Vox AC15C1 red plate problem

[sorin65]

Suddenly my AC15 stops to sound. I found that one of the EL84 burned. I put a new pair but I observed the one of them go red plate after two minutes or so. meantime the amp seems are working again, but still the problem with redplating. always is about the left position socket, even I inversed the tubes or I put a new other set.
also the voltage on the cathode resistor is not 9V or so as I remember corect value. is 12V and rise in time. when reach 13v the tube goes red plating and very very red plate when reach 14v.
the screen resistors are ok, cathode cap ok, cathode resistor. anyway I changed all with new components.
on DC, I read around 100 ohms for each primary transformer coil, and around 1 ohm on secondary.
I feel the OT have some problems...
Any ideas? thanks for help!

[Chuck H]

It's possible that the coupling capacitor feeding the left position is leaking DC voltage onto that power tubes grid. Another possibility is a bad connection for the grid to ground resistor on the left position. Maybe even a bad contact for the grid pin due to a dirty socket or bad solder joint.

[sorin65]

thanks Chuck. I suspected also the coupling caps and changed too. still the same.
on DC, the grid voltage on the left rise from 0.2 V on starf to around 6 volts when tube go red. on the right side, the DC grid voltage gos to 2 volts

[Helmholtz]

Suddenly my AC15 stops to sound. I found that one of the EL84 burned. I put a new pair but I observed the one of them go red plate after two minutes or so. meantime the amp seems are working again, but still the problem with redplating. always is about the left position socket, even I inversed the tubes or I put a new other set.
also the voltage on the cathode resistor is not 9V or so as I remember corect value. is 12V and rise in time. when reach 13v the tube goes red plating and very very red plate when reach 14v.
the screen resistors are ok, cathode cap ok, cathode resistor. anyway I changed all with new components.
on DC, I read around 100 ohms for each primary transformer coil, and around 1 ohm on secondary.
I feel the OT have some problems...
Any ideas? thanks for help!

Did you get sound with the new tubes?
If so there is no reason to suspect the OT. DCR readings look O.K.

I would think of some leakage (to EL84 grid) issue: coupling cap, carbonized EL84 socket or circuit board... To verify you may ground the EL84 grids (pin 2) - one after the other - and monitor cathode voltage. If cathode voltage drops when grounding the grid, there was leakage in the corresponding grid circuit.

Please post schem.

[Jazz P Bass]

"Please post schem. "

AC15C1_.pdf

[sorin65]

@Helmholz, thanks for sugestions. I put the grids to ground, same problem. cathode voltage rise suddenly to 12-14 Volts. and left EL84 goes red. till then, yes I have sound, (when no grounding G1), but started to crack sounds in a minute or so. tommorow I will receive a new output transformer.

[Helmholtz]

@Helmholz, thanks for sugestions. I put the grids to ground, same problem. cathode voltage rise suddenly to 12-14 Volts. and left EL84 goes red. till then, yes I have sound, (when no grounding G1), but started to crack sounds in a minute or so. tommorow I will receive a new output transformer.

I can't think of a way a defective OT could noticeably increase idle cathode current/voltage. Possibly bad tube socket contacts.

[Jazz P Bass]

Agree on a probable socket issue.

If the grid pins (pin #2) are knackered you would loose the bias & red plate the output tube.
Or a slight conductive short between the cathode (pin #3) & the grid would send the grid positive with the same results.

[Chuck H]

...a bad contact for the grid pin due to a dirty socket or bad solder joint.

And I'll add, a conductive socket. This sometimes happens with a catastrophic tube failure. Do we know if this may be part of the amps history? It hardly matters. Unfortunately testing the socket for insulation doesn't tell the story because conductive paths at HV can present at voltages much higher than any meter uses for measurements.

[Dave H]

tommorow I will receive a new output transformer.

Which sadly won't fix the problem

[sorin65]

@Chuck, yes it was a tube failure. I changed the pair with a very very nos EL84 Siemens, all seems fine, but in two minutes os playibg the guitar, the sound stops and I saw one EL84 goes in a very luminous blue neon light. on cathometer the tube shows no plate current.
the transformer arrived, but first will change the sockets - I dont have around PCB sockets, but I m waiting a friend to borrow sone in a couple of hours.

[Dave H]

@Chuck, yes it was a tube failure. I changed the pair with a very very nos EL84 Siemens, all seems fine, but in two minutes os playibg the guitar, the sound stops and I saw one EL84 goes in a very luminous blue neon light. on cathometer the tube shows no plate current.

If one tube has no plate current the other tube will most likely red plate because they share a common cathode resistor. This could also happen if one side of the OT primary failed open but both sides measure 100 ohm.

[Enzo]

If one tube has no plate current the other tube will most likely red plate because they share a common cathode resistor.

Um, I just woke up...and I am old... but that sounds wrong to me. If they share a common cathode resistor, one red plating tube carries way more current, that increases the voltage drop across the cathode resistor. That in turn serves to bias the OTHER tube colder, not hotter.

[Dave H]

...but that sounds wrong to me. If they share a common cathode resistor, one red plating tube carries way more current, that increases the voltage drop across the cathode resistor. That in turn serves to bias the OTHER tube colder, not hotter.

I think we are both right

I said if one tube is not conducting then the OTHER red plates.

[sorin65]

thanks guys, the problem was the socket. I changed and red plating disappeared. I let the amp working on for two hours. is back to life!
thanks again!