Hi all.
I gave my son a schematic of a tube preamp for a project at school…
At modest gain settings there were no problem but at high gain settings there appeared some nasty saturation from the output transformer.
I took it to the bench and with the volt meter I could measure some 4vdc after the last coupling cap… into the output transformer. This was at high gain settings. Reducing the gain the voltage came down to 0vdc.
I used the scope and found out that at high gain settings there were some serious interference on the signal at the secondary of the input transformer (he did not use shielded cables from the input transformer to the grid of the first pre am tube).
Changing the cable to a shielded one all the problems were solved. 0vdc after the last coupling cap and no saturation.
My question is how interference on the first input can cause vdc leaking through the last coupling cap into the output transformer?
Is this normal and if so why? It could easily been mistaken for a leaking cap. When measuring I could also see some strange leakage after other coupling caps within the circuit.
I am just curious about it all
Best regards
/John
I gave my son a schematic of a tube preamp for a project at school…
At modest gain settings there were no problem but at high gain settings there appeared some nasty saturation from the output transformer.
I took it to the bench and with the volt meter I could measure some 4vdc after the last coupling cap… into the output transformer. This was at high gain settings. Reducing the gain the voltage came down to 0vdc.
I used the scope and found out that at high gain settings there were some serious interference on the signal at the secondary of the input transformer (he did not use shielded cables from the input transformer to the grid of the first pre am tube).
Changing the cable to a shielded one all the problems were solved. 0vdc after the last coupling cap and no saturation.
My question is how interference on the first input can cause vdc leaking through the last coupling cap into the output transformer?
Is this normal and if so why? It could easily been mistaken for a leaking cap. When measuring I could also see some strange leakage after other coupling caps within the circuit.
I am just curious about it all
Best regards
/John