Hello Spencer,
Yes, I believe you that the preamp works on the other two amps. But the thing is, what is the input sensitivity of the two amps compared to the 845? It is possible that the ground loop is there already but not audible with the two amps while when using the 845 with higher sensitivity on the inputs, the hum becomes more pronounce.
For example, I have two amps running 2A3 output tubes. The first one uses 6SL7 as the input driver and needs 1 vrms for full ouput. The other one uses a type 76 tube as the first stage and 6SN7 as the second stage before the output tube. In this amp, it only needs 0.3 volts for full output. So what does that tells us, that if you compare the two, a 10 mV AC hum for example going to the first amp will be seen as 3.33 times higher as seen by the second amp. That could be the reason why the hum was not audible on the other two amps.
It is unlikely that you will hear a hum coming out of the preamp when you are using a Push Pull amp because it cancels at the output stage. Another point to consider is that both the EL34 and 6AS7G(OTL)are Indirectly Heated Triode (IDHT), while the 845 is a Directly Heated Triode (DHT) which can be more suceptible to hum since the cathode and the filament are one.
You mentioned that when the input to the 845 amp is shorted, there is no hum. This is the standard way of measuring the hum figures for SET amps, and is the prefered way to "null" out the hum if you have humbucking pots on the filament supply. If by shorting the inputs you do not hear a hum, then the amp is not the culprit. When you plug an input cable to the amp inputs, the ground of the preamp becomes a part of the overall ground path. This is where the hum loop can exist.
regards,
Abe
Yes, I believe you that the preamp works on the other two amps. But the thing is, what is the input sensitivity of the two amps compared to the 845? It is possible that the ground loop is there already but not audible with the two amps while when using the 845 with higher sensitivity on the inputs, the hum becomes more pronounce.
For example, I have two amps running 2A3 output tubes. The first one uses 6SL7 as the input driver and needs 1 vrms for full ouput. The other one uses a type 76 tube as the first stage and 6SN7 as the second stage before the output tube. In this amp, it only needs 0.3 volts for full output. So what does that tells us, that if you compare the two, a 10 mV AC hum for example going to the first amp will be seen as 3.33 times higher as seen by the second amp. That could be the reason why the hum was not audible on the other two amps.
It is unlikely that you will hear a hum coming out of the preamp when you are using a Push Pull amp because it cancels at the output stage. Another point to consider is that both the EL34 and 6AS7G(OTL)are Indirectly Heated Triode (IDHT), while the 845 is a Directly Heated Triode (DHT) which can be more suceptible to hum since the cathode and the filament are one.
You mentioned that when the input to the 845 amp is shorted, there is no hum. This is the standard way of measuring the hum figures for SET amps, and is the prefered way to "null" out the hum if you have humbucking pots on the filament supply. If by shorting the inputs you do not hear a hum, then the amp is not the culprit. When you plug an input cable to the amp inputs, the ground of the preamp becomes a part of the overall ground path. This is where the hum loop can exist.
regards,
Abe