Can A PT Fail Providing Too High Voltage?

[Daver]

Is it possible for a power transformer to fail in such a way that the output voltage is too high? I've never run across this, but maybe someone has. The transformer in question is in an Ampeg Gemini VI with 7591 output tubes. The schematic calls for about 435vdc on the plates and the tubes biased drawing 40ma each. The current draw seems pretty high... Anyway, the amp I have here puts out 503vdc with the tubes biased at 21ma and 485vddc with them at a too hot 40ma. That voltage seems too high even taking into account the differences in line voltage over the years. The PT stays cool and there are no visual signs of stress. No dripping guts, smells or signs of anything burning. Thanks.

Dave

[eschertron]

Does the amp in question have tube rectification? What's the secondary AC voltage, and is that spec'd on the schematic?

[Justin Thomas]

Is the PT replaced? Also, 7591s can take 550 on the plates & 19W. Can you get there? How's the amp sound? And this on is fixed bias, correct?
I guess it's a little harder to tell what might be original, since Ampegs don't have the same support base as Fender...

Justin

[g1]

Schematic here:
https://supportloudtech.netx.net/lou...ic/#asset/6499

[Daver]

The schematic is on the back of the amp, so I can't easily scan it. The one I found online is similar, but not 100%. It has diode rectification. It's fixed/adjustable grid bias, yes. The 7591's can take 550v on the plates but only 440 on the screens and they are only about 3 volts lower than the plates. The PT and OT are original. They have the correct part numbers on them. The secondary AC is not on the schematic. I'll have to measure it and get back to you. The amp sounds fine, now, but came in with very low output and distortion. It had fairly recent JJ 7591s in it that look like they were heated up pretty good. They test very weak on my Eico 667. It's been recapped before as well. Thanks!

[The Dude]

If that schematic's output section matches, there would be about 50 ohms per cathode to ground. What is your cathode voltage? You need to subtract that from plate voltage to get the voltage across the tube. The cathode is your reference- not ground, in this case.

[Daver]

The cathodes are tied to ground. This one doesn’t have that pot.

[bob p]

That amp was built when, back in the days when wall voltage was 110V, or 88% of what it is today?

Does that ~500V plate voltage that you're referring to correspond to a nominal wall voltage of 125VAC or have you corrected it to 110VAC?

An amp that was supposed to have ~435 volts on the plates/screens back in the days of 110VAC inputs could easily produce ~495 VDC using a 125V source.

I'm thinking there isn't a problem with the PT.

[Daver]

It was made in the mid 60’s. The schematic shows 117 vac for the input line voltage. I’ve got 123 here. I know what you’re getting at. It just seems to be a bit beyond what I’ve seen on other amps of the era. All vintage amps have higher voltages than their schematics show. This one has the biggest voltage disparity I’ve experienced. Thanks.

[The Dude]

Just to be clear, when you measure bias as in post #1, "the amp I have here puts out 503vdc with the tubes biased at 21ma and 485vddc with them at a too hot 40ma.", the mA readings are for each tube and not both combined? What is your bias method, probe, shunt, other?

[Daver]

Yes for each tube. Measured across a 1 ohm resistor put between cathode and ground.

[g1]

I'd be more suspicious of the schematic voltages being off than the PT being faulty.
Just the increase from 117 to 123 should take it to around 460VDC, so 500V doesn't seem that far off then.

[pdf64]

In these cases, I think that the best reference point is the heater voltage.
What Vac does the heater circuit actually measure, eg 6.3?

[J M Fahey]

Absolute worst case this amp was fed from an overwound PT, so it had higher voltage from Day 1 , most factories had more than one transformer supplier , there might be batch to batch variations or they might have tried something different and even on a Friday afternoon they might have fitted a transformer meant for another amp but which was handy, just to finish the d*mn amp and deliver it so as to fulfill an order or get some needed cash.
All these things quite normal in the real world production life.

Personal example: my meat and potatoes amp is the 2 x 12" SS 100W one, I donīt care much (read that as not at all) for bedroom rockers and such, all my amps are meant for live use with a loud drummer nearby .
That said, many demand something lighter and more compact , so sometimes I agree to make a single 12" 60W one which I consider the absolute minimum for Club work.
Power amp board is the same, amp is loaded with 8 ohms (1 speaker) instead of 4 ohms (2 speakers) , and uses a somewhat smaller and lighter PT.
*Many* times I have to custom wind a single 60W one for the odd order because thereīs none left and I simply grab a 100W one instead from the shelf just to avoid the bother; in such cases a puzzled Tech may find "higher than normal voltages" ....... now you know why.
Same might have happened here.

[Daver]

In these cases, I think that the best reference point is the heater voltage.
What Vac does the heater circuit actually measure, eg 6.3?

Yes, I had planned on checking that. I will do so tonight. Thanks.