AC at pin 8 on a 6V6

[morroben]

Hello everyone. I'm new around here but after looking around a bit, I think you folks might be able to help me with a problem I have.
The 6V6s on my 5E3 are redplating. The amp worked fine for 2 years, then all of a sudden started redplating. I've swapped out tubes, sockets, coupling caps, cathode resistor, and bypass cap. Still redplating.
Most everything seems normal voltage and resistance wise, except that in addition to the 22.5 VDC at pin 8 on the 6V6s, I read 48 VAC. From my limited knowledge of amps, that doesn't seem correct. Any thoughts or ideas on what might be causing this?
Thanks in advance for any help or advice.

[tubeswell]

Hello everyone. I'm new around here but after looking around a bit, I think you folks might be able to help me with a problem I have.

Hi morroben and welcome to the forum

The 6V6s on my 5E3 are redplating. The amp worked fine for 2 years, then all of a sudden started redplating. I've swapped out tubes, sockets, coupling caps, cathode resistor, and bypass cap. Still redplating.
Most everything seems normal voltage and resistance wise, except that in addition to the 22.5 VDC at pin 8 on the 6V6s, I read 48 VAC. From my limited knowledge of amps, that doesn't seem correct. Any thoughts or ideas on what might be causing this?
Thanks in advance for any help or advice.

Seems odd that you are getting a VAC reading on your cathodes with a cathode bypass cap in place. Are you also getting a VAC reading at idle on the plates and/or screens, (and/or any other plates in the pre-amp)?

My initial thoughts are possibly that one of the filter caps may be haddit. The cathode voltage would only vary in response to a varying plate and/or screen voltage (and if the cathode bypass cap was not connected). If one of the filter caps is gone filter cap is gone, then you could get oscillation of ripple of some kind on the B+, which could affect the cathode voltage. 2CW

[Mars Amp Repair]

Hello Morroben,
if you are inputting no signal to the amp & you have any AC on the cathode of the 6V6 then my guess is that the amp is oscillating.

There are many things that can cause this, however without a scope to verify that, you might be chasing your tail. I'm also not certain that your meter would respond to the frequencies that an amp oscillates at.

Can you hear a faint 'hum' through the speakers? If an amp is oscillating many times you hear a funny hum in the speakers.

Also have you recently removed the wiring to the output transformer to the power tube sockets & possibly got them reversed? That will cause feedback, too, but generally the amp will squeal like a pig if you have done that.

As an indicator you can also remove the negative feedback loop & sometimes that will cause the oscillating to stop or at least change. the negative feedback goes from the output transformer secondary back to the phase inverter circuit.

I'm not certain if that amp has neg feedback anyway.

Some of these amps have a negative bias voltage applied to the grids of the power tubes eventhough they are cathode biased. If this amp has that then you need to be certain that voltage is present on the control grid.

glen

[Jazz P Bass]

5E3

What does the plate voltage measure on the 12AX7. DC & AC
What do the 6V6 screens measure. DC & AC
What do the 6V6 grids measure. DC & AC
You just may have a bad power supply cap or one of the grid pass caps is bad.

[morroben]

What does the plate voltage measure on the 12AX7. DC & AC

I think this is pin 6?
If so, 435 AC, 211 DC

What do the 6V6 screens measure. DC & AC

V3: 832 AC, 366 DC
V4: 828 AC, 366 DC

What do the 6V6 grids measure. DC & AC

control grid? pin 5?
V3: 0 AC, 0 DC
V4: 2 AC, 4 DC

Can you hear a faint 'hum' through the speakers? If an amp is oscillating many times you hear a funny hum in the speakers.

There is a constant hum along with some crackling. This used to be the quietest amp I've ever owned.

Hi morroben and welcome to the forum

Thanks Tubeswell...I recognize that screen name. TDPRI?

[Jazz P Bass]

V4

I really do not know how you canme up with those Vac readings.
Something is going on at the grids of your 6V6 tubes.
Pull one tube at a time a see if the noise goes away.
I am not thinking a bad tube, more like a leaky 0.1 pass cap.

[morroben]

I really do not know how you canme up with those Vac readings.
Something is going on at the grids of your 6V6 tubes.
Pull one tube at a time a see if the noise goes away.
I am not thinking a bad tube, more like a leaky 0.1 pass cap.

Pulling both tubes quiets things down, but either one alone makes no change.