Diodes on the Power Tube Plates...

I've always wondered if putting a fuse in series would prevent shorting failures, but still suppress the voltage spikes...of course you would need to check the fuse every once in a while.

 

True, but the way they're daisy-chained makes it so extremely unlikely that it can basically be discounted. If they're 1N4007s, every one in one of the stacks (three each in this amp) would have to short for there to be any risk to the transformers.

And as usual, a HT fuse would remove even that risk (as well as several others).

BTW, that amp appears to still have a 'death cap' fitted on the ground switch, which is FAR more of a danger in every way.

It's funny how people will do one (largely unnecessary) reliability 'upgrade', and miss two much more important and useful ones...

 

I would snip it off and keep it for repairing a tone stack or something where the original-type cap has some potential value .

Then you can remove the ground switch entirely, fit a HT fuseholder in the hole, and not have to worry about a lot of nasty failures that take out trannys...

BTW, what's that arc burn in the edge of the chassis lip above the fuseholder from? Has the wire underneath it burned as well? If so, replace it. (It could also be from where someone was metering the voltage on the fuseholder and touched the chassis with the side of the probe though... you probably shouldn't ask how I know that, I'm supposed to know what I'm doing .)

 

I agree. Every amp I've owned for any length of time that has these eventually winds up blowing HT fuses due to shorting. I think they just can't handle the duty cycle.

 

would it possible to use higher current rated diodes(1N5408--1kV 3A, etc.) or put a cap in series(I have no idea if that would effect anything) to increase their reliability?

 

Reliability? Those diodes serve no purpose except to improve the odds of frying your output transformer. I'd like to believe the dude who proposed that absurd mod (K. Fisher) was just high at the time.

 

When diodes fail, they go short. Almost always a nice clean hard short. This blows the fuse right now. That saves output and power trannies.

I've seen a number of amps where they've done their job perfectly, sacrificed themselves blew the fuse and instead of having to find and replace arced, burned tube sockets or diagnose an arced output tranny, I just snipped and replaced the diodes.

Look at the circuit and the diode's characteristics. If you are replacing them, then you are replacing them instead of a larger, more expensive part. They do not blow unless something is wrong.

 

I'm assuming the "diodes" go from ground to each of the primary leads with the cathode on the leads. If you are measuring some resistance with the diodes reverse-biased they are probably TVS's. They are a better bet than using silicon diodes because they are designed more like a zener in that they start conducting at a certain voltage and keep much higher voltages from appearing across the tubes, or the windings of the tranny during unloading of the secondary. Usually amplifiers with these have a circuit breaker which will open during a fault and basically save itself. Is this a Traynor?

 

I don't mean to hijack, but is there any chance someone could point me at a schematic of some amp w/these diodes in place. I've never seen this before and am always fascinated to learn about techniques to make circuits more robust or protected from expensive/catastrophic failure. Thanks!

 

You have no way of knowing that. All you know is that you have seen a case where the diode went, shorted the B+ to ground through the transformer primary, and that the amp fuse happend to blow before the output transfomer was damaged. That is not always the case.
The "protection mod" can fail in amps that have nothing wrong with them and has the potential to take out the output transformer.

 

http://www.schematicheaven.com/newamps/rivera_knucklehead.pdf

 

IIRC, I think one of Ceriatone's amps has them on a layout.

 

They are also in some newer Fender amps, i.e the Hot Rod series and Blues Deluxe. But Fender uses 3kV reverse breakdown so they are less likely to fail.

 

I believe Ampeg is the origin of this mod.

Ken F. worked there and that is where he learned about it.

 

Check an old Ampeg V4 schematic.

 

Just a thought on the diode-string and the reported failures of individual diodes:

Would adding transient-averaging caps of small value across each diode in the string improve reliability? I've see this methodology used in HV discrete-diode bridge rectifier circuits used in amateur radio transmitter power amplifiers that use vacuum tubes to improve reliability. Maybe this method may also work here, removing or greatly-reducing one of the major drawbacks to using this type output-section protection.

Cheers!

Strat

 

May want to look at the heater wiring polarity as well on the power tubes. Looks to be out of phase from here.