Fender Super Reverb PR 469 Burning power tubes

[Boss X]

I got this amp with 2 burned up 6L6's and 2 burned up 470 ohm/1 watt resistors [R62/R63]...soldered in 2 new resistors, and put 2 new/matched 6L6's...brought it up slowly on the Variac...got it to 120 volts AC/.85 amps...set the negative voltage to -50 volts...sounded good...about 5 minutes in, the tube closest to the rectifier red plated and failed. The other tube was wasted as well, just not as badly. What am I missing, or should be checking...? Diodes #1,2,3,4,5 are in that part of the circuit as well as the 100v 100uF bias capacitor...and possibly the output transformer...? Thanks in advance for any help troubleshooting this...

[Boss X]

I should mention...this is a 2000 era reissue w/pcb's...and the bias capacitor is C36...

[Helmholtz]

Please post corresponding schematic.

A bad OT can't cause redplating at idle.

[TomCarlos]

I believe this could be the schematic - Fender Super Reissue Schematic. Boss X, please confirm.

Fender_65_Super_Reverb_Reissue_Schematic.pdf

[Helmholtz]

Link doesn't work for me.

[vintagekiki]

... ...

Given the described accident, check octal tube socket V7, V8 that they are not burned. This is manifested by a black soot between pin 2 and 3
If all is OK, bias voltage adjust with R58 (10k) to maximum.
Install the V7 in octal tube socket. Connect the DVM (range 200mVDC) to the TP37 and with the R58 set the quiescent current to 30-33mVDC

[nosaj]

Remove power tubes and place a meter probe in pin5 turn on and monitor voltage. See if it goes up and keeps going up or if it goes down.

nosaj

[TomCarlos]

I fixed the link and posted a pdf.... you should be able to access the schematic now. Thanks!

[Helmholtz]

I fixed the link and posted a pdf.... you should be able to access the schematic now. Thanks!

Thanks. Works now.

[Enzo]

Check the wires on the power tube sockets. See if any of the solder joints are weak or failed. I have found unsoldered joints in Fender amps right from the factory.

[Boss X]

Thank You TomCarlos...the schematic/layout are great...Thanks nosaj, the negative voltage stays steady...and I have it set to -50 v as per the Fender recommendation...vintagekiki, there is a small amount of soot all around the power tube sockets, not just between pins 2 and 3...and the reading at TP37 is 31mVDC and reading at pin 5 on both power tubes is -50 VDC...

[nosaj]

Any soot can be conductive causing voltage to leak where it should not. You can try cleaning them but it may be best to replace them. Soot indicates arcing. Photos would help.
nosaj

[vintagekiki]

best to replace

[Helmholtz]

Redplating is caused by excessive current. Increased tube current shows as increased cathode voltage (amp has 1R cathode resistors).
So no matter what is the root cause (defective tube, loss of bias, leaky socket, leaky coupling cap....) it makes sense to monitor cathode voltage.

[Chuck H]

All of the above. The problem could still be intermittent even though you didn't see deviation in the bias voltage. Monitoring cathode current and then chasing anything that may be causing it to rise is the best move. This way you can power down if you see current rising dangerously.

I'd replace the power tube sockets and inspect the board for heat damage around the sockets and any power supply traces and pads.

[vintagekiki]

replace the power tube sockets and inspect the board for heat damage around the sockets and any power supply traces and pads.

[Boss X]

I can only send one photo at a time...would just like to say, thanks for all the help tracking this down...readings just now w/6L6 in V7...492 VDC on Pin #3...-50 VDC on Pin #5...31.5 mVDC on Pin #8