30-09-2019, 06:25 PM
Congratulations on your progress so far - you are doing very well indeed!
I think the reason for that is that the two windings on the mains transformer are the same whereas in the Grundig circuit that you borrowed the idea from, the ratio between the two sections will have been designed to null both at the same time. Not much you can do to improve on that but full marks for trying.
Yes, as Kalee20 has already pointed out, 2n2 is rather a small value for an audio coupling capacitor - I would be inclined to experiment with 10n being the minimum and increase that if the bass response is still lacking
That's very big for an AF cathode decoupler! Particularly as you seem to be experiencing HF rather than LF instability.
It might be worth putting another resistor in series with the triode grid leak, 100k, say, or whatever comes to hand, and adding extra decoupling there instead. It would be much more effective than the current arrangement.
Also, setting the triode bias to 50% of the pentode figure seems a bit high to me - perhaps others might like to comment?
Keep up the good work - you really are doing very well!
(30-09-2019, 10:05 AM)Amie Wrote: Recent experiments with the balance circuit earlier in the thread using a half wave power supply (as I intended to use), show that it CAN be able to null hum, but that feature isn't coincident with the standing current balancing that I was aiming for.
I think the reason for that is that the two windings on the mains transformer are the same whereas in the Grundig circuit that you borrowed the idea from, the ratio between the two sections will have been designed to null both at the same time. Not much you can do to improve on that but full marks for trying.
(30-09-2019, 10:05 AM)Amie Wrote: The frequency response is better, though not going down quite as far as the mains transformer version it appears.
Yes, as Kalee20 has already pointed out, 2n2 is rather a small value for an audio coupling capacitor - I would be inclined to experiment with 10n being the minimum and increase that if the bass response is still lacking
(30-09-2019, 10:05 AM)Amie Wrote: I'm including the update on the attached diagram, one erratum is the cathode capacitor is 470uF not 47uF as the diagram, its needed to stop + feedback.
That's very big for an AF cathode decoupler! Particularly as you seem to be experiencing HF rather than LF instability.
It might be worth putting another resistor in series with the triode grid leak, 100k, say, or whatever comes to hand, and adding extra decoupling there instead. It would be much more effective than the current arrangement.
Also, setting the triode bias to 50% of the pentode figure seems a bit high to me - perhaps others might like to comment?
Keep up the good work - you really are doing very well!
