Single ended amplier

[Mark Hennessy]

Hi.
On an amp of this calibre thermal noise won't be a problem...

I agree - that's exactly what I said earlier.

...and I belive it's confusing the issue.

Sorry, but I don't think this is the case. There is no confusion here - he's chosen 50k because he plans to use it with modern sources, and has selected 3uF to work with that. Job done

The op has asked why there is a problem and what I have explained is to replace the incorrect  components with preferred values, then it is timely to then explain why.

The values you've suggested seem eminently sensible to me, as I said earlier. Speaking totally personally, I'd like to learn more about how one chooses those values - beyond "that's what was typically used" - so I'm guilty of leading the thread astray in that regard. Give me transistors and I can design an amplifier and justify the presence and value of every single component, but I'm not there with valves. But keen to learn.

Notwithstanding that, it's important to establish if the input stage really is causing a problem. Re-reading the original post suggests that actually, all might be well with that:

I tried to bias the triode on the straight part of the curve so it's OP was as linear as poss, so little THD and used a 12k anode resistor so I didn't have much gain, IE, 25. The measured OP is 25v and is low distortion.

So perhaps focusing on the input stage is what's confusing the issue?

Turning to the output transformer, I did find some data on it here: http://www.vintage-radio.net/forum/showt...hp?t=25657 - I've taken the liberty of stealing the datasheet from that thread:

   

This confirms what Andy said in the OP - the primary is rated for 40mA. It has a DCR of ~1k, and he says it's dropping around 20V DC in use, suggesting that he has biased it fairly well (though 7.3V across 220R is 33mA). In which case, I wonder if it has a shorted turn or similar? Or perhaps a construction fault that isn't apparent from the picture?

Either way, we need to simplify and focus on each stage of the amplifier in isolation. How many times have we said that now?

For anyone learning electronics, here is a good bit of advice: https://www.youtube.com/watch?v=_pGk6GYDnes (jump to 1:40 and watch until 3:17). In short, you learn most when it doesn't work! Ain't that the truth