One of my KT88 output tubes red plated this morning. I caught it pretty quickly, but I want to query the board to make sure I check what I'm supposed to.
I know in the normal scheme you should check the 10 ohm resistor connected from pin 8 to ground and the 1K resistors connected on pins 5&6.
However I have the AB board installed and the 10 ohm resistor is taken out of the circuit (or at least moved to the AB board). I'm pretty sure it's replaced by the 10ohm resistor that sits on top of the AB board (4 of them of course--I've sent a note to Pavik to check).
I checked each of those and they measured 10ohms back to pin 8 of the socket. I also measured all 4 of the 1000 ohm resistors on pins 5&5 just to be sure.
Lastly, I moved the output tube that red plated to a different socket to see if it followed the tube or the socket (after cleaning the sockets and tube pins)....but since reinserting the tubes the amp has been running fine.
Kind of unsettling to see that thing red plate and then not-any advice would be appreciated.
A tube that has redplated is not to be trusted. It should be replaced !
Of course I ran it after it red plated!!
Fortunately it seemed to run fine. I take your point completely though and won't repeat my foolishness--I have pulled all four KT88 OTs and will be replacing them with the original set of 6550s that came with the amp (which have very low hours on them). The KT88s are over two years old and have been run pretty regularly for 4-6 hours at a time (I'm retired and this amp is in my personal space where I spend 90% of my awake time in the house re: so as not to get in the way of my more productive wife...)
Before I run the amp again however, I plan on inserting the varistor and fuses that Pavel sent. These apparently are to ensure that the AB board does not suffer any damage in the event of a shorted OT. OF course I DIDN'T insert them right after he sent them to me last month...do you detect a pattern here?
I also plan on contacting Erhard Audio, their US distributor, per Pablo's advice above, just to ensure I know how to check of the AB board is OK before turning back on and to confirm my understanding of how I apply the varistor/fuses.
Lastly, the space is very very tight inside the VTA 120 with the AB board as you know--if anyone on the forum who uses the AB board and has already added the varistor/fuse upgrade, I would greatly appreciate a photo to see how you have inserted these components into the unit.