RESOLVED: Original Dynaco ST-70 with VTA driver board bias Problem - One Output Tube Glows Cherry Red

knotscott wrote:I'm finishing up the 2nd of my original Dynaco ST-70 rebuilds with the VTA driver board.  (First one sounds amazing BTW!).  The right channel is giving me bias problems.  The 0.40 bias holds up for a few minutes, then the bias on the front right channel output tube skyrockets and the tube glows cherry red.  That also drops the bias on the rear tube.  I've tried three different tubes of two brands, and the story is the same.  Left channel is fine.  Everything except for the PT and OPTs is new, including the quad cap.  I don't see any issues with the socket or the solder connections....what else can I check

Edit: I put the amp back on the bench, and rechecked every connection, resoldered several, and measured most of the components.  I basically found no smoking gun, but decided to give it another go hooked up to the system.  It held bias well, played, and actually sounded pretty good for nearly 20 minutes. After around 20 minutes, it started to distort badly, and the bias skyrocketed again.  I shut the amp down, and am not sure what to do next.  Any help would be greatly appreciated.

Bob Latino wrote:

knotscott wrote:I'm finishing up the 2nd of my original Dynaco ST-70 rebuilds with the VTA driver board.  (First one sounds amazing BTW!).  The right channel is giving me bias problems.  The 0.40 bias holds up for a few minutes, then the bias on the front right channel output tube skyrockets and the tube glows cherry red.  That also drops the bias on the rear tube.  I've tried three different tubes of two brands, and the story is the same.  Left channel is fine.  Everything except for the PT and OPTs is new, including the quad cap.  I don't see any issues with the socket or the solder connections....what else can I check

Edit: I put the amp back on the bench, and rechecked every connection, resoldered several, and measured most of the components.  I basically found no smoking gun, but decided to give it another go hooked up to the system.  It held bias well, played, and actually sounded pretty good for nearly 20 minutes. After around 20 minutes, it started to distort badly, and the bias skyrocketed again.  I shut the amp down, and am not sure what to do next.  Any help would be greatly appreciated.

Check to see if the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground on that right front tube socket does measure 10 ohms ? If that resistor is open or it measures something other than 10 ohms, that might be what is wrong ?

Bob

knotscott wrote:Update: Still no clue what's causing this.  The situation is worse now than before, because it will no longer bias on that socket at all vs playing for 20 minutes before.  Out of desperation I did do the following:

- swapped out the bias trim pot - no change
- replaced and rewired the entire faulty socket - no change
- disconnected, stripped, and resoldered OPT ground wires - no change

Since I have no idea what else to look for, can anyone offer any suggestions?

knotscott wrote:Hi Peter,

Thanks for replying.  Voltage to pin 5 of the offending tube is ~ - 62VDC, all others were near - 40VDC.

About the time you were replying, I was replacing the 0.3 uf cap that goes to pin 5....is that the coupling cap?.

knotscott wrote:Thanks again, will definitely try it with the switch, and will reduce the caps to 0.22.  

BTW - those voltage readings were all done with the output tubes removed, so I'm assuming all would be a bit lower once the outputs go back in.