I was checking tube bias on my ST 120 this morning, after several months since the last check, and found that there was no bias voltage when checking V2 (back left tube) and that the bias on the other power tubes was, not surprisingly, high. I have been using Black Treasure KT88 tubes. And the amp seemed to be functioning normally.
I had previously noted that the filaments seemed brighter on V2. I chalked this up to differences in tube construction. I checked the V2 bias resistor and it was fine at 10 ohms. I put my backup Gold Lion KT88's in the amp and bias voltage returned to V2, however, again the filaments on V2 seem brighter than the other power tubes. The left-hand power tubes and the left-most driver use different transformer heater windings from those on the right. Maybe this is simply a coincidence but, I think I need to check the filament voltages. I have the TDR module installed but I don't think this affects the heater circuitry. So, with the power tubes removed can I just check the AC voltage on pins 2 and 7 with a multimeter probe on either pin and the second to chassis ground, or would one lead be on each pin? Then, if there are voltage differences between the two heater windings, is the power transformer faulty? Thanks in advance for any help the forum can provide.
Bill
I had previously noted that the filaments seemed brighter on V2. I chalked this up to differences in tube construction. I checked the V2 bias resistor and it was fine at 10 ohms. I put my backup Gold Lion KT88's in the amp and bias voltage returned to V2, however, again the filaments on V2 seem brighter than the other power tubes. The left-hand power tubes and the left-most driver use different transformer heater windings from those on the right. Maybe this is simply a coincidence but, I think I need to check the filament voltages. I have the TDR module installed but I don't think this affects the heater circuitry. So, with the power tubes removed can I just check the AC voltage on pins 2 and 7 with a multimeter probe on either pin and the second to chassis ground, or would one lead be on each pin? Then, if there are voltage differences between the two heater windings, is the power transformer faulty? Thanks in advance for any help the forum can provide.
Bill