'59 Bassman RI eating power tubes

[methodofcontrol]

hey guys, i'm looking at a friend's 59 bassman reissue. everything seems to work fine, but V4 red plates and emits a burning smell within 5 minutes of turning it on. first thing i tried was testing the old ones. (i tested all of the tubes. pre's and rectifier good.) V5 tested almost new, V4 tested almost dead. i put in a used but known-good pair and turned the amp back on looking for clues and checking voltages. V4 started glowing cherry red. weird thing is, all the voltages seem great (except one) and plate and grid resistors are fine.
the only discrepancy i found was on pin 3. according to the schem, it should be about 257v. but this one reads at about 450v! actually, pin 4 and pin 3 on BOTH power tubes are reading the same voltage.
so, i'm wondering...why is V4 getting taken out and NOT V5? if all of my components are seemingly fine, why is this happening? could it have anything to do w/ the OT? definitely need some help w/ this.
thanks!

[stokes]

Pin 3 is the plate and should not be 257v,the 450v is probably right.check the bias voltage at pin 5 of the power tubes.

[MWJB]

That 257V you are reading from the schematic is in fact AC signal at max clean W RMS, not dc plate voltage. Plate voltage, at idle, should be in excess of 450vdc with a GZ34 or SS rectifier (possibly less with a 5U4G). Did you check negative voltage at pin 5 of eack power tube? What plate current do you get (in mA)?

[Enzo]

Please look more cloesly. That 257v is in a round test point, that means AC volts. And further that means AC volts of signal when teh test signal at the input is as specified.

Look down at the power supply portion of the schematic. See the square test point voltages. Note the plate and screen B+ voltages are +491 and +490.

When the tube starts to red-plate, check the voltage on pin 5 of that tube. It ought to be about -55vDC. Is it?

[Enzo]

Ahhh, beat me by THAT much...

[methodofcontrol]

pin 5 is -51 last i checked. which i wouldn't think is far enough from spec to be a concern.
thanks for catching my AC vs DC error. oops. i'm still learning.

[g1]

What does pin 5 measure on the good socket? For bias, -51 is quite a bit off from -55. I would not want to see a difference more than a volt or so between the 2 tubes pin 5 voltages. If they are different, you likely have a leaky coupling cap.

[Enzo]

Pin 5 is -51 NOW, and that is fine. What I want to know is WHILE IT IS REDPLATING, what voltage is there.
I don;t want to know last time you checked, I want to know when the problem is at hand.

Just an opinion, but 51 or 55, either way that is not going to result in red plates. I suspect you are right about his cap being leaky. That is where I am headed with my questions.

[methodofcontrol]

pin 5 is -51 or so at idle and i measured -50 while V4 was red-plating. V4 and V5 voltages have always been matched thus far. V5 has never red-plated. i don't understand why, given the same voltages, V4 would get eaten up and V5 would remain ok?
as always, thanks for the help guys!

[MWJB]

OK, so now we need to check that the socket is in good order & that the tube pins are making good contact with the socket, power down unplug from the wall and throw the standby into play mode to drain any voltage. With a small watch makers screwdriver or similar gently bend in the socket contacts so that the tube pins are a snug fit.

It's worth noting that the early RI (pre bias pot, pre LTD) had a different voltage divider in the bias circuit to the vintage amps, the PT kicks out a little more negative voltage than the old ones & the RI used colder tubes as stock...but there are plenty on NOS/current production tbes that will run seriously hot in a stock RI, so we really need to know what plate current you have in mA at idle.

[methodofcontrol]

i'll put my bias socket on there and get some readouts across the 1 ohm resistor. good thinking.
i don't understand how a loose tube socket would cause red plating? but, of course i agree that tube sockets should be well tensioned.

[Alex R]

i don't understand how a loose tube socket would cause red plating? but, of course i agree that tube sockets should be well tensioned.

If pin 5 was not making good contact? Have a think about it.

[methodofcontrol]

bias probe says 95 mA on V4! only 23 mA on V5. we're getting somewhere! what's next?

[pdf64]

What's next - a pair of tubes that bias a lot closer.
Just to confirm - the 95mA is in the normal condition, before the red plating starts?

[stokes]

Switch the tubes around to see if the 95ma's stays with the socket or follows the tube.