speaker jack shorting or not?

[R.G.]

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?

[R.G.]

@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..."

[trem]

@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

[loudthud]

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.

[trem]

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

[loudthud]

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.

[Tage]

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...

[R.G.]

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.

[trem]

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

[J M Fahey]

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 , 180ºC 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.

[R.G.]

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 , 180ºC 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.

[trem]

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....

[J M Fahey]

@ 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".

[trem]

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

[Shane]

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