ST-120 power tube red plate issue

calauver1 wrote:Good morning,

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. https://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.

tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

calauver1 wrote:

tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

Best to change them out then, correct?

tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

corndog71 wrote:

tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

Does this also apply to M125's?  What about using KT120's?

corndog71 wrote:Ah, yes.  That’s what I have in my M125s.  

Looking over my VTA120 docs from 5 years ago I got 150k.  Just ordered some 100k resistors.

I thought I read a while ago that KT120s should use 50k.

peterh wrote:

corndog71 wrote:Ah, yes.  That’s what I have in my M125s.  

Looking over my VTA120 docs from 5 years ago I got 150k.  Just ordered some 100k resistors.

I thought I read a while ago that KT120s should use 50k.

That is correct, the documentation stated 51k


http://www.tungsol.com/specs/kt120-tung-sol-specs-curves.pdf

MechEngVic wrote:I would listen to Peterh on this one, his comments mirror my experience also. A tube can red-plate for several reasons, but once it does, it is damaged. It will more easily short and arc in the future even though it seems to be working fine at the moment. It will also start taking other components out with it if you leave it in. Once you replace them, I would re-do my voltage checks. Hopefully, no resistors or capacitors are damaged. Three years is a good amount of time for a set of tubes.