12AT7 intermittent channel

[Bartguy]

Okay, so here's the weird thing. My Stereomour has been working fine for several weeks. Now, suddenly, after rolling 12AT7 tubes to try to find the one I like, the right channel is coming on and off intermittently — and it seems to be directly related to how the 12AT7 tube(s) are seated in the tube base. There's a certain "springiness" to the tube base, like a tension between the pins and the holes they touch that gives the tube a bit of play. When the channel goes out, I can rock the tube a smidge and it comes back on. But then it will go out again. I've also sometimes noticed that turning the amp off and then on again can sometimes make the channel come back on.

I'm going to resolder the tube base and the power switch, but I got the right resistances and voltages when I finished it and the problem has only recently started. So those joints were working fine originally.

Anything else I should look at? Could the issue be a bad tube base?

Thanks.

[Grainger49]

If the 2A3s still glow when the sound dies the power switch is fine.

Before resoldering just use the tube socket to clean the socket and tube at the same time.  Pull the tube out, insert it and repeat 10 times.  That will clean both. 

If it persists touch up all the solder joints on the 12AT7 socket and anything that could have been moved by inserting new tubes.   

[Paul Birkeland]

I think you have a cold joint at or around the 9 pin socket; best to just reheat those joints.

[Bartguy]

Thanks, everyone. Both sides of 12AT7 tube glow whether the channel is playing or silent, so it appears that power is getting there all the time.

I'll reheat the joints around the socket and clean it as described. Then report back.

[Bartguy]

No luck. I reheated and added a little solder to all the 9 pin socket connections and all connections on board above it, plus the terminal strips that connect to it. They didn't look bad to start with. Anyway, the issue remains. It really feels like an issue with the connection between the tube pins and the base slots. I can make both channels play reliably by tilting the tube to the right in the base (see pic). Any other options to troubleshoot this?

Thanks.

[Paul Birkeland]

It might not hurt to clean off the tube socket pins if they appear to be a little oxidized, there may just be some crud on there that needs to be brushed off.

[galyons]

I had a similar issue on my Paramour 45's, plus a nasty intermittent "finger nails on chalk board" growl.  Did everything you have tried to no avail.  I finally replaced the tube socket and problem solved. Of course my amps are much more experienced than yours! 

The problem may not have been the socket and I just solved the issue in the process of replacing it.  But the end result was the same.

Cheers,
Geary

[Grainger49]

Check the FAQ thread:

http://bottlehead.com/smf/index.php/topic,2408.0.html