'67 twin reverb power tube cherry red

[jrfrond]

First post "I have swapped power tubes to see if a bad tube but i confirmed it to be that position"

Sockets can and do carbonize and or become contaminated, especially that old Bakelite. I see it all the time. Remember, there's a lot of heat running up into the sockets on inverted tube designs. In addition, high-current contacts e.g. heater connections, can build up resistance and heat up, causing local heating in addition to the overall heating.

My next step, eliminating practically everything around the affected tube would be to replace the socket. You did try a NEW tube in there too, didn't you? I'd do THAT first, not just on of the other used tubes.

[gromrock]

Joe: i believe you are referring to post 22 and no i have not yet. but that is a very good point which points back to the socket.

John: we're 5 blocks from each other! and yes im back on the socket bandwagon. at my old shop the engineer would believe that a totally innocuous relay socket couldnt go bad, but i got to prove him wrong a couple times. those were good days. i should take my own advice. the socket is on the way, so it will certainly get replaced. maybe thats my problem on my other twin.......hmmmm......

[jrfrond]

Joe: i believe you are referring to post 22 and no i have not yet. but that is a very good point which points back to the socket.

John: we're 5 blocks from each other! and yes im back on the socket bandwagon. at my old shop the engineer would believe that a totally innocuous relay socket couldnt go bad, but i got to prove him wrong a couple times. those were good days. i should take my own advice. the socket is on the way, so it will certainly get replaced. maybe thats my problem on my other twin.......hmmmm......

Five blocks away? You could have walked over and we could sell you a socket, or any other parts for that matter.

[gromrock]

yeah i realized that as i was writing it. bummer. but now i know for the next time!

[überfuzz]

yeah i realized that as i was writing it. bummer. but now i know for the next time!

"... next time!" Did you nail it?

[gromrock]

no i dont have the parts yet. im going in to the studio in a few hours to take a couple measurements though. ill be swapping the socket on thursday sometime.

[gromrock]

So with no power tubes installed and the amp in standby, pin 5 on the power tube sockets: v1 is -44.45, v2 is -44.45, v3 is -44.5, v4 is -44.5. Still thinking a bad socket?

[52 Bill]

Now with no power tubes turn off the standby switch. Do the voltages at pin 5 stay the same or does the voltage on the affected sockets drop?

[Tom Phillips]

So with no power tubes installed and the amp in standby, pin 5 on the power tube sockets: v1 is -44.45, v2 is -44.45, v3 is -44.5, v4 is -44.5. Still thinking a bad socket?

Actually. I'm thinking along the lines of my post #28 but I wouldn't totally rule out the socket. Maybe there is a leakage path from pin 3 or 4 to pin 5.
Whatever the problem is it is affecting both the V3 & V4 positions. The results of the test suggested by 52 Bill will help. If the bias voltage changes when you switch the high voltage on with the tubes removed then I'd lift the coupling caps from the phase inverter and re-measure the bias voltages.

[gromrock]

Now with no power tubes turn off the standby switch. Do the voltages at pin 5 stay the same or does the voltage on the affected sockets drop?

Turning the standby switch on and off makes no difference. The voltage stays the same, its around -45vdc across all pin 5's.

[gromrock]

sorry double post

[52 Bill]

As far as I can tell, you've proven that the bias supply is probably ok, the coupling caps are ok and the tube sockets are probably ok. That only leaves the tubes. Try installing one tube at a time and measure the voltage at pin 5 in the suspect socket. Does one tube cause the bias voltage to drop?

[gromrock]

yeah i will start swapping out tubes tomorrow night into that socket and have the meter leads clamped on pin5 and ground and see what happens. I think though it still could be the socket. if swapping tubes around doesnt prove out then i will change the socket and hopefully thats it.

[Tom Phillips]

... I think though it still could be the socket.

Ask yourself why you think that. Can you explain the reasoning to yourself?

...if swapping tubes around doesnt prove out then i will change the socket and hopefully thats it.

Re: "hopefully that's it". We can always hope but it would be better to have some proof.
As you know a common reason for redplating is under bias or the total loss of bias (which is an extreme under biased condition)
Consider the evidence that BOTH tubes on the same side were reading -25 V on pin 5. So what would cause that? Assuming that the problem is not intermittent then I think it's more likely one of the tubes. I hope you marked the tubes so you know which socket they were previousl installed in.
I hope to hear good news from you soon.
Good luck,
Tom

[JoeM]

Sometimes if I suspect a tube(s) I'll label them with a sharpie 1,2,3,4 or A,B,C,D so later I'm not confused which tube was which. Make a note what socket they are in and the test results, and then swap one tube at a time to the other side and see if the low socket changes side.