Ahhh...that explains it :-)

If they glow, this is the place to be
Post Reply
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#1 Ahhh...that explains it :-)

Post by Cressy Snr »

There is a thread on DIYAudio that really does give a good insight into why pentodes with plate-to-grid (Schade) feedback sound as wonderful as they do, when applied properly.
Now of course we know that schade fed back pentode/beam tetrode circuits cause the output stage to appear to take on triode characteristics, whilst still giving pentode output power, but the curve traces on this thread really do ram the point home.

http://www.diyaudio.com/forums/tubes-va ... ent-3.html

If you are at all interested in push pull pentode amps, this thread is a good read.

Posts 26 and 27 start off the curve traces done by "Smoking Amp" and for a 6550 user like me, post 27 showing a KT88
(pink trace) in Schade mode is particularly informative. Combine that trace with a Dr Gizmo inspired power supply and you have the reason why a SET fan wanting more power might like push-pull pentodes configured in this way.

Have a look. :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#2 Re: Ahhh...that explains it :-)

Post by Paul Barker »

That does explain what I heard at your house.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#3 Re: Ahhh...that explains it :-)

Post by Cressy Snr »

Yes it explains a lot.
Most of the sets of Schaded pentode curves put up by Smoking Amp do seem to show that Schaded pentodes can make better triodes than triodes :shock:

When I took my own amp to Steve's last month, we were talking about how these very old, pre-war ideas often seem to be the best ones, and that as time went on, things got progressively worse because of the quest for cheaper power.

Witnessing first-hand, those old wartime VT75 pentodes of Steve's, in push-pull, imitating the sound of SE PX4s, but with a power and solidity of which the PX4 could only dream, was, er...interesting for both of us. :wink: and those traces on DIYAudio, show, pretty convincingly, what was going on.

It's a shame that the full sonic potential of PP pentodes was never really realised in the post war world of mainstream hi-fi electronics
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#4 Re: Ahhh...that explains it :-)

Post by Nick »

When I took my own amp to Steve's last month, we were talking about how these very old, pre-war ideas often seem to be the best ones, and that as time went on, things got progressively worse because of the quest for cheaper power.
I think the ideas and understanding continued to improve during and after the war, but the requirement they were designing for changed. They didnt forget how to do things, they just did different things.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#5 Re: Ahhh...that explains it :-)

Post by Paul Barker »

I'm wandering how a hybrid of this would affect a 211 output stage. Have you ever thought of it Nick? I'm thinking more of the affect on impedance than linearising the curves, as those are good enough anyway. But what happens when the 211 inevitably goes into grid current?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#6 Re: Ahhh...that explains it :-)

Post by Nick »

I don;t know about the 211 case, but unless I am missing something, the main point is "negative feedback linearises an amplifier", I thought we all knew that?

I do something very similar in the LCR phone, use g2 on the pentode as a feedback point from a low impedance source. But IMHO, showing the curves only gets us so far, showing the transfer function as a spectra of distortion products would be more useful, you have to watch the higher order distortion and that wont be easy to see (if at all) on a curve tracer.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Post Reply