Yesterday while listening to my VTA ST-120, I suddenly noticed a drop in volume and looked to see one of the KT88's glowing red. I immediately shut down and headed to this forum. I found a thread that indicated some things I should check. http://dynacotubeaudio.forumotion.com/t3871-serious-red-plate-event?highlight=kt88+red+plate
These are the things I checked.
A. Socket is fine
B. All 10-ohm resistors from pin 1 & 8 to ground read correctly
c. All 1K-ohm resistors between pins 5 & 6 read correctly
d. R29 - 32 (mine are 150K) all read fine
e. Checked the grid voltage at pin 5 with all tubes removed and voltages are in line and smooth change when the pot is rotated.
f. Checked the tube for any shorts and found none. ( I don't have any way to test the tubes ).
My amp is exactly 3 years old and the only issue I have had is the 5AR4 rectifier tube went out about 1 1/2 years ago. At that time I did the Yellow sheet Mod and put in a new rectifier tube.
About two or three weeks ago I notice a little spark inside the rectifier tube when I powered it on, but it did not go out and the amp worked fine and has worked fine up until yesterday.
Since everything checked out OK and the only thing I could think might be the problem was the rectifier, I put in a new rectifier, the same KT88 tubes, and powered up as a new start up. Everything seems to be working fine although it does seem like it takes a while for the bias to stabilize, like 10 min. or so. Also I have checked my voltage from my wall outlet and it varies between 121-124 vac.
My questions are.
1. Can the rectifier tube cause this issue?
2. Is there anything else I should check?
3. Should I change the R29-32 resistors to 100k as they are in the newer kits.
4. Also thinking now about adding the auto bias board. Yes or No?
Thanks for advice.