How to protect the OT?

[Steve Conner]

MWJB: Yes, if the amp is running flat out and the speaker goes open, the screen grids of the power tubes will be destroyed in a few seconds like I mentioned. This is not the same as blowing the OT though, and the flyback diodes and resistor wouldn't prevent it. When I was testing this, I never took it all the way to destruction, but I saw the screen grids light up white-hot and there was still some way to go on the master volume.

Shorting jacks are still useful, I just wanted to point out that they're not 100% effective. Only 50% as the speaker cord has two ends and you could forget to plug in either one.

Neutrik now make "Silentjack" 1/4" jack plugs that short when they're disconnected, so maybe you could put those on both ends of all your speaker cords too...

As for the flyback diode rating, I think it should be as low as possible: if there are spikes flying around, you want the diodes to go before the OT does.

[daz]

As for the flyback diode rating, I think it should be as low as possible: if there are spikes flying around, you want the diodes to go before the OT does.

So do you think i should use 2 instead of three?

[Steve Conner]

If three is what the rest of the industry generally uses, then you should probably use three.

[daz]

If three is what the rest of the industry generally uses, then you should probably use three.

That was just a number i thought would be right from what i read in this thread. but you said....

"As for the flyback diode rating, I think it should be as low as possible: if there are spikes flying around, you want the diodes to go before the OT does."
So i figured maybe i should use less.

[Gregg]

Below is a very simple circuit which will not allow you to switch your amp ON without load/speakers connected. Use the relay for switching your HV winding.

[Hardtailed]

If three is what the rest of the industry generally uses, then you should probably use three.

Well having 3 also add protection in case you're unlucky enough that 2 diodes fail and you have a short

[pdf64]

'the mod showed to remove the flyback diodes the classic 30 has. My question is, why was this done?'
Some people feel that they add capacitence to the plate circuit and roll off high frequencies to ground. So try it with and without.
R G Keen is also doubtful that they do anything useful -
'Q: I've seen circuits that use reverse biased diodes connected from ground to the plates of output tubes as "transient spike preventers". How does this work?

A:The 1N4007's serve mainly as an amulets against the voltage gods in this case. An inductive flyback pulse will go to literally ANY voltage until it finds a discharge path. Ideally, transients that would cause very high positive voltages on one push-pull plate would cause high negative voltages on the other plate, and the diodes on the negative going plate would clamp the voltages on the positive going plate through the output transformer. This does indeed happen for small, extremely-tightly coupled transformers. However, any leakage inductance between the two primaries prevents the tight coupling that would let the negative going diodes protect, and worse yet, it's the leakage inductances that cause the spikes on transients anyways.

What really happens is that the first few flyback pulses that occur will break over the 1N4007's rather than than arcing the plates on the positive side, so there really is some protection, it's not just where it looks like it is. If you're lucky, the 1N4007's break over before the transformer insulation punches through, and all is well until the 1N4007's go leaky or short. Probably better than nothing, but not a whole lot of additional protection, either. Heck, amulets are not harmful, I guess. '
Peter.

[daz]

man, you guys really have me confused now. i guess there is no definate way to do this, just suggested ways that may or may not protect it?

[Enzo]

Gregg, that is a nice circuit, but it doesn;t protect against no load, it only protects against not having a plug in the jack. Bad cord, intermittent voice coil, or cab wiring problems won;t be protcted against.

Daz, lots of little things add up to protection for your circuits. But ask yourself this: how many bad output transformers have YOU experienced in your life?

[daz]

None till i started building amps, then two ! Hey, i never professed to know WTF i'm doing !

[loudthud]

Output spikes can occur when the output stage is driven hard enough to cause a crossover dead zone (a short amount of time when both tubes are off). The coupling caps to the power tubes take on a charge from grid current and cause a bias shift. You won't see this testing your amp with a dummy load. Put in your ear plugs and connect a speaker. You'll be looking at some nice square waves across the speaker and suddenly spikes will shoot off the screen as you dime the volume. IMHO this is what kills Marshall OTs. Look at some of the newer Fender stuff and you'll see a snubber network across the primary, an R-C network meant to catch the spike.

But, it's not just the OT that creates the spikes. Speakers can create spikes when given the right drive with a crossover dead zone. I've done it with a solid state amp.

What happends when one side of a push pull amp dies say from open screen resistor? Won't that create spikes?

[daz]

What do you suggest for protection loudthud? so far i haven't seen anything mentioned that wasn't then said to be uncertain by another poster. At this point i'm unsure what to do, but i really would like to be certain of one thing....that the OT i just bought for my friend's amp will be the last one i'll have to buy for it.

[loudthud]

I don't think 100% protection is possible. The resistor across the speaker in conjunction with a B+ fuse probably gets to the high 90%s for cheap with a low increase in failures caused by the protection circuit itself. The diodes to ground provide more protection but the diodes can fail and kill the transformer. KOC recomends using 10 ohm 1WFP resistors in the cathodes of power tubes because they act more like fuses that 1 ohm 1W.

See if you can hear the difference with a 270ohm 10W resisitor across the speaker.

[Enzo]

Daz, my point wasn;t that you are not doing something right. I was just pointing out that one maybe shouldn;t worry too much about the transformers, since really, most people go through their whole lives without ever having to replace one. Ther are a lot of other things far more likely to fail and they might warrant the concern rather than the transformer.

[Bruce / Mission Amps]

I wouldn't get too hung up on this either.
With common sense players, I don't see blown OTs very much at all.
I know you are sensitized to the failed OT thing now but it isn't a real serious common failure.

However, for amp heads with multi tapped OTs, you know 4-8 and 16 ohms, I frequently use a 470 ohm 5 watt cement resistor across the highest impedance tap of the OT's secondary.
That will pretty much take care of most problems with the OT arcing over from a speaker load disconnect.
Why 5 watts? ... because they're smaller and it's doubtful a 50 to 100 watt amp will blow a 5 watt 470 ohm resistor,
The tubes simply can't make that much power at that primary impedance and its probably still enough load that internal arcing will not occur.

Another thing you could try is to lift each power tube's cathode from it's
biasing resistor... (I think you are still using cathode biasing)... with a small 1/4 to 1/2 watt, 10 ohm resistor.
Think of it like this:
say one of your tubes is crappy and it starts to draw something crazy, like 250ma to 350ma of plate/cathode current.
That runaway current added to the other good tube's current still might not be enough to blow your OT center tap fuse (also a good idea to use a 500ma fast blo there) ... but if you make all that tube's current go through a singular 10 ohm, low wattage current limiting cathode resistor too...
Ohm's Law:
I^2 X R = P
.3a x .3a x 10 ohms = .9 watts
Maybe a 1/4 to 1/2 watt flame proof resistor would go open... like a really slow SLOW blow fuse.
Wait, maybe just a simple 250ma fuse on each tube's cathode would be OK.
Wait, since all the current from that point is also going through the OT's center tap, then I am back to a fast-blo, OT center tap fuse... like just about all Marshall amps used.
Speaking of which, I wish I had a dollar for every 500ma and 1a fuse I've replaced in Marshall heads....
When those fuses go it is usually a pretty good sign the power tubes are about to blow up or there already is a blown up power tube.

Why, just the other day I had a studio engineer bring their non-working small box 50 watter with a "blowing the 500ma HT fuse" complaint.
He said the only way he could stop it from doing that during the last session was to put in a bigger amperage fuse! YIKES.
The knuckle head had put a 20 amp fuse in it from Checker Auto and played it until it smoked and then wouldn't play anymore.... and now it has two blown up El34 power tubes and an open primary side on the original but defunct, OT.

Wow I can't believe I just typed all that drivel....