG is an X-Y display of an amp (~100W) driving a 16 Ohm 4x12 speaker cab. The vertical (current) deflection factor 200mV/div is the Voltage across a 0.1 Ohm sense resistor. So that is 2 Amps per division. The Horizontal (Voltage) deflection is 20V per division.

    Back to an earlier point JM made, when the plate to ground diode is conducting, where is the current going? One side of the OT goes to ground through the diode and current that wants to flow goes out the center tap to the power supply. The diode just completes the circuit. This relys on transformer action and does not load the leakage inductance that is on the side of the OT that just turned off. It's not like the case of a relay where the diode shorts the inductive spike and current never really flows back into the power supply, it just flows in a loop back to the relay coil.

    When an OT is coupled to a speaker, from the primary side the transformers primary inductance is in parallel with the load so it is hard to separate the two. I think they look like one inductor to the tube.
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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  23. #23
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by loudthud View Post
    I think what JM is referring to is post 37 in this thread: How do the diodes protect the OT?
    Note: The only things I know of that will play these video files is Window's Media Player and RealPlayer.
    Real quick: VLC plays the video, but in case anyone has any issues, I combined them and converted them to a H264/mp4 video.
    Here you are -
    MVC-531W • 081W • 080W .mp4.zip
    (sorry, it's a little larger file than I wanted, but it was a quick and dirty conversion)
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  24. #24
    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by R.G. View Post
    ...
    Here's the secret that makes transformers hard to understand. A second winding on a "charged up" core also sees the same back emf per turn as the primary that is charging it up. If you let current flow in the secondary coil, that energy has to come out of the M-field. That drop in energy in the M-field decreases the primary back EMF, and lets more current in the primary to balance out the lower back emf and keep the M-field in the core constant. In effect, the energy going out the secondary passed through the core, but did not affect the size of the M-field in the core. The M-field in the core kind of sits to one side of the action (in the very special case of AC to AC transformers) and keeps its M-field constant, and energy flows from the primary to M-field and to the secondary without changing the charge level of the M-field in the core. ...
    Good explanation from R.G.

    Just to add something that might help with one of the tricky bits:

    The energy transfer through a transformer is electrical energy to magnetic energy and then back to electrical energy. For electrical energy transfer we need to have both voltage and current. For magnetic energy transfer we again need two quantities at the same time. These are magneto-motive force (measured in ampere-turns) and magnetic flux (in webers).

    Magnetic energy input to the core is the magnetic flux multiplied by the ampere-turns of the primary (or primaries). Magnetic energy output is the (same) magnetic flux multiplied by the ampere-turns of the secondary.

    This helps to explain why the magnetic flux can stay the same, whether there is energy being taken from the secondary or not.

    (Ampere-turns is the current in a coil multiplied by the number of turns.)

    Edit: It’s analogous to the way that energy is stored in a capacitor by virtue of the voltage across it, but to transfer energy in or out of the capacitor we need current as well.
    Last edited by Malcolm Irving; 10-18-2016 at 12:38 PM. Reason: improvement

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    Malcolm is right about both voltage and current being needed for power transfer. There is a degenerate mode of energy transfer out, where the M-field is sucked out by magnetic means, as in a solenoid or by mechanical means, as in a motor, where a conductor or magnetic material is physically moved by the field, but we hope our power and output transformers are not doing much of that.

    Here's another quirk I should have put in. The voltage generated in a conductor by a magnetic field is proportional to the *rate of change* of the magnetic field, not the field itself. So a conductor has no voltage induced by a static "dc" field. Sine waves are great for this, as the input field is changing all the time (except those two peaks) and so the input voltage creates a constantly changing V*dt drive for the stored magnetic field, which then changes constantly, and couples the changing field into the secondary windings so they have a constantly-changing and similarly sine voltage. This comes right out of the inductor equation, where V = L *di/dt.

    My engineering compatriots chuckled at my fascination with transformers. They thought they were dull. I was sitting there visualizing toroidal fields with leakage and dissipation as 1/R^2 out to infinity. Well, OK, it was a kind of idiots' delight, I guess.
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    Senior Member SoulFetish's Avatar
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    I've been meaning to get back to this for some time

    I've been meaning to follow up on a couple of points for some time. But first I want to sincerely thank all of you who consistently take the time to write thorough, thoughtful responses to my questions in this forum. I really appreciate the experience and depth of knowledge you guys bring to this joint.
    So, when you write half a page explaining something to me -- yeah, I read it. Often more than once... and sometimes end up having look up a couple of things
    (like this for instance: "current in the core is the integral of Vdt/L from zero up to the voltage". I'm not sure I even got a clear answer as to what Vdt is. I think it stands for Voltage in a differential transformer, but I'm not certain)

    Anyways, I bought some MOVs for my output transformer and I want to double check that I'm looking at the right criteria and these look suitable to you guys.
    The recommendation was a MOV in the rang of 600V-800V. I found one rated for 680V RMS and an energy rating of 440J (@ 2ms) with a max capacitance of 350pF. I like those stats, so I bought a few. However, there are a few different voltage characteristics which seem significant. When I got the invoice receipt the item description stated it was a 1100V MOV. I thought MOVs were most commonly designated by the max RMS working voltage. I guess I'm not sure if the 600V-800V as specified above describes the RMS voltage or the clamping voltage.

    Here is the MOV part number which I purchased: Panasonic ERZ-E14A112
    Here is the Datasheet:
    Panasonic ZNR_MOV transient surge protection.pdf

    Did I f*ck it up?
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  27. #27
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by SoulFetish View Post
    I've been meaning to follow up on a couple of points for some time. But first I want to sincerely thank all of you who consistently take the time to write thorough, thoughtful responses to my questions in this forum. I really appreciate the experience and depth of knowledge you guys bring to this joint.
    So, when you write half a page explaining something to me -- yeah, I read it. Often more than once... and sometimes end up having look up a couple of things
    (like this for instance: "current in the core is the integral of Vdt/L from zero up to the voltage". I'm not sure I even got a clear answer as to what Vdt is. I think it stands for Voltage in a differential transformer, but I'm not certain)

    Anyways, I bought some MOVs for my output transformer and I want to double check that I'm looking at the right criteria and these look suitable to you guys.
    The recommendation was a MOV in the rang of 600V-800V. I found one rated for 680V RMS and an energy rating of 440J (@ 2ms) with a max capacitance of 350pF. I like those stats, so I bought a few. However, there are a few different voltage characteristics which seem significant. When I got the invoice receipt the item description stated it was a 1100V MOV. I thought MOVs were most commonly designated by the max RMS working voltage. I guess I'm not sure if the 600V-800V as specified above describes the RMS voltage or the clamping voltage.

    Here is the MOV part number which I purchased: Panasonic ERZ-E14A112
    Here is the Datasheet:
    Panasonic ZNR_MOV transient surge protection.pdf



    Did I f*ck it up?
    For an inductor V = L. (di/dt) i.e the voltage is proportional to the rate of change of current in the core i.e i = integral( V/L.dt) is just the same thing - the current is proportional to the integral of the voltage over time.

    800 Vrms == 800*sqrt(2)=800 x 1.41=1128 Vpeak

    That's probably higher than you wanted, but much better than nothing.
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by nickb View Post
    For an inductor V = L. (di/dt) i.e the voltage is proportional to the rate of change of current in the core i.e i = integral( V/L.dt) is just the same thing - the current is proportional to the integral of the voltage over time.
    Thanks Nick. I understand "conversational" math well, but I suppose I suffer illiteracy when i see expressions i can't define. So "rate of change" is defined as the change in current "i" in amps over time "t" is expressed as a derivative (di/dt)?
    I was an art student and my strategy was to avoid any math course which wasn't an absolute requirement, and then put that off for as long as possible. My algebra teacher used to say "Corey, you're smiling... you're not doing algebra!". She was correct, I wasn't, but it certainly not a very inspirational way of being introduced to do something. If I knew I was going to enjoy electronic design so much, I would've gone to class! I guess I need to go back into brush up on some calculus and trig, or formulas will look like Zapf Dingbats for the rest of my life.
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  29. #29
    "Thermionic Apocalypse" -JT nickb's Avatar
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    If it's any consolation, I'm really, and I'm mean really, bad at art
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    Senior Member trobbins's Avatar
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    MOVs of same type/model can be connected in series - this means you could use MOV's commonly used for say 240VAC applications. The MOV spec rating you want to identify is the minimum Vdc rating at 1mA - that identifies the voltage across the MOV at which the MOV resistance is just starting to reduce and starting to softly load the voltage across a primary half-winding. Eg. if the MOV has a 330Vdc min, 1mA rating, then its resistance is no lower than 330kohm at that voltage (ie. not much of a loading).

    I typically use 2 or 3 series connected MOV's with that 330Vdc 1mA rating for primary half-winding protection. As with capacitors, a series connection will have a lower capacitance than a single part (ie. 50% or 33% of nominal part capacitance).

    It is worth trying to appreciate that the winding has self-capacitance, and so any voltage spike or extension from leakage inductance (as per loudthud's video) shows voltage waveforms where the leakage energy forces the self-capacitance (from winding and plate circuit) to a certain peak value Vpk (0.5 x C x Vpk x Vpk). If a MOV device is used to restrict voltage across that winding, then it acts as an additional means of soaking up the energy in the leakage inductance, and the MOV doesn't by itself get hit with all that energy but just the incremental energy left after charging up the self-capacitance. If 3 MOV's in series are used, then any energy soaked up by the MOVs is spread across 3 devices.

    MOV's have been developed to not degrade at all when they soak up limited levels of energy. MOV's can get a hard time on mains AC side applications, as the mains can provide high surge transient current levels. For valve amp protection, the level of peak energy able to be applied to a MOV is quite limited/constrained - a very different application for assessing service life and degradation.
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    Senior Member SoulFetish's Avatar
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    Trobbins, thanks for the tip about connecting them in series. That actually helped me solve another issue I had run into on the mains side of my Power transform. I wanted the ability to switch means voltages so I designed it to have primary connections for 120, 220, and 240V. I wanted to be able to protect against transients for each of those taps so I initially had water protection across the primary windings in from each line to earth. Like this:

    img_2515.png

    But I realized I can't do that because that MOV for 120V neutral side is all we switched in and would conduct to early when connected to the 220 and 240 taps. I realized I'm probably going to lose my line to ground mov protection unless I can find use another type of switch. But I can still have proper protection across each leg of the primary windings using appropriate series Connected MOVS bridging the taps. I don't see why this wouldn't work seeing that it will always maintain its voltage divider ratio. Does that male sense?
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    In general, it would be better to alleviate any AC mains disturbances at or near a domestic main switchboard (or similar upstream point of coupling), and not within a particular item of equipment. Any MOV that is full-time connected to the AC mains becomes a local sinkhole for significant disturbances 24/7, such that high loop currents could circulate through the establishment to that item of equipment, and could cause neutral voltage disturbances on the way.

    I can see the benefit in applying a MOV to the PT primary winding, as that could alleviate AC mains switch contact pitting, depending on the timing of switch on/off, and could alleviate damage to the PT if something bad did befall the mains. I guess you could put a MOV on each of the PT primary windings for that purpose, if you know the tapping is being manually changed.
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  33. #33
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by trobbins View Post
    I can see the benefit in applying a MOV to the PT primary winding, as that could alleviate AC mains switch contact pitting, depending on the timing of switch on/off, and could alleviate damage to the PT if something bad did befall the mains. I guess you could put a MOV on each of the PT primary windings for that purpose, if you know the tapping is being manually changed.
    It's really there to protect the PT in the event of a large transient spike/surge which could destroy it. The mains selector would half to be manually switched and the chassis pulled out of the head (or I may put in a small screw access panel).
    This is the set up here:

    voltage-selector-iec-1.jpg

    voltage-selector-iec-2.jpg

    Windings and MOV connection:
    pt-windings.jpg
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    Hi Guys

    I never use MOVs nor buy any of those "spike protector power bars" that have them because MOVs degrade as they absorb energy, and their ultimate point of zero-protection is unknown. The diode protection method for OTs seemed a bit pointless in some regards, too, as RG has commented. Instead of using active devices or reactive ones, I use resistors to damp the OT. A simple R from anode to ground on each side and a load resistor on the secondary is all one needs to impose a maximum resistance to the tubes if the load opens. The Rs absorb energy benignly and last the life of the amplifier. My amps have had no blown OTs since I began building nor has anyone bulding the designs shown in TUT3 or TUT5.

    JF: Thirty ohms is a common value for pure resistance in the reactive load boxes built by various manufacturers. Thirty ohms dissipates a lot less power than a true 8R resistor, so one can use a smaller part and deal with less heat. The reactive components bring the nominal value to 8, 16, whatever,over most of the frequency range, but you see 30R even when no reactive elements are present, as in the Ultimate Attenuator you showed in post-18. That design was shown in TUT4 and was designed by a tech in Vancouver BC, named Ho. It changes the sound fed to the speaker because the EF stage has lower output impedance than most tube amps and the load is fully isolated from the tube amp.

    Have fun

  35. #35
    Senior Member trobbins's Avatar
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    Quote Originally Posted by KevinOConnor View Post
    I never use MOVs nor buy any of those "spike protector power bars" that have them because MOVs degrade as they absorb energy, and their ultimate point of zero-protection is unknown.
    Kevin, if you checked out the lifetime and degradation performance literature of MOVs with any effort, the reality would be that there is no degradation effect for modern MOV's subject to less than 'moderate' impulse energy levels (compared to their impulse energy rating). Operating a MOV in an energy limited situation, and with normal margin for selection of parameters, is effectively as reliable as a resistor degrading with time.

    Ciao, Tim
    nickb and SoulFetish like this.

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