27-06-2017, 08:36 AM
It should be pretty easy to sort out with a dual beam scope and a bit of tinkering.
I get the impression that you have added some silicone to this amp.
First disconnect the feedback loop and connect it to one of the channels of your scope and also connect the other channel to the input of the amp and set it to lock to this signal.
Shove it a sine wave in and look at the phase shift at the end of the feedback loop and note the phase shift. Then move the scope probe to the intended connection point for the feedback loop and note the phase.
The feedback signal should be 180 degrees different. Anything else and the amp will oscillate. This must apply for all frequencies over the intended bandwidth of the finished amplifier amplifier.
Be careful with those silicone bits in the cathode circuits as they might have too much gain and may try to get ahead of the valves phase shift wise causing an unwanted phase shift and oscillation.
I get the impression that you have added some silicone to this amp.
First disconnect the feedback loop and connect it to one of the channels of your scope and also connect the other channel to the input of the amp and set it to lock to this signal.
Shove it a sine wave in and look at the phase shift at the end of the feedback loop and note the phase shift. Then move the scope probe to the intended connection point for the feedback loop and note the phase.
The feedback signal should be 180 degrees different. Anything else and the amp will oscillate. This must apply for all frequencies over the intended bandwidth of the finished amplifier amplifier.
Be careful with those silicone bits in the cathode circuits as they might have too much gain and may try to get ahead of the valves phase shift wise causing an unwanted phase shift and oscillation.
