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Reply With Quotethey are usually 120-130VAC but a few people here recommend using 150VAC minimum as they last longer, see RG article here
The Immortal Amp Mods, Pt. 3 - Premier Guitar
RG also recommends very high voltage MOVs on the OT as protection
That little disc that goes on the power switch... I don't really understand how to select a rating.
they are usually 120-130VAC but a few people here recommend using 150VAC minimum as they last longer, see RG article here
The Immortal Amp Mods, Pt. 3 - Premier Guitar
RG also recommends very high voltage MOVs on the OT as protection
Thanks for that link! I did pickup a few 150V MOV's at a 395 clamping voltage. But I wasn't sure if the clamping spec would be right. It's amazing to me that the AC line could ever get up to 395v. But it must happen really fast, eh?
V150LA10AP Littelfuse Varistors
Does the MOV short itself out when it fails?
I usually think of them as shorting to ground at high voltages; they don't conduct until their clamp voltage gets exceeded. They are said to "fail" after repeated surges move they clamp voltage >10%, and parallel MOVs will take more surges.
That layout shows 3-MOV's installed after the power switch. Do you see a problem mounting then on the back of the IEC? It would be easier to ground them out that way.
I like to put these things after the switch and certainly after the fuse. The reason being they wear out and eventually fail short. If you leave your amp plugged into an outlet, you don't want the voltage on the MOV all the time when you're not using it.
However, there are many IEC outlets and power strips out there with built in MOVs. I remember seeing a lot of burnt ones in a UL bulletin, though.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
Transients of up to 1kv and more are fairly common. They're spikes, but they can puncture insulation. That's one reason that the 1.4kV test for primary-secondary insulation has moved up to 2.5kV, then 4kV.Originally Posted by leadfootdriver
MOVs conduct when the voltage exceeds their clamping voltage. Each breakover eats up a little of the insides of the MOV, and the breakover voltage gets a little lower. Eventually, if it conducts often enough, the breakover voltage drifts down to the peak of the normal AC power line. Then it breaks over 100 (or 120) times per second, and the voltage drifts lower, and lower, and soon it's conducting all the time, the power from the conduction gets huge, and you have yourself a Light Emitting MOV (LEMOV). Well, some people would call it a DED (Darkness Emitting Diode).Does the MOV short itself out when it fails?
Whether it opens or shorts in its Light Emitting period is open to discussion. Eventually it opens or the breaker does. This is why fuses in series between the MOV is a good idea. Using 150Vac instead of 130Vac (on a 120Vac nominal line) is a good idea because it gives you more room to drift down before the MOV's lifetime is over. What you want is for the line to open before the resistive (volts times amps) power generating phase of the death can start fires.
Thanks for the practical answers!
I couldn't search anything that made sense to me for an amp build.
They only short to ground if they are wired to ground. Neutral isn't ground, though it may be connected to it at the service panel.
Education is what you're left with after you have forgotten what you have learned.
Hear Ye! Hear Ye!
Read All About It.
Here ia a well written AppNote from Littlefuse, on voltage suppression devices and applications.
I read somewhere that MOVs are known to explode and set equipment on fire which doesn't sound like something that I need in my equipment...
Put a short section of heatshrink tubing over a MOV on the AC input if the position of the MOV cannot cope with the risk of a 'failed' MOV. This is also appropriate to NTC surge thermistors.
The PT primary can also cause transient voltages on the primary circuit after the mains power switch is turned off - the MOV across the primary alleviates any stress on parts that result (eg. stress on an old power switch).
The MOV across the Output transformer half-primary is effectively a shunt capacitor - so the MOV size should be kept as small as possible to reduce the capacitance (eg. 7mm disk size) - and preferably a resistor is placed in series with each MOV to make an RC zobel network (eg. resistance similar to OT primary loading). The DC 1mA voltage rating of that OT MOV should be somewhat above the DC HT level of the amp, and so would vary with the amp type/model. MOVs can be connected in series to achieve high DC voltage ratings. I reckon this form of protection is more effective than using a 'speaker side' pre-load resistor for making an amp more rugged to open-circuit output conditions.
Ciao, Tim
All Boogies have them, there Ralphie boy! I remember that was part of their advertising. I didn't even think to look at a Boogie schematic...
Last edited by leadfootdriver; 01-01-2012 at 01:07 AM.
Not only Boogie has them. I don't know how often they explode but since it was mentioned I would assume they were not talking about a couple of cases only.
The other thing about NTC surge thermistors was they work as specified only if your device has a constant current draw which is not the case with a guitar amp so at this point I'm not really convinced I should use both.
Thanks for the reply.
You mentioned capacitance in the OT mod. But what about just the AC lines? Does the capacitance of the MOV have a snubbing cap effect for the whole amp?
I was reading the TVS file jazz p bass posted, and it did say that they could explode but only if the max spec is exceeded. That would be 4500V for what I bought. But I never heard of it until you mentioned it.
I'm not interested in putting them on my OT secondaries tho. I haven't seem a schematic that uses that.
I find that when I start writing "I read somewhere..." that I need to go check that out.
MOVs are known to explode and set equipment afire; this is true. It's also true of capacitors, resistors, diodes, and wires (see "exploding wire" on google; they're used as blasting caps to set off explosives). All these things can be made to explode and start fires in exactly the same way as a MOV: you put in massively more electrical energy than it can take, and it heats up to the temperature where the materials inside melt and vaporize. You can explode nearly anything that can conduct this way.
The reason MOVs are known to explode and/or catch fire is because they're connected to electricity. They won't go off sitting on the shelf. You can pretty much guarantee your equipment won't explode or catch fire by disconnecting the AC power cord. But that's not all that useful, either.
Like everything used for electricity, applying MOVs requires understanding what they do and thinking about what failures would do. MOVs get connected to high-power electrical supplies as part of their normal job, so it makes sense that if you get it wrong, lots of electricity can get funneled into them, with various bad results. On the other hand, they are intended to absorb damaging transients which could cause other damage. The trick is to use them to protect against transients without adding problems. This is possible; otherwise, MOVs would not be commercially available. For instance, those "power line conditioners" you buy in power strips? Several MOVs (or tranzorbs, similar thing) in each one.
I put MOVs across the primary of the Workhorse amps for transient protection. They do have some capacitance. But I found that I could not detect any change in the frequency response of the amp in the audio band with them there or not. For amps with overall negative feedback, this may or may not be an issue, but my conclusion was that they were something that could be ignored as far as the sound of the amp goes. They're a lot more reliable than the string of high voltage diodes on each side of the primary.
They do different things. MOVs and similar devices do essentially nothing unless there's a voltage transient, which they eat, converting the transient energy into heat.
NTC surge thermistors are specified for much more than constant current use. They are a resistor that decreases in resistance as it gets hot. As long as the amp pulls X current, they insert a negligible amount of resistance in series with the power line, because there is enough current to get their resistance low enough to be negligible. When the amp is cold, they have a bigger resistance. The trick in designing with NTC thermistors is to learn from the datasheet what their resistance-versus-temperature and resistance-versus-current behavior is, and use the fact that when the amp is off (and presumably the NTC is cold) the NTC will have a high resistance for a few AC power cycles before heating up and becoming negligible in the power circuit. You also have to provide for the fact that the NTC stays hot as long as the amp is on.
They do different things. You may or may not need both, depending on what abuses you want your amp to be immune to.
A MOV on the incoming AC power line affects only the AC power in. There are some quite large caps (i.e. the power filters) between the AC power line and the audio circuits that completely swamp any capcitance of MOVs on the power line.
And as I mentioned, I found no change in the audio response from MOV capacitance on an OT. It depends on the capacitance and the impedances of the circuit it's attached to. If the self-capacitance of the OT primary or the wiring/tubes/secondary reflected load/etc connected to it is large compared to the actual MOV capacitance, the MOV does not materially affect the audio response.
Can one horse run faster than another one? Absolutely. But WHICH ONE? Details matter.
Great post! Thanks for that.
It looks like it was the Wiki. I know it's not reliable but it's a hint to investigate further:I find that when I start writing "I read somewhere..." that I need to go check that out.
Varistor - Wikipedia, the free encyclopedia
It turns out that thermally protected MOVs (TMOV and iTMOV) are available that will take care of the "exploding" problem:
US - Electronic Components Distributor | Newark.com
I'm aware that MOVs and NTCs will handle different issues. This looks like a good article about inrush current limiting:
Inrush Current
Your incentive may depend on whether you can get a replacement OT, how expensive a replacement is, and whether you've 'lost' an OT in an amp before and suffered the pain. This type of OT failure is not prevalent, but it is 'common', and amp designers/restorers are including OT protection, with the main OEM form being a pre-load resistor on the speaker side, as that is the simplest and lowest cost option to statistically reducing failure rates. Some older amps also used just C, or RC networks, across the half-windings, or from anode-to-anode, both to tailor high-frequency response and to provide OT protection.
I know of one PA amp manufacturer using plate-to-ground valve diodes for such protection back in the early 60's - it's not a simple addition to squeeze in two 6AL3 under the chassis! Peavey alert their owners to the risk as well (• View topic - Just a question....).
Hi-fi amps would notice the added capacitance of a MOV, and inadvertantly using a 'large' MOV in say a 10k PP is likely to start pulling in the treble end of even a guitar amp. For hi-fi'ers the aim would be to use the MOV-R to dampen say the main transformer self-resonance, but be negligible in the audio range.
Ciao, Tim
I suppose I'd be biased to do it if I had a meltdown!
It would be easy to do since I have a perfboard style turret board coming. I bought this style of board so I could have freedom of construction.
The schematic I'm building from specs a B+1 of around 395V. We'll see what it ends up as when I get it running. But assuming that's what I'll end up with, should I spec a MOV the same way as the AC line; giving myself a safety margin for degradation?
I found 429VDC MOV that clamps at 650V.
TMOV14RP250E Littelfuse Varistors
The next MOV in the radial style is a 473VDC that clamps at 710V.
TMOV20RP275E Littelfuse Varistors
For a PP OT half-winding MOV, the 1mA DC voltage, at minimum tolerance end needs to be safely above the high tolerance end of your B+ (eg. mains high, no load) - so you should be looking for probably a 320E model or higher for starters (eg. 459VDC 1mA rating). Those parts are 14mm/20mm disks - they have pretty high capacitance - I'd suggest trying to stay within 100-200pF. The joule rating for this application is way over the top no matter what size common MOV you choose.
I think the 1mA VDC rating is the best design guide to use. The clamping voltage is not particularly useful, as it is based on a particular test circuit and is likely to be fine for the transformer as the main aim is to keep the primary winding from stressing voltage insulation levels.
Smaller diameter MOVs may not be stocked in the voltage rating you want, so one workaround is to use two or three MOVs in series. It's best to place the MOVs or MOV-R parts as electrically close to the relevant primary half-winding as possible - as that is the root cause of any damaging transient.
Here's a 505vdc. It's the 14mm/20mm you speak of. It has a capacitance of only 40pf.
VDRS05C385BSE Vishay/BC Components Varistors
That part number is a 7mm diameter part - which should be fine - and yes it has low/negligible capacitance. The 1mA 620VDC rating is +/- 10%, so would be fine for amps with B+ max up to about 500-540VDC, which may be equivalent to say a nominal 450VDC B+ at idle.
I have a batch of 7mm 330VDCmin movs, which have come in handy for a wide range of amp restorations, including up to 3 in series for 800V B+ amps.
Right! 7mm... I was trying to make sense of the data sheet, and somehow thought it was the larger ones.
Thanks!
Yes. Even more, give yourself some margin for AC power line variation.
What you're trying to protect against is a transient spike puncturing the insulation inside the OT. These things are caused by things like sudden interruptions of current flow in the inductance of a speaker load (somebody yanked the cable out), a power tube trying to instantly stop the current flowing in the leakage inductance on the primary of the OT, and perhaps even a (VERY!) stout electrostatic discharge on the secondary wiring. All of these have in common a sudden voltage rise between one of the OT windings and another winding or ground.
We're not too interesting in putting in just enough voltage resistance. The layer insulation in most OTs is probably good for 2kV. I say "probably" because no OT maker is going to either test it or tell you the results if they did test it.
On the primary side, the "on" power tube pulls down on its half-primary, forcing it to within 50-100V of ground if it's really cooking, or honestly to ground if the tube shorts. Since the primary of the OT is an autotransformer, pulling one side down causes the other side to go up above B+ as much as the active side went down. So each half of the primary has to be insulated to at least twice B+ just to work normally. And B+ varies. It's reasonably constant at any given operating condition, but the AC line varies +/- 10% at least, so "normal" B+ may be 10% high all the time. And it could rise even more in fault conditions where a tube opens up.
All that's a way of saying that I think a transformer's internal insulation is good for much more than 110% of two times B+. You want MOVs to protect the insulation, so the voltage on the MOV breakover needs to be bigger than the 2.2*B+ and smaller than the breakover voltage of the insulation. So far, so good; but we have no idea what the insulation breakover really is.
So add in some padding. Make the MOV voltage be ... um... three times B+. 3.241 times B+. Pick a number. But don't pick it too low.
The window of acceptable MOV breakovers is easy to hit in this application because OT insulation is usually very good. You just want something to keep V = L* di/dt from going to infinity, punching an insulation hole along the way. T.R.'s right - the energy rating of any MOV you're going to put in there will be massively more than needed to clamp the transient energy. And that series connecting lower voltage MOVs is a good way. I had to use two in series on my amp line.
R.G. - your 2.2 and 3.3 ratios are for full primary winding protection :-) Half-winding MOV configuration is half.
Yep. One could certainly do this same kind of calculation to get half-primary values, then use two of them. It's probably not a good idea to only MOV-protect one half. I was mostly doing the calcs to show what things to consider. In all cases, you want a MOV value that's comfortably above the voltages that happen in normal operation, but below the level at which damage happens. This is only tricky when those two voltage levels are close together.
When I was a kid, and we are talking 50 some years ago, I have a scope tube given to me, and i was trying to build a scope around it. I needed something like 1500v for the third anode - or whatever number the side of the tube was in a 5UP1. Having enough knowledge to be dangerous to myself, I figures, gee, if I send 120VAC into the 12v winding of a 120/12v transformer, I ought to get about 1200v out the primary. I could then rectify that for my tube. By golly it worked. The little transformer lasted even a couple months befor it self-destructed. Litle arcs showing through the fish paper.
But I got a trace on my screen and some deflection. In retrospect, pretty impressive the abuse that little transformer took before it croaked.
Education is what you're left with after you have forgotten what you have learned.
I recall powering up a 5UP1 in high school physics - ahh they don't make 'green' like that anymore!
For OT flyback protection, are three series 1n4007's reverse biased frowned upon? I thought I was safe adding those, never used MOV's.
"Tell them I said something." - Pancho Villa's last words
For Portuguese speakers: Amplificador Valvulado
As I see it, catch diodes are a good easy means to protect the OT. But they don't directly clamp the over-voltage transient that would be the cause of an insulation failure, as they clamp the voltage of the 'other half' of the primary winding in a PP stage, and so rely on the coupling performance between primary half-windings.
All methods will have their pro's and con's - I don't think its a big deal in the long run. I guess the main point is to at least include some protection if you have the chance.
I always thought of the three diodes as a single use MOV. You can buy 100 1n4007's for a few bucks, thus my question if they're ok because I'm solder-happy joining them together and modding amps to include these.
The way I see it, and I may be wrong, if any of the extremities of the primary develops > 3kv, the diodes short and it's a blown fuse, and you need diode substitution. I've seen Fender Hot Rods use this, I think all Trainwreck amps do(I got this trick off Ken Fischer's tips on a book) and I've done it on some experiments of mine, I've never seen them fail so far.
The center tap on the OT comes directly from the power supply reservoir, I figured no component on the PS would stand several kv.... Having each end of the winding clamped I thought I was safe, thus the question.
"Tell them I said something." - Pancho Villa's last words
For Portuguese speakers: Amplificador Valvulado
A PP primary CT is heavily clamped to B+ (unless an in-line fuse fails), and so won't effectively move from that voltage level.
If secondary, or primary winding currents are abruptly changed (eg. speaker plug pulled out at high listening level, or one of the PP valves fails), then the dI/dt level effectively causes a high voltage across all transformer winding inductances. One half-side of the primary would be forced to go below 0V, and the other half-side would go higher than 2x(B+), as the CT is clamped at B+. The diodes clamp the negative going end to just under 0V, and the inductive energy in the transformer is dissipated through that clamped winding as it allows current in that winding to 'continue'.
Yes, that's true for energy stored in the transformer's magnetising inductance, or a reactive speaker load. It is safely returned to the power supply, diodes conducting in the forward direction.
But for energy stored in the leakage inductance between half-primaries, the diodes get stressed in reverse, perform like the 3kV ghetto zeners and eventually (not instantly, IME) burn out. In guitar amp OTs that leakage inductance can be substantial, and when the amp is cranked up full and abused with a Big Muff, the spikes can be pretty substantial too.
You could use diodes and MOVs. Or TVS diodes. Or a RCD snubber with the capacitor voltage monitored and hooked up to a shutdown circuit, as I once did with a heavy duty IGBT switch where mistimed switching would generate very destructive spikes, and I couldn't find a MOV big enough to eat them.
None of this will prevent you from smoking your screen grids and/or screen resistors on an open circuit load. Bean-counted commercial designs can't bear the cost of anything more than the diodes, unless you were lucky enough to get one of RG's mythical amps.
Last edited by Steve Conner; 01-03-2012 at 08:33 AM.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
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