I recently played a gig with no problem in low power.
Last week amp was switched to high power and the tubes glowed cherry red and the smell of plastic started.
I quickly turned the power off.
After 2 days I played the amp on low setting to test and everything was fine.
The plastic base of the tubes and tube sockets had dark burn areas on them.
My amp has the 12ax7 tube.
Any ideas would be much appreciated, so glad to have found this site.
Eddy
Fri, 03/27/2015 - 07:46
#1
Well, where do I start?
Well, where do I start? Ok- how old are the power tubes? How old is the PI? How hot was the amp biased? Not trying to be a pr!ck here but you aren't giving us much to work with. First, if they have red plated, you're PROBABLY going to need tubes. (OK- you NEED to replace them). Then, the original mm's had a tendency to do this so they converted them to SS drivers BECAUSE of it. There are only a few tubes that I would risk in a MM. So you are going to need to replace the driver with a NEW good quality 12ax7. You should replace the sockets since they are crunchy. THEN, you need to check the bias supply and see how much bias control voltage is going to pin 5 in high. Replace the resistors on the sockets as well. Bias around 22-23mA PER tube and fire the mutha up. If you can measure the control grid voltage BEFORE you take it apart, it will help. THEN, when you get ready to crank it up, INCREASE the NEGATIVE voltage going to pin 5 BEFORE you fire it up. This will give you the cushion to adjust UP. Questions? Ask away. Mike.
crispy tube sockets
Thanks a bunch, I will heed your advice asap.
Thanks Mike.
Sockets
Hi Eddy,
the tube sockets that have become discolored may have deterioated enough to become partially conductive at the high voltages found in a tube amp. In other words, replace the socket(s) first. It would be a shame to have other parts go south due to an arcing socket.
Cheers,
Lars Verholt
crispy tube sockets
Thanks Lars, will do.
Eddy