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RE: Thank you.

75.168.123.245

In Reply to: RE: Thank you. posted by Tre' on October 18, 2024 at 15:49:47

Did you intend to imply that a 6l6 in UL could have plate curves with the grid lines evenly spaced along a proper load line and be usable without feedback if only the screen taps were in the right place?

That is what I thought you were saying.

When you said "....misplace the tap slightly, resulting in less than ideal operation, which was then 'cleaned up' by feedback." did you mean to say that placing the tap in the right stop would allow "ideal operation" and there would be nothing to "clean up" with feedback?

That is what I thought you said.

All I am saying is that, no matter where the UL taps are placed, the plate curves will not become linear enough. A 6l6, in UL, single ended, will still need to be "cleaned up" with feedback.

That is indeed what I was saying up to your last paragraph above. If you look at the article by Langford-Smith about UL operation, you see the distortion quite a bit lower than even a triode (see pages 57 and 58). How that works exactly when single ended I can't say with precision, but having built such amps (although not using a 6L6) they seemed to sound good without feedback so I suspect the principle works with single-ended in a manner as described by the patent.

So what I am saying is you might be pleasantly surprised to find out how good a 6L6 can actually sound when operated UL single-ended, in particular if the tap is optimized. You might try it and see!

Follow Ups:

RE: Thank you. - tube wrangler 22:59:14 10/27/24 (1)
I have tried it, but it was necessary to mentally
discard a lot of linearity theory-- not to eliminate that,
but instead to explore the whole "could be" SE Pentode
amplifier as a new set of sonic opportunities in itself-.

I adopted an attitude where one imagines what will
sound the most accurate, instead of relying too much
on what calculates as the most "accurate".

I have put that word "accurate" in quotation marks because the
human ear is decidedly not linear, but even more interesting is
the fact that no two humans hear even remotely alike anyway.

Building a great audio amplifier is an art, but
included in that must also be total reliability and
long component life, including the tubes.

I am currrently amazed at what the "premium" Hammond SEA
series of SE output transformers can do-- did their engineer
just get the taps (for ultralinear) right-- maybe it's
a special model that they got right_deliberately, or
is this one item just a piece of good luck? Or do we
have some excellent metals and winding techniques here,
or is it just what they say-- a "premium" piece? Who knows?
I do know that this one model works musically, like the
best transformers out there.

One could argue over that for decades, or he could
instead learn how to use the results to his sonic advantage.

That one can build a GREAT SE amplifier using a 6L6GC,
run in transformer tapped G2 mode is a fact as of this writing.

As many of you probably know, I used DHTs for many a
year, and won more than a few audio shows with those.

They are certainly low distortion and linear.

I just don't listen to DHTs anymore. The pentode SE
simply trashes them in the ways that count the most--
having more of the musical artifacts fully there and
fully fleshed-out during the actual enjoyment of the music.

These are conservatively run Zero feedback SE Pentode amplifiers.

Personal preference? Fidelity trumps theory and
calculation? It's not that simple-- it's more like
having ALL of the musical expressions there ALL of
the time. In other words, it's a better WORKHORSE.

Do any of you have good explanations for this? I
would love to hear those.

Thanks, Guys!

--Dennis--
- If you read the UL patent I think it says why :) nt - Ralph 11:35:44 10/28/24 (0)
  -
RE: Thank you. - Tre' 18:15:29 10/19/24 (1)
I will have to look into that. What I have read says that a 0% tap is pentode and a 100% tap is triode. In triode tubes are more linear than in pentode and UL is operation somewhere between pentode and triode so I always assumed that the linearity of the tube in UL would be somewhere in between pentode and triode.

If what you are saying is true, then why did Hafler include feedback here?

Push pull cancels the even harmonics but single ended doesn't cancel anything. I would think feedback would be required more in a single ended amp than in a push pull amp?

One other thing. Langford-Smith also said this.

"distortion quite a bit lower than even a triode"

So maybe he was talking about the amplifier as a whole (including GNF) not the linearity of the tube (in UL) itself?

I say that because at 43% the tube itself is not very linear at all so even if 43% is a little off the tube should still be very linear in UL if it is the tube itself that is "quite a bit lower than even a triode" when used in UL. Again, he said the tapping point is not at all critical but the 43% tap UL 6l6 is not even close to the triode 6l6 in terms of linearity.

Can you understand my logic?

Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
- I understand your logic - Ralph 11:11:57 10/28/24 (0)
  Actually working with the circuit and its variables is likely to bring a greater understanding.
  
  Without feedback the distortion of the amp rises with the output power. So if you want the amp to be neutral, the usable amount of power goes down. With SETs this is typically about 20-25% of full power at clipping. This is why Hafler used feedback.
  
  The other thing to understand about feedback is what happens to the output impedance of the amp. When EV and MacIntosh were introducing the Voltage rules for loudspeaker drive, the get the desired Voltage source effect out of the amplifier, feedback was a requirement for any tube amp. So if you want your tube amp to be compatible with speakers designed under the Voltage rules (these days that's about 99% of speakers made) then you had to have feedback. This would allow the amp to cut back its power into impedance peaks, such as the one in most box speakers at low frequencies, in order to obtain flat FR.
  
  The trick with feedback is proper application. Outside of the audio world its known as 'control theory' and the rules of use are well understood.
  
  Its often not applied properly in audio. Crowhurst and Baxandall both pointed to problems associated with linearity issues at the feedback node but neither really offered a good solution, although it was just right there all along. The trick is to wrap it around the amp in the same manner as seen in an opamp, since any amplifier is a primitive opamp anyway. IOW it should be applied to a grid rather than a cathode so as to prevent the feedback signal from being distorted by the tube.
  
  I happen to not agree with Langford Smith as the UL patent says something else- I go with that. Langford Smith was referring to a zero feedback circuit; look at the rest of the article as he discloses his test procedure.

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