Fun with Feedback

[andrew Ivimey]

I don't like the look of it, schematic wise, makes me wonder about the strange components, why for example 200pf - it ain't right all this feedback stuff - what's wrong with simple triode into simple triode with as few capacitance as possible ...and feedback - this lady's not for turning, no, no No! stamps foot.

Does it really sound that good ( yeah I know NVA is rubbish) ?

ducks and runs - so long suckers- Happy New Year. I'm in Lviv.

[pre65]

Happy New Year. I'm in Lviv.

Beware of drones.

[Nick]

I did just run the numbers and cant help thinking the 200pf may be a bit more than is needed, unless I am wrong that will start to reduce the gain from around 8kHz. But I guess you have scoped it all out.

[Cressy Snr]

I did just run the numbers and cant help thinking the 200pf may be a bit more than is needed, unless I am wrong that will start to reduce the gain from around 8kHz. But I guess you have scoped it all out.

Hi Nick,
Yes, there is scope (s’cuse the pun) for reducing the cap value, maybe down as far as 68p. I was being a little bit belt and braces until I could be sure that nothing untoward was going to occur with the pentode conversion. It scopes out quite well but I think there is still room for improvement to the corners of the 10KHz square wave and the rise/fall time. A bit of further tweaking of the compensation network might get it better.

[Cressy Snr]

…it ain't right all this feedback stuff - what's wrong with simple triode into simple triode with as few capacitance as possible?

Everything?

Does it really sound that good?

Yes.

[Cressy Snr]

From what Ant says you've got a good amp there Steve.

Would you agree with Ants appraisal ?

Yes I would. It is very good indeed. However, I’m not about to start getting all evangelical about pentodes. The only way for folks to decide whether or not they like these thermionic outcasts is to build something. Enter this realm and all that minimal or zero feedback stuff goes out the window. A whole set of wizarding skills are required. but I found it interesting and ultimately very rewarding.

[andrew Ivimey]

Who's a thermionic out cast here?

Most of us have been, are still up to our ears in it.
(A few may look back fondly remembering the joy)

But why set up an Aunt Sally just to shoot her down to an audience who knows a great deal about all this thermionic stuff. It's daft.

Audio Talk wouldn't exist if nobody knew what you were doing. We'd just think you were bonkers.

Apologies if I sound aggressive or rude Steve but...

[Cressy Snr]

Power Pentodes were what I was referring to as the thermionic outcasts of the hi-fi world not people. Don’t understand your anger.
Anyway I can’t be arsed to argue anymore. I’ve built something I like, I put a huge amount of work into learning about the maths and engineering stuff required to achieve what I’ve achieved. The mistake was posting about the bloody thing. I really have no idea what I’m supposed to have done. I thought I was among friends here.

I’m afraid it’s over and out. I don’t need the aggravation.

[pre65]

I thought I was among friends here.

You are Steve, just remember that.

But being friends does not mean we automatically agree with everything you do, or say.

[Cressy Snr]

OK
But I still have no Idea what I’ve done wrong. Perhaps someone could explain the nature of my apparent faux pas.

[Nick]

I don't think you have done anything wrong Steve and I don't think Andrew is angry. I think though there is maybe a suggestion to make. I suspect you forget which forum this is (as Andrew said) you don't need to make everything a narrative
We can understand what you have done and why and find it interesting without having to make it into a story. Other places may need that but not here. It can get in the way of the interesting part. Just my take on it. Likewise you don't need to create conflict for the sake of the narrative. It just creates imaginary tribes and aggression where it's not needed.

Also, friends don't have to agree all the time. It can be fun to argue.

[IslandPink]

I don't think you have done anything wrong Steve and I don't think Andrew is angry.

That was exactly what i was thinking of posting as I read this just now !

[Cressy Snr]

OK point(s) taken. Thanks chaps.

[Cressy Snr]

Anyway, using the only polystyrene cap values I have in the parts box, I managed to work something else out for the step network on the input stage anode and the one in the feedback loop. So here is the diagram:

Heretikpentode 2.0.png (72.3 KiB) Viewed 6488 times

Works nice.

[Cressy Snr]

Was revelling in Pat Metheny’s “As Falls Wichita etc, then came a smell of burning, followed shortly after by smoking wreckage. My beautiful sounding creation! I’m devastated, no really. It was the most exquisite sounding thing and now it’s a pile of dead scrap.
No doubt I’ll get it going again following an investigation, but Karma can be a right bastard sometimes.