Steve looking again at your sketch you have both a 1uF cap to ground and a 100k resistor which I don't have. Could you help me understand the purpose of these please ?izzy wizzy wrote: ↑Thu Dec 27, 2018 10:13 pmBloody knackering this kind of thing. Did you have the same bias arrangement on the other one with the wings?RhythMick wrote: ↑Thu Dec 27, 2018 9:30 pm Well it was a good effort. All 6 bias trimmers are now out of circuit, top resistors changed to set bias. Interstages mounted back in amp.
Squeals like a stuck pig.
I recognise I'm pretty tired. Stopping now. Will check wiring tomorrow and make sure no mistakes.
Did you decouple the grid bias from the cathode; something like pic below?
26bias.JPG
Cheers,
Stephen
The cap I can understand, to stabilise the bias against fluctuations - as it's a balanced long-tailed-pair I'd expected there to be little need for that but it's something I can try.
What's the 100k for ? Grid resistor ? Without it, each grid will see a coil and then the lower half of the cathode stack to ground ? Again something I can try easily.
Geoff's amp has neither and is singing beautifully.
I think first course of action will be to bypass the balancer pot with clip leads so the virtual cathodes connect directly to the top resistor. Quick and easy. If that doesn't do it I'll try both the 100k and the cap in the bias.