akulikow42
Tele-Meister
Hi everyone...
Working on a 5A3 Deluxe that has ~ 20vdc difference in plate voltages on the 6V6s with one plate being equal to HT / center tap voltage. Thus, the bias is wildly different for each tube. It will be ok with equal voltages for ~ 30 sec once off the current limiter, then voltages start to fluctuate and eventually settle with the imbalance. Problem stays with the socket (i.e. swapping OT leads, tubes ect. doesn't affect which tube has the high voltage that = CT) and I can instigate the behavior by probing the grid of the tube with the higher voltage with my multimeter.
Things I've investigated with no luck:
- swapped in three sets of tubes & rectifiers
- confirmed cathode connections and appropriate bias voltage on each tube - stays balanced throughout but voltage drop across the cathode resistor doubles when behavior starts
- Disconnected grids to rule out something going on under the board after the PI
- Swapped reservoir cap
- Jumped in a new cathode bypass cap
- Tried two different OTs (one with a near perfectly balanced CT)
- Swapped out the socket with the higher voltage / grid probing behavior.
Frustrated since this is the second time this simple circuit has thrown tough stuff my way... any thoughts are much appreciated!
Working on a 5A3 Deluxe that has ~ 20vdc difference in plate voltages on the 6V6s with one plate being equal to HT / center tap voltage. Thus, the bias is wildly different for each tube. It will be ok with equal voltages for ~ 30 sec once off the current limiter, then voltages start to fluctuate and eventually settle with the imbalance. Problem stays with the socket (i.e. swapping OT leads, tubes ect. doesn't affect which tube has the high voltage that = CT) and I can instigate the behavior by probing the grid of the tube with the higher voltage with my multimeter.
Things I've investigated with no luck:
- swapped in three sets of tubes & rectifiers
- confirmed cathode connections and appropriate bias voltage on each tube - stays balanced throughout but voltage drop across the cathode resistor doubles when behavior starts
- Disconnected grids to rule out something going on under the board after the PI
- Swapped reservoir cap
- Jumped in a new cathode bypass cap
- Tried two different OTs (one with a near perfectly balanced CT)
- Swapped out the socket with the higher voltage / grid probing behavior.
Frustrated since this is the second time this simple circuit has thrown tough stuff my way... any thoughts are much appreciated!