I have one of Bob's ST-120 amps (been running for around 2 years) and just the other day had a KT88 red plate on me which took out the rectifier tube as well.
I have had tubes red plate a couple of times before and on one of those occasions it took the fuse out, his time the fuse was ok.
On those other occasions I simply replaced the faulty tube, rechecked the bias and everything is good again.
This time however looks like some damage has been done in the left channel where the tube red plated.
After replacing the faulty KT88 and rectifier tube I now have a hash type of noise comming through the speaker and this is accompanyed by the woofer cone moving back and forth very slowly ... probably 5 to 8 mm of movement at random times.
Also the bias voltage is now unstable and swings between 0.470 and 0.560 ... trying to adjust the bias will just see the swing continue at either a higher or lower value.
The right chanel is silent ... no woofer movement and the bias is rock solid with no fluctuation at all.
Swapping the pair of KT88's from one channel to the other makes no difference ... the left channnel still has the problem.
And it still plays music fine from both channels.
Any ideas what might have gone wrong?
Has it fried a bias resistor or a cap?
I haven't taken the bottom cover off yet ... thought I would ask the question here first.
Thanks for any help.