'67 twin reverb power tube cherry red

[gromrock]

So I have been having issues with a 67 twin reverb (AB763). it had been eating fuses, each time it blew it was after about 10-15 minutes of playing it. I have replaced all of the following:
-470ohm on power tubes
-1.5k on power tubes
-all rectifier diodes with 1n4006
-bias diode with 1n4006
-bias cap
-all "blue tubular" caps with orange drops
-100k resist just before power tubes
-82k resist just before power tubes
-i didnt, but someone has replaced the filter caps with atoms
-220k's just before power tubes

now after about 5 minutes max of playing the 2nd from left power tube's core (if looking from the rear) starts glowing cherry red. i have swapped power tubes to see if a bad tube but i confirmed it to be that position. it is the socket with one of the leads that comes directly from the OT. Do I have a bad OT? what else can I check? MUCH thanks in advance.

[dumbassbob]

Your grid resistors may be bad..Do not turn amp on until a pro looks at it...Red Plate is BAD

[gromrock]

all four 1.5k grid resistors were replaced as i said.

[g1]

The 1.5K resistors on the power tubes are the grid resistors.
Remove the tube from the bad socket. Check the voltage on pin 5. It should be the same negative voltage as you have at the other power tube pin 5's.
The socket itself could be bad, or a connection leading to pin 5.

[52 Bill]

Your grid resistors may be bad..Do not turn amp on until a pro looks at it...Red Plate is BAD

Yes, red plating is very bad for the tube as well as the amp.

If it is only happening on that one socket regardless of which tube is inserted in the socket, then you need to check the wiring going to pin 5 of that socket. It should be one of the 1K5 resistors. You also need to check for the negative bias voltage at that same pin. Also check the socket itself to be sure that the pin contact is good and tight.

[gromrock]

now there is some useful advice. thank you gents. i do remember there was a super loose contact on one of the sockets when i was in it earlier tonight. who wants to bet its the cherry bomb tube? will check it tomorrow and report back.

[Amp Kat]

Yes, red plating is very bad for the tube as well as the amp.

If it is only happening on that one socket regardless of which tube is inserted in the socket, then you need to check the wiring going to pin 5 of that socket. It should be one of the 1K5 resistors. You also need to check for the negative bias voltage at that same pin. Also check the socket itself to be sure that the pin contact is good and tight.

Yup ^^^^^ monitor pin 5 at idle and make sure you are getting a good steady negative bias voltage for a couple minutes. Pin 8 should have a good solid connection to ground so verify it is grounded good. A loose contact inside the tube socket itself could also cause it so if it still redplates make sure the tube is pushed in well.

[MWJB]

The most common reason for a tube to redplate is too much plate current, this is information that you have left out.

Report back with dc voltages at idle for all power tubes pins 3, 4 & 5. If you have repeatedly redplated the tubes you have they are screwed, before you install the new set turn the bias pot to max negative voltage, say -55v for a B+ of <450vdc, -60vdc for <480vdc, -60 to -65v if B+ is higher than that.

[gromrock]

I didnt have the chance to open it back up and meter anything, but i certainly will later this week. No one thinks this could be a bad OT though? just trying to rule it out or put it at the bottom of the list of things it could be.

[Amp Kat]

No one thinks this could be a bad OT though? just trying to rule it out or put it at the bottom of the list of things it could be.

No, I bet there is not one of us that thinks it's a bad OT.

[g1]

No, I bet there is not one of us that thinks it's a bad OT.

Agreed, any possible OT fault would have to involve at least 2 power tubes.

[gromrock]

Agreed, any possible OT fault would have to involve at least 2 power tubes.

awesome!! that is excellent (ie. cheap) news! ill be back in the studio tomorrow to go through it. thanks gents.

[gromrock]

The most common reason for a tube to redplate is too much plate current, this is information that you have left out.

Report back with dc voltages at idle for all power tubes pins 3, 4 & 5. If you have repeatedly redplated the tubes you have they are screwed, before you install the new set turn the bias pot to max negative voltage, say -55v for a B+ of <450vdc, -60vdc for <480vdc, -60 to -65v if B+ is higher than that.

Looking from the back at the amp and starting all the way left:
Power tube 1: pin3=419vdc
Pin4=419vdc
Pin5= -43.5vdc
Power tube 2: pin3=419vdc
Pin4=419vdc
Pin5= -43.5vdc
Power tube 3 (the one that red plates): pin3=412vdc
Pin4=412vdc
Pin5= -26.48vdc
Power tube 4: pin3=408vdc
Pin4=408vdc
Pin5= -26.5vdc

What could be causing this difference in voltages?

[52 Bill]

What could be causing this difference in voltages?

Read the voltage at pin 5's with the standby switch off (no high voltage to circuit). You need to find out if the bias supply is low to those sockets or if it is being pulled down by a bad tube, bad socket or a leaky coupling cap.

If the voltage comes back up on pin 5 to equal the other two tubes, then pull the two tubes on that side, and see what happens when the standby switch is turned on.

[gromrock]

With the amp in standby mode and all power tubes installed and amp powered on. V1 and v2 pin 5 = around -43vdc. V3 and v4 pin 5 = around -25vdc

EDIT: this -25vdc is also measured at the joint of the 220k resistor and 0.1 cap on the eyelet board.