UL for EL84. 22-24% is better than the dogma....40% is lowest distortion.... - Page 2 - diyAudio
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Old 31st March 2011, 08:54 PM   #11
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Triode mode and UL lose power capability in the output stage due to the feedback, and both suffer from plate or screen grid feedback not tracking grid1 non-linearity exactly. (triodes have the same non-linear issue as screen feedback since it is g1 gm that suffers the island effect, not g2 or plate) The best approach would appear to be global feedback around the OT up till the top of the audio band, then a frequency crossover to a primary side feedback (still to the input stage) to avoid the OT frequency limitations.

Several ways to implement that, but easiest is probably just using an inner loop that kicks in at 20 KHz or above (so greatly reducing the outer loop gain as a result). Putting that freq. crossover lower (in-band) would require careful design to prevent phase changes at the output within the freq. transition region, but the low performance/ low cost OTs around may require this.
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Old 1st April 2011, 04:34 AM   #12
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Here we go. Output transformer physics is the business end of tube amps.
For those who are looking for trouble, here it is in plenty.

Frequency distortion: Caused by too small prim L, too much leakage induct or resonant effects. These days not so much a problem.
Phase distortion: Caused by differences in phase when global nfb is used. Symptoms, HF parasitic oscillation caused by combination of phase differences from both high leakage inductance & high self capacitance leakage. Most amps Iíve seen have not been correctly optimised for this ! The trick is to design the closed loop response for equal stability at both frequency extremes. Too high global feedback generally makes this harder to achieve.
IM and THD: In o/p stage when primary L is too low causing overloading at low frequencies, this creates non linearity between I mag and B flux in the transformer core. Cure, for standard transformer sheet keep Bmax value low. This implies high Ae.
THD: Too high DC resistance of primary. Problem; power transfer load curve efficiency drops.

So generally the design should be aimed at,
High Prim L
Low leakage inductance and capacitance, this is the most serious conflict nub in modern day UL.
Peak Bmax in core allowed for.
Efficiency. Low losses in windings and core.
Matching of windings.
Sum up;, The resonant frequency caused by leakage L & C's determines the quality of the transformer.

There is alot more to all this; By coincidence, in a Williamson, the optimum closed loop Q or transient response damping occurs when o/p trans LF F2 THD occurs at 1.4% for a given F3 cutoff freq v.s core size and has a marked influence on sound quality. This depends on the type of transformer iron/alloy used. This implies the coupling cap values from the concertina and the o/p stage inputs should be properly calculated and not guessed at !
The Radiotron Handbook hasn't discovered the importance of the closed loop issue in detail, although the transformer math is physics clear. For coupling caps, see Morgan Jones, Valve amps 4th ed, p-83, p-190,p-585.

richy
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Old 1st April 2011, 12:01 PM   #13
tinitus is offline tinitus  Europe
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hey, generally speaking, how much does UL coupling change tube load resistance ?
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Old 1st April 2011, 01:04 PM   #14
tinitus is offline tinitus  Europe
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why 40% UL tap ?

well, maybe just a natural consequence of trafo construction
caused by the shorter inner layer

22-30% UL tap may need a slightly different construction
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Old 1st April 2011, 01:29 PM   #15
Yvesm is offline Yvesm  France
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The "canonical" 43% is only the result of the winding scheme.
If each half primary is constitued with 7 layers and the screen tap is done at the 3rd, then 3/7 = 42,857%

That's how legend spawn

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Old 1st April 2011, 05:55 PM   #16
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That's why when order from Edcor you always get PP trannies with screen grid taps, even if it is 10K 100W or above.

(Imagine 1000V on screen grids and smile)
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Old 3rd April 2011, 07:36 PM   #17
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Yvesm View Post
The "canonical" 43% is only the result of the winding scheme.

Yves.
hey, I just read a claim that the amount of UL percentage is related to the physical arrangement inside the tube
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Old 3rd April 2011, 08:07 PM   #18
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Quote:
Originally Posted by tinitus View Post
hey, I just read a claim that the amount of UL percentage is related to the physical arrangement inside the tube
It is called pseudo-science. You may create any pseudo-scientific theory relating almost anything.
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Old 3rd April 2011, 09:04 PM   #19
tinitus is offline tinitus  Europe
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yeah, I was a bit 'reluctant'
ultra-linear

also read original paper by Williamson, from 1953 or so
typical, can't find it now
he sure didn't like ultra linear very much
looked to me like he connected the UL tap to the catode
and cross referenced the PP plates
ahead of his time ?

and found this
http://www.angelfire.com/electronic/...L6_Config.html

edit, well, this would be a Williamson schematic, yes ?
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Old 3rd April 2011, 09:08 PM   #20
almarcy is offline almarcy  United States
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Default dogma, pragma and kharma

I am now listening to P-P EL34.

The factory had used UL and fixed bias. I converted it to triode and cathode bias with Blumlien Garters. NFB network disabled.

The same OPT transformers now provide wider bandwidth and less distortion.

There is less power. Oh, the humiliation

I only use amplifiers to listen to Music. OK, and sound effects and test tones. I am not a purist.

I like to hear more of the Music. One way is to hear less of the amplifier.

Blumlien Garters automagically set the operating points of the opposing tubes to a point halfway between the normal for each, in this circuit. This minimizes the stress on each tube.

The currents through the two halves of the OPT primary are balanced at idle. This minimizes the magnetic distortion caused by unequal currents at idle. The OPT now does less when idle.

It was interesting to find that getting the amp to do silence better improved the sound. The sound continues to improve. The magnetic distortion done by unequal idle currents does not all vanish at once. Instant gratification is not the only kind.

Part of engineering is understanding how the system works and what can be done to improve the part I want, the sound. YMMV ;*)

Happy Ears!

Al

PS I can swap the output tubes to the opposite sides of the OPT and they still are balanced. Try that with fixed bias. For grins, I put an EL34 on one side and a 6550 on the other. Still balanced. Awesome.
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