More silverface bandmaster issues - intermittent vibe channel

[MikeH]

Hello folks,

I've been "restoring" a 69 silverface bandmaster reverb for a little bit now; when it came to me it had no reverb tank, no tubes, and a host of leaky caps. All of which I have taken care of, but it has one lingering problem that I can't nail down:

When I first fire it up, the vibe channel passes no signal. I can hear the ticking of the vibe, and the reverb will rattle, but no signal through. With the volume up after a minute or two, some staticy crackle starts coming through, when the crackle is gone, the vibe channel works fine. And it seems like it would keep on working too, I've played for about 20 minutes or so, and the signal never cuts out.

I've chopsticked the dickens out of the vibe section; when I redid the power section cap, I also replaced all of the ceramics and electrolytics on the board, as well as one of the film caps that looked dodgy (there was a lot of corrosion around one of the legs).

Any ideas as to how I should proceed with the debugging?

[Enzo]

Yes.

Isolate the problem.

Get out the schematic. From the input jack, there is a flow from stage to stage until you get to the point where the other channel joins in. From that point on we know it works because the other channel works. Right? SO look at the tubes in that section. First check power supply. Is there B+ on each and every plate pin? That is pins 1 and 6 on the 12AX7. Looking for something within 50v of 200v I guess. Now look at the cathodes, pins 3 and 8. ANy with zero volts? If a triode stage has zero volts on the cathode, and probably very high voltage on the plate, then the tube is not conducting. Are BOTH heaters glowing in each tube?

If all the voltages look OK. Then apply a signal to the input, and trace it through the stages. Got it at the grids? Got it much larger at the plates?

Does touching any grid pin with your meter probe or a scope probe trigger the amp back into life? That would probably mean that grid was lacking termination, ie and open grid return resistor.

And you said nothing about swapping out tubes. By all means try different 12AX7s in the sockets for that channel. No spares? Trade places with the tubes for the other channel, we already kniow they work.

And of course, the tube sockets. Does wiggling any tubes sidde to side make noises or make the amp wake up? And inspect the solder on the pins of those sockets. Or just resolder them anyway.

[MikeH]

I forgot to mention; yes I did swap tubes with known working tubes, so I do not believe it is a tube issue.

I checked voltages, which was a little tricky because I had to work fast before the amp started 'working' again. Anyhoo, I think you're on the right track; the plate voltage on v2 pin 1 was high (around 380) and the cathode was sitting at 0V.

The tube in V2 is brand new, but just to be sure I swapped it with V1, and then the voltages were correct on V1 AND V2. Now I'm thinking it might be socket-tension related? The socket does not feel all that tight... could that cause this issue? I would expect a socket that didn't make a good connection to have a consistently bad connection, as opposed to one that starts bad, and then works perfectly after about one minute, and for seemingly the rest of the time the amp is on, like clockwork.

At this point I've been swapping tubes, and letting the amp cool down as well as discharging it before firing it up again, and I cannot reproduce the issue no matter what I do. Wiggling v2 does produce some snap, crackle, and pop but does not cause the channel to cut out, or produce the static sound from earlier.

[Enzo]

If your socks lose their elestic, do they instantly fall down and stay? Or can you pull them up and sometimes they stay ther a while and sometimes they start right bacjk down. Loose tube sockets don;t have to be continuously bad, in fact usually they go intermittant. As heat and vibration cause things to shift around, continuity comes and goes. Metal warms up and expands, tightening the grip perhaps.

There is an old TV repair trick from the tube days. It was a major job to remove the chassis, drill out the rivets, replace a socket and wire all the components to the new one. SO if a socket for a small tube like a 12AX7 went loose, they would stick just the very tips of the tube pins in the hole, just enough to catch the edge of the hole, and they would twist the tube just a little, as though screwing it in. WHat that did was to set all the pins at a sort of angle. Now when pushed into the socket, those angled pins push more against the sidewalls of the socket pins. He could do that in seconds, and from outside the chassis.

I don't recommend that because it also then stretches the socket out a little more. But it illustrates what may have happened. When you swapped tubes, first just pushing a tube in and out of a socket cleans the contacts by abrasion, but mainkly, the pins on the two tubes may sit at slightly different angles, and so make contact in the socket at different points, so the two tubes both find themselves comfortable in their new socket. Plus the act of moving them around can also slightly bend pins, and that may be all it took to get past the problem.

Socket cleaning and retensioning is a common part of amp repair and maintenance.

[Jazz P Bass]

If your socks lose their elestic, do they instantly fall down and stay? Or can you pull them up and sometimes they stay ther a while and sometimes they start right bacjk down.

Socket cleaning and retensioning is a common part of amp repair and maintenance.

Or is that sock cleaning & retensioning

[MikeH]

Thanks for all of the tips!

The socket isn't totally blown out, so I was able to take a pick and gently bend the contacts inward a bit. It seems to be working great now, so I don't think I need to replace the socket... yet.