The Steel Guitar Forum

I have a '69 Twin Reverb and recently installed a NOS Mullard 12AT7 for the phase inverter. Lately I've noticed that sometimes as I play, I loose volume and bottom end all at once. If I turn the amp off then back on again often it will work fine again.

I suspect the tube I recently bought but haven't had it locally tested yet.

Is it possible for a tube to work intermittently? Since I noticed this soon after installing the tube this is what I suspect, but is there anything else I should consider besides the tube?

I'm not an amp guru by any stretch...any ideas would be great before I take it to my amp guy and spend some dough!

First principle of tube amp troubleshooting is substitution... try another 12AT7. Still happening? Not the tube.

Wiggle the tube... thump the tube, while someone's playing through it. Can you induce failure? It could be the tube, or its connection...

It's possible that your tube socket has become worn... you can re-tense the leaves that grab the pins using a large needle or other sharp implement to push the leaves back together. Hi-temp Pro-Gold is also good for tube pins... the normal stuff won't work, though, you have to get the hi-temperature formula.

It's a good idea to drain the capacitors in the amp when you perform this... your Twin Reverb should not need this, since this amp has bleeder resistors installed (some Fenders (Princeton etc) don't)... but at least check each pin to chassis with a voltmeter to make sure.

An amp of this vintage probably should see a technician... they do the re-tense socket thing, clean the pots, test the tubes, put new filter caps (if needed)... this is common for an amp this old.

Stephen,
your comment about the age of the amp and the filter caps brought to mind that a capacitor somewhere in the amp, probably a bypass or coupling cap may be old enough to be drying out and when you turn the amp off and back on, it charges up. (for a while) Most likely an electrolytic capacitor. Jeff, as you know, it's hard to troubleshoot an amp miles away. All we can do is offer our best guess.

Best regards,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024

I appreciate the advise...I put another 12at7 in there and it hasn't acted up since. Pretty sure it was the tube. I thought a tube either worked or it didn't. At any rate..seems like the tube at this point anyway!