Cooked power tranny? (Vibrolux Reverb)

[slidincharlie (Carlo P)]

You have checked tube sockets & power supply wiring.

Besides looking for carbon traces that might cause maybe a current draw (?), what else should I check in a socket? With a meter, I mean.

[MWJB]

Any possible shorts accross pins/from pins to chassis...you already know plate & screen draw for power tubes & they seem normal. Visual inspection should do it.

[slidincharlie (Carlo P)]

How much current is drawn accross the standby with just the GZ34 in place? Then try with a SS rectifier if you have one. Do a similar test & see what is "normal" in another, similar amp. Establish a datum line. I would really worry about the current draw test if it was massively out of whack.

I tested everything again.The current draw with only the rectifier in place (GZ34, 5U4GB or ss) is a fraction of one mA.
Adding the power tubes brings the current draw up to 123mA with GZ34 and 110-111mA with 5U4GB and a ss recto.
Adding the preamp tubes one by one adds about 15 mA more. This means around 140mA with the GZ34, and around 125mA with the 5U4GB or ss.
I don't have other similar amps to compare unfortunately, but the PT is a stock late 70's unit and, as you said, I expect it to last for many more years if 140mA isn't too much... But isn't it too much?

[pdf64]

'Adding the power tubes brings the current draw up to 123mA with GZ34 and 110-111mA with 5U4GB and a ss recto'
That's with the power tubes adjusted for 35mA each each time?
Or the bias adjust left the same for both?
Whatever, I don't understand how even with the 5U4, power tubes can have total of 75mA (including screens), but the B+ current is measuring 110mA.
There must be a measurement error somewhere.
Suggest that you break into a power tube cathode ground return and measure the cathode current directly, to se if we can find the missing 35mA somewhere. Peter.

[slidincharlie (Carlo P)]

That's with the power tubes adjusted for 35mA each each time?
Or the bias adjust left the same for both?

Adjusted each time to 35mA each.

Whatever, I don't understand how even with the 5U4, power tubes can have total of 75mA (including screens), but the B+ current is measuring 110mA.
There must be a measurement error somewhere.
Suggest that you break into a power tube cathode ground return and measure the cathode current directly, to se if we can find the missing 35mA somewhere. Peter.

I put a 1 ohm 1% resistor between the cathode of each 6L6 and ground. The direct current reading taken with the OT shunt method are confirmed (that is, about 38mA from each tube, as expected across the cathode resistor reading).
Any other hint to chase that excess current draw?

[MWJB]

Voltage creep accross the circuit board? Measure various points on the board for voltage, especialy hi voltage eyelets, also try stuffing an extra layer of insulation between the top circuit board & the lower board. Voltage creep on the board is usually accompanied by some unwanted noise, but have a look see anyway.

PT should take 140mA happily, at higher demands the B+ will just drop, if it sounds OK to you is it necessarily a problem?

[pdf64]

In the spirit of investigation, put 1 ohm resistors between the OT centre tap lead and the B+ capacitor, and between the screen grids and their B+ cap.
Then, with just the power tubes in, compare the volt drop across the 1 ohm B+ resistors, 1 ohm cathode resistors, and your normal current measurement (current meter across the standby switch).
If the OT and screen 1 ohm resistor volt drops add up to equal the cathode resistor volt drops, but the current meter is still higher, then I would put it down to being a measurement error.
Because we already know that with no tubes in, there's almost no current.
The measurement error possibly caused by the high level of ac ripple, and the higher equivilant resistances of a 5U4 than GZ34.
I'd assume that your current meter isn't giving accurate measurements in that situation.

[Steve Conner]

While we're clutching at straws, have you checked the accuracy of the mV range on your meter? Maybe it underreads.

If 35mA were flowing through a leaky tag board, I'd expect clouds of smoke.

[slidincharlie (Carlo P)]

I've been away from home for two weeks now... Hope somebody is still willing to follow me...

Voltage creep accross the circuit board? Measure various points on the board for voltage, especialy hi voltage eyelets, also try stuffing an extra layer of insulation between the top circuit board & the lower board. Voltage creep on the board is usually accompanied by some unwanted noise, but have a look see anyway.

The only place where I can read some AC voltage is on the OT center tap eyelet (5.3Vac).

PT should take 140mA happily, at higher demands the B+ will just drop, if it sounds OK to you is it necessarily a problem?

I've been told above that this amp, with its tube complement, is expected to draw around 100mA. So there are about 30-40mA that are being drawn in excess with the GZ34 (or about 20-30mA with the stock 5U4GB). I ignore the maximum continuous current rating of this PT... You say it should handle 140mA happily... I guess I am happier with the GZ34 tone-wise... So if you guys believe that I am not stressing the PT too much, I'd be incline to leave it alone.
BTW, I play this amp only a few times per year 'cause it's too loud for most local venues.

Re: meter misreadings. I have a good old (ICE brand) needle meter. Is it good to check my DMM against this meter?

[MWJB]

You should be looking for dc, not AC on the board, between the eyelets.

Under large signals the PT's current rating is going to be exceeded anyway, when this happens the secondary voltages drop...you would just seem to have a little headstart on that front, so I'd live with it.

[slidincharlie (Carlo P)]

No dc anywhere on the board, just around .5V near the B+ eyelets.
I'll leave it alone since it hasn't given me any problem in the last check trials.
Thanks to everybody .