ST120 HiGain to Octal board Problem

Hi Folks. Well, I broke my perfectly working ST120 amp. I replaced the board with the Octal board. When starting the amp following the directions it says to install only the 3 driver tubes and see if the filaments light up. They do. Then I pulled the driver tubes and install the SS rectifier and the left side two power tubes. After the Relay board clicks the tube bias runs away very quickly. It hit over 4 VDC before I powered it off. The bias pots are turned full CCW per the instructions. My Hi-gain board was the opposite where the bias pots are full CW. Anybody have an idea what I could have messed up. I'm pretty sure it is put together correctly. Yeah, famous last words

geode wrote:Hi Folks.  Well, I broke my perfectly working ST120 amp.  I replaced the board with the Octal board.  When starting the amp following the directions it says to install only the 3 driver tubes and see if the filaments light up. They do.  Then I pulled the driver tubes and install the SS rectifier and the left side two power tubes.  After the Relay board clicks the tube bias runs away very quickly. It hit over 4 VDC before I powered it off. The bias pots are turned full CCW per the instructions.  My Hi-gain  board was the opposite where the bias pots are full CW.  Anybody have an idea what I could have messed up.  I'm pretty sure it is put together correctly.  Yeah, famous last words

I do not know what you have messed up. But as advise you could remove the power tubes and
measure pin 5 ( grid) with a DC meter. They should have a negative in the range of -25 -45 Volt
adjustable with the bias pots. If they dont have this you have reduced the problem area to the
bias circuitry.

Hey Peter
I found the problem. I forgot to clip the wire lead on one of the output wires and it was grounding against the chassis. Snipped it off and all is well.