EmRR said:
all but the lowest gain settings and It starts to appear in the first tube stage, upstream of VR1 sounds contradictory. I don't know if I'm reading that correctly.
Sorry. Should have clarified. I started out looking at the final output on the scope, and when I saw things getting ugly, I turned down the gain and observed that I had to turn it down pretty low to get rid of the distortion. It was then that I started to work my way backwards through the circuit to find the source of the distortion. I got as far back as right before VR1.
EmRR said:
You are still missing the part where BALANCED DC current is OK, and that's what you have.
Balanced DC is even OK across a transformer winding that isn't supposed have any DC across it?
EmRR said:
It's an output stage with no plate resistors and small value cathode resistors, I would expect it to get very hot. Measure current.
Now that you mention it, that makes perfect sense. Will definitely measure current. Would it make sense to either add plate resistors or increase the size of the cathode resistors?
EmRR said:
I'm wondering why you went with a cathode follower output stage over a traditional plate coupled stage, especially given the transformer in question, which has gobs of inductance for use in plate circuits.
As I mentioned, I stumbled across this hack on the intertubes and it intrigued me. Thought I'd give it a try. Not too surprised that it's shaping up to be a dud.
EmRR said:
I don't see mention of the power supply voltage or current draw. As drawn there's no isolation at all between the PSU and the plates of the output stage. A traditional plate coupled stage at least would act as isolation, since it's essentially a pair of inductors coupling the plates.
Don't have my notes in front of me at the moment, but the plate voltage is around 250-280VDC.
In response to your following comment regarding that other dog of an idea I submitted...
Noted, and sh*tcanned!
The values stated were bodged together using the ampbooks.com online calculator for LTPs as well as some notes from Aiken Amps' site. I must remind myself that those all pertain to guitar amps, so "apples and oranges."
The hardest part of this project (for me) is figuring out how to implement the interstage transformer, which is why I dove into it in the first place (and landed right on my head!). It seems almost everyone uses IS transformers for power amps, and not much else, if the internet is any indication.
Obviously, the intent is to feed the grids on a push-pull pair, so now I'm having to educate myself (always a good thing!) on the intricacies of push-pull and how to get from there to an output transformer.
One more question....
Is there any possibility that the distortion in the first stage is a result of some sort of undesirable loading from the mess going on downstream?