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Thread: speaker jack shorting or not?

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    speaker jack shorting or not?

    I have seen both versions. It it better to make the jack shorting or not? Of course as long as the speaker cable is inserted it doesn't make a difference and it should be plugged in all the time. It's just in case someone forgets to plug in the speaker ... is the risk of damage smaller with the output wires shortend or open?

    thanks!

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    Tube amps and solid state amps have opposite reactions to load impedances.

    In simple designs, solid state amps die almost instantly from shorted outputs. It's so fast and total that almost all solid state amps have designed-in protection from shorts that in turn, they're essentially immune to any load resistance from an open to a short.

    Tube output stages can die from high voltage spikes generated when too high a resistance is connected to their output. In simple tube designs, they're almost immune to shorted outputs, but can die from open circuits, or any load resistance above some amount. The exact amount is hard to determine, but open circuit is generally bad for them.

    I think the best thing might be to put only NON-shorting output jacks on solid state amps. Tube amps might use dead-short or resistance-connecting jacks. Both would be better than non-shorting jacks for a tube amp.

    The problem with speaker cables is that external forces can either short the output cable by squashing it and cutting the insulation, or cut it open entirely, depending on Mother Nature's mood at the moment, and how Her assistant Murphy feels about it.

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    Thanks, that makes sense!

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Bluefinger View Post
    I have seen both versions. It it better to make the jack shorting or not? Of course as long as the speaker cable is inserted it doesn't make a difference and it should be plugged in all the time. It's just in case someone forgets to plug in the speaker ... is the risk of damage smaller with the output wires shortend or open?

    thanks!
    Solder a 360 ohm (if an 8 ohm winding) to 470 ohm (if a 16 ohm winding) 5 watt power resistor across the highest impedance secondary to chassis ground. Then forget about it.

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    On a tube amp Id leave the jack switch open but have a 220-470ohm resistor wired permanently across the OT secondary (not through the switch). Theres always the possibility that the speaker cable could become disconnected at the speaker cab but still be plugged into the amp. A shorting jack on the amp wont offer any protection in that case.

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    Bruce must have posted as I was typing. All my amps have a 470ohm resistor across the 16ohm tap.

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    Quote Originally Posted by Bruce / Mission Amps View Post
    Solder a 360 ohm (if an 8 ohm winding) to 470 ohm (if a 16 ohm winding) 5 watt power resistor across the highest impedance secondary to chassis ground. Then forget about it.
    If I can just piggy-back onto this question.....
    I rebuilt a BF/SF Bassman for a friend. His OT was bad. I installed a 4-8-16 Ohm OT from Mag Comp.
    At present there is no "protection" from an open circuit.
    So, with that OT, I should run a 360 Ohm, 5 watt resistor from the "tip" of the 16 Ohm speaker jack to ground.?
    Thank You

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    ok, thanks. that's easy, I'm going to do that.

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    Quote Originally Posted by Dave H View Post
    Bruce must have posted as I was typing. All my amps have a 470ohm resistor across the 16ohm tap.
    I do this too.

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    I was just thinking about this general topic. That's always dangerous.

    I did some reading about tube amp outputs, open circuit loads, OTs and such. I did not find a specific cause of death for tube amp outputs open circuited in a way that was definitive. I speculate that output death in tube amps with open circuit loads comes in one of two ways, sudden opening of the output, and oscillation of the output stage.

    I *know* that simply opening the output side of an OT in an amp without feedback is not uniformly deadly or even something to write home about. I know this because I did this accidentally on one of the prototypes for the Workhorse amps for about an hour a couple of times. There was no change in the amp at all, no bad effects whatsoever.

    I speculate that there are two causes of death for an unloaded tube amp: high voltage spikes and over-dissipation of the output tubes. The high voltage spikes would be a consequence of the leakage inductance and perhaps primary inductance of the OT primary having its current suddenly interrupted or reversed, and the overdissipation would be caused by RF oscillation in the power tubes, with perhaps similar high voltage effects in the leakage inductance. The actual death syndrome for spikes would be arcing of the power tube, tube socket, or internal insulation of the OT itself. Small, low energy spikes would arc and cause minimal damage, but repeated arcing would reinforce the damage until it was so bad that normal operation would be impossible. Symptoms would be short tube life (from internal arcing), arc trails on the socket, and OT performance between "ugly sound" and death.

    Leakage inductance is by definition inductance that does not and cannot couple to the secondary, so the mechanism of it killing power devices is well known in power-conversion circles. Switching power supplies in particular have to use snubbers and other spike-catching mechanisms to prevent sudden-death to power devices attached to magnetics. But leakage inductance is relatively independent of the secondary, since by definition it's not coupled to the secondary, so secondary open or shorted should not matter.

    But if the output tubes themselves try to turn off the current into the OT too fast, both the leakage and primary inductances go all V-equals-L-dee-I-dee-T on it and make big spikes, with punctures, sickness and death resulting.

    The primary of a push-pull is wound with two different windings for the "primary". Actually, there are two primaries, one for each tube, and these have leakage inductance between them, flux that is not coupled between the two half-primaries, as well as flux that is not coupled between each half-primary and the secondaries. Each of these leakages may be and usually are different values unless the winder is either lucky or extremely skilled *and* lucky.

    In the normal course of operation, a power tube pulling its half-primary voltage down towards ground makes the other half-primary go up above B+ by transformer action. The ends of the OT always go in opposite directions from B+.

    This is the origin of the protection circuit which puts 3000V worth of diodes from ground to the ends of the OT. The diodes are there, not to break over at 3000V on positive voltage spikes, but to clamp the negative-going half to between ground and B+. This forces the opposite side to not go above two times B+ by transformer action. The high voltage breakdown on the diodes is to keep the high-side diodes from being killed. It's the low-side diodes that do the protecting.

    That's all well and good, but transformer action cannot clamp leakage inductance, by definition. These diodes protect against sudden death from spikes from interrupting the primary inductance, but can't help against leakage inductance spikes. This is valuable, as the primary inductance may be 10,000 to 100,000 or more times the leakage inductance in OTs, but the smaller E = (1/2)L*I*I spikes from the leakages, which can add up over time.

    A capacitor between the output tube plates, or a capacitor and resistor between the output tube plates can act like a snubber for both primary and leakage spikes. The cap converts the spikes into part-sine wave rings (which they are anyway from the internal capacitance of the primary; but slower with the external cap) that cut down the speed of the voltage change, and hence the peak voltage of spikes. A cap+resistor actually damps the ringing down faster as well as cutting the peak. This is good for saving transformers, but may lower frequency response and possibly upset Nyquist stability.

    My favored solution is a string of one or more MOVs from primary to primary. These have some capacitance when open, but suddenly snap over to conduction at their protection voltage and not only keep spikes from going much higher, but eat the energy from the spikes. This seemed to work well in the amps I've tried it on.

    RF oscillation is a special case. I think this primarily (... sorry... ) happens in power stages with feedback. Feedback around a transformer is always chancy from a stability standpoint. The usual feedback from secondary to PI means that the forward gain of the output stage must be limited to keep it stable, and so to get benefits from feedback, the output stage is always near the edge of Nyquist stability, at least for a design where this has been thought about.

    The gain of a power tube can be thought of as its transconductance times the load on the plate. The plate load is normally the leakage inductance in series with the parallel combination of the primary inductance (which is as big as it can be made within the economics of the design) and the secondary reflected load. When the reflected secondary load gets huge, as it does when the secondary is open circuited, the plate load seen by the output tube becomes just the primary inductance. This is not only much larger than the normal load, but is essentially an undamped inductor/capacitor load.

    The gain rises hugely at the resonance of the unloaded primary winding. The phase shift is very different on each side of the resonance, and the feedback connection to the secondary is not unloaded, so the secondary gets feedback unshunted by an external load. It is very likely that this sets up a runaway oscillation at or near one of the resonances of the primary winding. L-C resonances can build to very high voltages, limited mostly by any internal resistances (that is, the wire resistance of the winding). These high voltages can easily build up until the catch diodes or MOVs limit them. A primary-side capacitor doesn't help, it just changes the frequency of resonance, unless it moves the resonance down to a frequency where the loop gain is below unity. A primary side cap-resistor may well help, as it adds loading and a lower Q at high frequencies.

    In all of these cases, the death mechanism is probably high voltages on the plates of the tubes. It can kill the tubes, arc over the sockets, or punch through the insulation of the OT.

    Putting a resistor at the output jack may help or may not. It depends on the value of the resistor. Since this resistor is always connected to the feedback point as a load, it both damps the primary resonances through transformer action, and lowers the load seen by the output tube plates, which lowers the forward gain when there is no speaker attached, and may keep the amp out of RF oscillation.

    I suspect that a parallel resistance may work GREAT in some situations, but not in some others. The "others" will be amps that are already near a cliff of stability, and any change is enough to let them run away. But it sure can't hurt. The only down side to this would be the loss of power to the resistor and the heat given off by the resistor in normal operation.

    I wonder if a series capacitor+resistor on the output jack might be juggled to load the transformer at above-audio (or above-guitar!) frequencies and avert oscillation-death, much as zobels protect solid state amps.

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    Quote Originally Posted by R.G. View Post
    I was just thinking about this general topic. That's always dangerous.

    I did some reading about tube amp outputs, open circuit loads, OTs and such. I did not find a specific cause of death for tube amp outputs open circuited in a way that was definitive.
    OK, I am glad YOU wrote this.
    Will be the first to admit that some of it is beyond my understanding, but I get the gist.
    Anyway.....
    back to your word DEFINITIVE.
    I think we have all read claims of an OT burning down "because the bass player tripped over my speaker cord". And how not even the heroic efforts of the guitar player, to shut his amp down in 10 seconds, was enough courage to save the OT of the amp.
    I have never personally seen that happen. I have played amps with either one side of the speaker cable disconnected or the other (which would create either an Open or a Short, correct me if I am wrong) and have never seen any catastrophic failures. I am not saying there was no, subtle internal damage to some part of the winding(s). But the amps all seemed to go on living.
    Is there a Real Answer.?
    What stars really have to align to kill an OT.?
    On a side note, I recently repaired a BF Bassman and a Tweed Deluxe that had a bad OT.
    There was a newish Heyboer in the Bassman and newish Merc Mag in the Tweed. Both amps were bought used, and the cause of OT abuse is not known. So I am aware that it does happen.
    Thank You

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    Just do it the way Bruce does it....He knows what he is doing....

    -g

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    Quote Originally Posted by trem View Post
    I think we have all read claims of an OT burning down "because the bass player tripped over my speaker cord". And how not even the heroic efforts of the guitar player, to shut his amp down in 10 seconds, was enough courage to save the OT of the amp.
    I doubt it would burn out in 10 seconds if the guitar player stopped playing immediately. I once had an amp on for an hour without a guitar or speaker connected and it was fine. Here'a a simulation of an over driven amp (single ended for max effect) into 16ohm, 470ohm and open circuit loads. I expect something would break down before it got to 45kV!


    VJ 16ohms.pdf
    VJ 470ohms.pdf
    VJ open.pdf

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    Another excellent and thoughtful post (as ever) from R.G.

    A few of my own observations on the subject.

    Although this thread is about OTs, it's worth noting that the screen grid current increases significantly with increased loading, so this is another good reason for not running your amp without a load.

    A perusal of the load lines for any pentode clearly demonstrates the effect of increasing the loading dramatically, as a horizontal load line, ie infinite impedance, would result in infinite positive voltage swing with very little signal voltage. I assume this would encourage oscillation as only a very small signal would need to be required to swing a very large voltage out. Presumably this would not be Nyquist instability, but caused by some form of parasitic oscillation (assuming of course you are employing negative feedback).

    Open loads was certainly a know issue in the 30s; indeed one of the advantages of the "ultra-linear" output configuration that Alan Blumlein mentions in his 1938 patent, is that greater signal voltage swing is required to generate such high voltages with very high loading (again easy to see from the load lines), so an ultra-linear output stage should immune to open loads.

    My guess is that wire insulation in the 30s was not as good as it is now, so transformer damage due to no loading may have been a more common occurrence than today.

    It's also occurred to me that the "conjunctive filter" (ie Zobel network) described in the RCA Receiving Tube Manual should provide some protection for the transformer from an open load.

    I see quite a few amps with damaged OTs.

    When I have the inclination to investigate the nature of the failure (by dissembling the transformer), there is usually some evidence of arcing, which implies an insulation breakdown.

    The other cause of transformer failure is see is a broken wire, which I assume either due to poor construction or mechanical damage.

    In amps known to run with open loads (customers can be quite coy regarding the events preceding amplifier failure), I see both damaged OTs, and valve with evidence of external arcing.

    I put a 470 ohm resistor across the 16 ohm tap of the amp I build as a precaution again an open load (although of course the load is much higher than is "should" be). The VOX AC50 also does this too; this amp was designed in an era when valves were the prevalent technology.

    Regardless, it's cheap and makes me feel good......

    Of course I've accidentally run amps without a load, and no harm has resulted, so I would have to conclude that whilst running and amp without a load is a bad idea, it doesn't inevitably lead to destruction of the OT (or indeed valves), although have seen examples of both these failure modes in amps.

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    Lifetime Member Enzo's Avatar
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    It is important to note that getting away with something does not mean it is a good idea. These potential failure mechanisms for transformers are just that - potential. A transformer can punch through and arc in an instant. It is not a sort of failure like slowly warming up until it melts. Any particular unloaded peak MIGHT damage the transformer. If it does not, great, you dodged a bullet. 10 minutes? Not a factor, other than the amp has 10 minutes worth of opportunities to fail the transformer. There is no one-to-one do this and your transformer burns out every time.

    SO if an amp is working fine over time, then we forget to plug in a speaker one time and POOF the tranny dies, it is not unreasonable to think them related. On the other hand, "I did it before and nothing bad happened" is no guarantee.

    MY analogy is driving home from the bar while drunk. Many people do that every night of their lives, and do not kill themselves. That does not mean it is a good idea or that there is no risk because "I done it a million times." It will be the one time all the factors add up against that tragedy happens. It won;t be because the drive was 10 minutes instead of 12.

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    Quote Originally Posted by mooreamps View Post
    Just do it the way Bruce does it....He knows what he is doing....
    Do you have anything of any substance to add to the discussion, Gary?

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    @jpfamps and Enzo:
    Good points. I think that repeated small doses of high voltage may go on a long time before the flaw becomes a permanent issue. It may be that the transformer also has to be hot for the final failure, which would allow easier breakovers, or the amp may have to be dirty - dust, congealed bar funk, distilled cigarette smoke, etc. might well form the first arc trail on a socket, things like that. As you said Enzo, it may take a combination of things. It may also be a long life of many insults. Liver cirrosis might be a good analog in humans. Each run with an open speaker line may not amount to much, but in the end, it gets you. And each drive home inebriated may not kill you (or someone else) but your bag of luck starts getting more empty each time.

    The young ones won't remember, but there was once a T-shirt with the quip on it: "I survived Three-Mile Island. I think..."

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    Quote Originally Posted by R.G. View Post
    @jpfamps and Enzo:
    Good points. I think that repeated small doses of high voltage may go on a long time before the flaw becomes a permanent issue. It may be that the transformer also has to be hot for the final failure, which would allow easier breakovers, or the amp may have to be dirty - dust, congealed bar funk, distilled cigarette smoke, etc. might well form the first arc trail on a socket, things like that. As you said Enzo, it may take a combination of things. It may also be a long life of many insults. Liver cirrosis might be a good analog in humans. Each run with an open speaker line may not amount to much, but in the end, it gets you. And each drive home inebriated may not kill you (or someone else) but your bag of luck starts getting more empty each time.

    The young ones won't remember, but there was once a T-shirt with the quip on it: "I survived Three-Mile Island. I think..."
    I am sure that stuff enters into the equation with some of, or maybe even most OT failures.
    However, the two I recently replaced were "only a year old" according to the previous owners. Both OT were in much older amps, and certainly looked quite "new".
    Just makes me wonder what would ruin a fairly new Merc Mag, and Heyboer OT.
    In the case of the Tweed Deluxe, with the Merc OT, one half of the primary was shorted. That is to say from CT to Blue was normal resistance, but CT to Brown was less than 1 Ohm.
    The Heyboer in the Bassman measured "normal" as far as DC resistance is concerned. But the amp had a strange distortion as the notes decade. I tried everything to fix the problem. I could not believe that a "new" Heyboer would be bad. But when I swapped in an OT from another Bassman, the problem was fixed. But who knows what the previous owners did to these amps. Maybe they ran the Tweed with 6L6 for a year.
    Maybe they dropped the Bassman in a bathtub.
    Just wish a knew what DID cause the failures.
    Thanks Again to everybody

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    Last edited by trem; 12-02-2012 at 05:34 AM.

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    Supporting Member loudthud's Avatar
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    Inductive spikes can occur while a speaker is connected if you drive the output tubes hard enough. The bias shifts to where there is a dead zone in the crossover region. I showed this in a previous post where the plate voltage on one side spikes to high voltage and goes negative on the other. Spike was 30uS wide. The resistor across the speaker or the conjunctive filter helps control those spikes.

    The Fender Blackface feedback network, usually 820 and 100 ohms might be why you don't see as many OT failures in those amps.

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    Quote Originally Posted by loudthud View Post
    Inductive spikes can occur while a speaker is connected if you drive the output tubes hard enough. The bias shifts to where there is a dead zone in the crossover region. I showed this in a previous post where the plate voltage on one side spikes to high voltage and goes negative on the other. Spike was 30uS wide. The resistor across the speaker or the conjunctive filter helps control those spikes.

    The Fender Blackface feedback network, usually 820 and 100 ohms might be why you don't see as many OT failures in those amps.
    Inductive Spikes.?
    So this would be from the OT itself, if there is a "dead zone" in the bias as you say.? Power to the OT would be shut off, and then then the OT would spike (inductive kick) for that period.?
    Is that kind of what you are saying.?
    This is kind of maxing out my grasp of electronics, obviously.
    thanks

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    Supporting Member loudthud's Avatar
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    Look at post 27 in this thread: http://music-electronics-forum.com/t28096/

    Under hard overdrive the coupling capacitors to the power tubes take on additional charge (in the form of voltage) from grid current. This shifts the bias into class C territory where there is a short time when neither tube is conducting. The plate current of one tube cuts off abruptly, the other tube is not conducting yet and the result is an inductive spike from the OT.

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    Quote Originally Posted by R.G. View Post
    My favored solution is a string of one or more MOVs from primary to primary. These have some capacitance when open, but suddenly snap over to conduction at their protection voltage and not only keep spikes from going much higher, but eat the energy from the spikes. This seemed to work well in the amps I've tried it on.
    How do you go about rating the MOV's? Do you just go with 2x the B+? or 3000V like with flashover diode strings? I think a couple MOV's would be alot more tidy than 6 diodes...

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    Quote Originally Posted by Tage View Post
    How do you go about rating the MOV's? Do you just go with 2x the B+? or 3000V like with flashover diode strings? I think a couple MOV's would be alot more tidy than 6 diodes...
    You have to estimate what a damaging voltage is to the OT, and for what normal operation might need, then rate the MOVs inside that range. In normal operation, the voltages on the OT primaries are equal-and-opposite around B+, at least if we're ignoring leakage inductances for the moment. Since one tube can only pull its side of the OT down to perhaps 50V of ground from B+, and certainly not below ground, then we're pretty sure that the other side can't get much above 2X B+ for theoretically "normal" operations. It's not that pristinely clear, of course, as there are leakage inductances and speaker spike reactions, etc. But since we're only trying to get in a range, it's a good starting point for a minimum.

    We already know that an OT, tube, and socket have to withstand more than 2x B+, because they do that and live a long time until it becomes really bad. The issue of where breakover happens is foggy, but fortunately, we can say its "big". Normal, simple transformer winding practice with layers and insulating papers can result in power transformers which easily pass 1500V high pot testing. It's done in power transformers all the time. You have to do something really wrong not to make well over 1000V.

    If I were doing this, I'd go for well over 2x B+, but under maybe 1500V, just as a guess. You want it well over 2x B+, because if normal operation ever starts breaking over the MOVs, they'll die. You want them there just for the nasty spikes. So ... 1200V, just to pick a number. Or 600V-700V per half-primary. That IS the design procedure I did with the MOVs in the Workhorse amps, and we've never gotten one back with dead OTs or shorted MOVs that I know of, but the numbers were not all that huge, of course.

    Since B+ is in the middle, one could install two MOVs with the middle tied to B+, which is where it would be (to a first approximation) anyway.

    Notice that flashover diode strings do not rely on the reverse breakover of the diodes for protection. They rely primarily on the bottom-side diode clamping the B+ - to - ground side of the primary to B+, and from there transformer action to clamp the volts-per-turn of the windings so the high side does not get over 2x B+. There may be some benefit from the normal protection diodes being over 3000V for clamping leakage inductance spikes above 2X B+, but clamping at voltages that high may still let punchthrough happen. They're 3000V so the diodes themselves don't get killed by spikes, then short. At least that's my best speculation at the moment.

    MOVs do have internal capacitance, so if that gets too big, it might affect frequency response. There are other devices with similar characteristics, like Transorbs that might do much the same thing. I even suspect that one might be able to use something like gas-discharge tubes in calibrated voltages or even Neons. Anything to eat energy at a voltage over the normal operations, but below punchthrough.

    Update:
    I went off to look for suitable devices. In the zener-style transorb family, I found that one device per half-primary was chancy to get good normal operating, and breakdown clamping and maximum transient voltage all at the same time. The trick was that the normal operation voltage had to be big enough, and that pushed the max transient voltage up too. I think that for power supplies below about 520-540V, I'd go with a string of four 256V rated devices, two on each half-primary. Mouser lists the Littelfuse P6KE300 for $0.48 each, and there are alternatives at nearly the same rating in bidirectional/non-polar like the Littelfuse 1.5KE300CA and the Vishay 1.5KE300CA-E3/54 at about $1.00 each. These list capacitances of about 100pF, so a series string of four would get down to about 25pF, which shouldn't throw off the frequency response of an output transformer.

    >>>Note that I have not tried these devices. I just think this will work.<<<

    There are a pot full of new MOVs and I haven't found a likely target in a casual look. I'd pick one for about 525-550V DC normal operating and as low a clamping voltage over that as possible, which could easily be 800-900V. If I didn't find one device, I'd do the same thing, and stack two on each half-primary.

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    Last edited by R.G.; 12-02-2012 at 11:46 PM.

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    Quote Originally Posted by loudthud View Post
    Look at post 27 in this thread: http://music-electronics-forum.com/t28096/

    Under hard overdrive the coupling capacitors to the power tubes take on additional charge (in the form of voltage) from grid current. This shifts the bias into class C territory where there is a short time when neither tube is conducting. The plate current of one tube cuts off abruptly, the other tube is not conducting yet and the result is an inductive spike from the OT.
    Thank You

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    Old Timer J M Fahey's Avatar
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    Well, I have repaired many failed OTs.
    And not by replacing them, but by rewinding, so I could also "autopsy" them.
    Normal failure is perforated insulation, clearly because a high voltage spike .
    Not too common in "old" OTs, but rampant in "modern" ones.
    I have repaired JCM900 which went through 3 original Marshall supplied OTs within a 6 month period!!!
    In this case, same as in Mesas and even Laneys , the *big* problem is that they use "self soldering" wire, the kind that has a *very* thin layer of insulation, designed to evaporate without solid residue at soldering iron temperatures.
    That kind of wire was designed for small cheap disposable wall warts where saving a few seconds assembly time means the difference between profit or failure, but it has NO business inside a Tube Amp, simple as that.
    I rewind using Class F , 180C rated wire, designed for electric motors, and they never come back.
    The wire is a bitch to solder, enamel has to be burnt on a gas flame first and then the blackened remains (which still cling tenaciously to the copper) have to be scratched off with a sharp box cutter.
    That's the proof of high quality.

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Well, I have repaired many failed OTs.
    And not by replacing them, but by rewinding, so I could also "autopsy" them.
    Normal failure is perforated insulation, clearly because a high voltage spike .
    That's a good bit of data in support of the "high voltage spike" theory. Not too many people perform the post mortem.

    the *big* problem is that they use "self soldering" wire, the kind that has a *very* thin layer of insulation, designed to evaporate without solid residue at soldering iron temperatures.
    If that is the case, yes, there's a high likelihood that the insulation itself is getting a little decomposed each time it gets hot, then punctures.

    I rewind using Class F , 180C rated wire, designed for electric motors, and they never come back.
    The wire is a bitch to solder, enamel has to be burnt on a gas flame first and then the blackened remains (which still cling tenaciously to the copper) have to be scratched off with a sharp box cutter.
    We used to use a strip of medium-grit sandpaper. We'd fold a 1/4"/6-7mm wide strip around the wire, pinch it between thumb and finger, then rotate it around the wire a few times. It worked pretty well. The insulation loads up the sandpaper, but one sheet of sandpaper makes many strips. Note - to cut sandpaper without destroying a knife, fold it with the grit on the outside, then slide the knife along the fold from the paper side. The grit is not forced against the blade and does not destroy it, at least as fast.

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    Quote Originally Posted by J M Fahey View Post
    Well, I have repaired many failed OTs.
    And not by replacing them, but by rewinding, so I could also "autopsy" them.
    Normal failure is perforated insulation, clearly because a high voltage spike .
    Not too common in "old" OTs, but rampant in "modern" ones.
    .
    That is very intriguing.
    I imagine it can be pretty expensive (for the customer) to have you rewind a tranny.?
    I have 2 dead OT right now. I am disabled, and have LOTS of time. Maybe I should unwind this Merc Mag and see what caused its death....

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    @ R.G. : the wire I use has such a tough high temperature Epoxy Enamel that starting with sandpaper is real slow.
    Surface is so hard and smooth that sandpaper sort of "slides" over it.
    Burning takes care of that.
    And on thick wire (1 mm to 1.40 mm) a sharp cutter blade is faster and more precise than sandpaper.
    Although on 0.65 mm and thinner yes, sandpaper is preferred, a blade can nick the wire.

    "Self soldering" wire, by Law, *must* be coloured different.
    Usually Pink or Green Fluo, the typical wire you see on Wallwarts, Sega power supplies and such, where they just insert the wire in a PCB hole and simply soldering (with an iron, don't think wave soldering lasts long enough) removes it, leaving behind perfect shiny copper.
    Same wire in a "big" transformer may be a fire hazard and on a motor, which overheats when stalled or under load, plus vibrating and spinning at high RPMs under heavy load, is criminal.
    But now wire manufacturers supply (on customer's responsability) self solderable with almost transparent colour, which can cheat you.
    Look at these transformers which I usually rewind:
    Left is 80's Twin Reverb Power Transformer.
    Big, heavy, very high quality, wound with good classic wire, fully mounted (EI iron + covers) and then they filled it with Polyester Rosin throuh a cover hole.
    And indestructible transformer (unless you short the load and use a nickel as a fuse, the only way to damage them) ; this one still works, I had to rewind it (actually made a new one) because it's 110V only, we have 220V and the amp owner was *sick* of carrying a 12lB autotransformer.
    Right is a modern Marshall one (ok, 1992 , then's when they became cheap minded), you'll clearly see the typical bright Pink wire.
    Center is a MB *Triple* Rectifier OT (6x6L6), I was very disappointed when I saw the carbonized bright pink and green wire inside .... although they used paper, not plastic bobbins and added a thin strip of wood to take up free space.
    Very "ecologic", very "green" .... and very dead.
    By the way, I also rewound the OT for the MB combo shown below.

    Marshall transformer closeup:

    MB transformer closeup
    Unfortunately my camera chose to focus on the Tolex, instead of on the coil top.
    You can see it has all the "Mojo" stuff: wound on brown paper and cardboard ... but with the "un-Mojo" wire


    @ trem:
    I imagine it can be pretty expensive (for the customer) to have you rewind a tranny.?
    I have 2 dead OT right now. I am disabled, and have LOTS of time. Maybe I should unwind this Merc Mag and see what caused its death....
    Yes, it's expensive, but remember one of them went through *3* original Marshall OTs, the customer was desperate; for others just one was enough.
    As of the MB one, it had to be ordered from USA, its weight made shipping expensive, it had to clear Customs (paying Duty, of course and probably bribing somebody) and the delay would be at least 2 weeks, so my customers were *happy* to pay and have the amp working within the week.
    By the way, I have these here because I preferred to build new ones
    I'd say: study those you have.
    If you have to work at home for any reason and have time available, it might become an interesting side line.
    I'm sure some people must prefer their vintage 40's or 50's Fender/Gibson/Silvertone/Ampeg/Magnatone transformers rewound on the same cores, "the original way".
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Juan Manuel Fahey

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    Once Again.....
    Thanks for the info and your encouragement.
    Some of us are "lucky" to live where we do. I understand how some things can be very expensive in different parts of the Globe.
    best

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    Has anyone tried 'resitive tethering' ie adding a 330K or so resistor from the power tube anodes to ground?
    This is something recommended by Kevin O'Connor.
    Supposedly it's better than the string of diodes often used.

    Cheers
    Shane

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    Quote Originally Posted by J M Fahey View Post
    The wire is a bitch to solder, enamel has to be burnt on a gas flame first and then the blackened remains (which still cling tenaciously to the copper) have to be scratched off with a sharp box cutter.
    I use a Dremel with a fine stone grinder tip on it. A single slight pass by the enamel and the wire's shiny copper shows through. I have a butane minitorch that I used to use for that but I don't anymore.

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    Quote Originally Posted by Shane View Post
    Has anyone tried 'resitive tethering' ie adding a 330K or so resistor from the power tube anodes to ground?
    This is something recommended by Kevin O'Connor.
    Supposedly it's better than the string of diodes often used.

    Cheers
    Shane

    I've never tried it, but the resistors will surely have an impact on tone whereas the diodes only leak microamperes. Ken Fischer used the string of diodes and nobody's complaining about the tone of his amps

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    FWIW, a bunch of people over on AmpGarage have reported some unpleasant distortions that seem to be caused by the diode strings, and go away when they are ripped out.

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    Quote Originally Posted by markr14850 View Post
    FWIW, a bunch of people over on AmpGarage have reported some unpleasant distortions that seem to be caused by the diode strings, and go away when they are ripped out.
    I'm guessing that the diodes that were causing unpleasant distortions were not correct for the application in some way. These would include:
    - diode(s) not properly rated for the reverse voltage
    - diode(s) damaged by earlier spike events
    - diode mods improperly selected or installed

    The thing about protection circuits and devices is that if they're properly designed and installed, they do *nothing* in normal operation. Protection events can sometimes damage protection devices - after all, the protection stuff should be simpler and cheaper to replace than what it protects.

    The fact that there are distortions which go away when the diodes are removed is itself evidence that the diodes were not doing the intended thing. In this application, the diodes are always reverse biased, and do not break down or flash over, as people think. They are intended to be nonconducting unless their side of the primary goes below ground, at which point they go into forward conduction and clamp both sides of the primary to no more than B+ across it. The output tubes simply cannot make this happen in normal audio operation. Well, OK, except for that grid-conduction-bias-shift-class-C thing, but that's not normal operation either, and in itself causes audible distortion.

    Humans being what they are, they think that if removing the diode "fixes" the amp, then the intent of the diodes was wrong. This is a simple thing to think - and wrong. For some ideas about how the human mind works like this, see http://failblog.cheezburger.com/thereifixedit

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    Quote Originally Posted by J M Fahey View Post
    @ R.G. : the wire I use has such a tough high temperature Epoxy Enamel that starting with sandpaper is real slow.
    Surface is so hard and smooth that sandpaper sort of "slides" over it.
    Burning takes care of that.
    And on thick wire (1 mm to 1.40 mm) a sharp cutter blade is faster and more precise than sandpaper.
    Although on 0.65 mm and thinner yes, sandpaper is preferred, a blade can nick the wire.

    "Self soldering" wire, by Law, *must* be coloured different.
    Usually Pink or Green Fluo, the typical wire you see on Wallwarts, Sega power supplies and such, where they just insert the wire in a PCB hole and simply soldering (with an iron, don't think wave soldering lasts long enough) removes it, leaving behind perfect shiny copper.
    Same wire in a "big" transformer may be a fire hazard and on a motor, which overheats when stalled or under load, plus vibrating and spinning at high RPMs under heavy load, is criminal.
    But now wire manufacturers supply (on customer's responsability) self solderable with almost transparent colour, which can cheat you.
    Look at these transformers which I usually rewind:
    Left is 80's Twin Reverb Power Transformer.
    Big, heavy, very high quality, wound with good classic wire, fully mounted (EI iron + covers) and then they filled it with Polyester Rosin throuh a cover hole.
    And indestructible transformer (unless you short the load and use a nickel as a fuse, the only way to damage them) ; this one still works, I had to rewind it (actually made a new one) because it's 110V only, we have 220V and the amp owner was *sick* of carrying a 12lB autotransformer.
    Right is a modern Marshall one (ok, 1992 , then's when they became cheap minded), you'll clearly see the typical bright Pink wire.
    Center is a MB *Triple* Rectifier OT (6x6L6), I was very disappointed when I saw the carbonized bright pink and green wire inside .... although they used paper, not plastic bobbins and added a thin strip of wood to take up free space.
    Very "ecologic", very "green" .... and very dead.
    By the way, I also rewound the OT for the MB combo shown below.

    Marshall transformer closeup:

    MB transformer closeup
    Unfortunately my camera chose to focus on the Tolex, instead of on the coil top.
    You can see it has all the "Mojo" stuff: wound on brown paper and cardboard ... but with the "un-Mojo" wire


    @ trem:
    Yes, it's expensive, but remember one of them went through *3* original Marshall OTs, the customer was desperate; for others just one was enough.
    As of the MB one, it had to be ordered from USA, its weight made shipping expensive, it had to clear Customs (paying Duty, of course and probably bribing somebody) and the delay would be at least 2 weeks, so my customers were *happy* to pay and have the amp working within the week.
    By the way, I have these here because I preferred to build new ones
    I'd say: study those you have.
    If you have to work at home for any reason and have time available, it might become an interesting side line.
    I'm sure some people must prefer their vintage 40's or 50's Fender/Gibson/Silvertone/Ampeg/Magnatone transformers rewound on the same cores, "the original way".
    Juan, excellent post.

    I'm always surprised how little good info there is on the net about transformer construction.

    It seems that transformer construction took a nose dive in the 80s. I've seen many transformers with no insulation between the layers.

    I've always assumed the reliability of older transformers was due to this additional insulation, which of course adds to the cost, not only in the construction of the winding, but presumably due to the fact you have potentially to use a larger stack to get the bobbin in the winding window.

    What is the prevailing view of using paper vs polyester interwinding insulation?

    A transformer winder I spoke to, expressed the view that paper reduced tracking as it had a rough surface compared with polyester.

    Also, does the paper have to be thicker than polyester tape for the equivalent insulation, and what effects would there be on distributed capacitance? Presumably the thicker insulation will have lower capacitance assuming equal dielectric constant (I've not been able to find a dieletric constant for paper, but I would be surprised if it larger than polyester).

    Thanks in advance for any answers.

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