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Old 3rd May 2021, 6:25 pm   #1
G6Tanuki
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Default Improving the 'Ultra-Linear' push-pull amplifier.

I've never really been a fan of Ultra-Linear push-pull; with many valves it imposes an unnaturally-low and indecent constraint on the HT-supply in order not to exceed the static maximum G2-voltage.

I instinctively think that for best linearity/power-output you need a nice high anode-voltage!

It occurred to me that by including a suitably-rated Zener-diode, or a classic "purple glow" 0D3 regulator-valve in series with each screen-grid-to-transformer-tap you could then beneficially wind the overall HT up by at least 150V.

OK, I know Zeners and glow-discharge tubes can be noise-generators - but a bit of parallel-capacitance should sort that. [Don't put too much parallel-capacitance across a glow-tube or it can become a relaxation oscillator].

What say you, good people? I like my 807s to have a least 600V on their anodes!
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Old 3rd May 2021, 8:35 pm   #2
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

How about capacitively coupling the ultra linear tap to a resistor fed screen grid, DC conditions ensured, AC as per ultra tap?

edit: The resistor feed could be the anode.
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Last edited by merlinmaxwell; 3rd May 2021 at 8:38 pm. Reason: had a thought
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Old 3rd May 2021, 8:57 pm   #3
PYE 405
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Certainly worthy of experimentation, it would be interesting to see if the screen DC voltage can successfully be made independant of the AC signal with stable results. I have a feeling there could be trouble with unwanted phase shifts introduced with capacitive coupling.
As you probably well know, Peter Walker had an alternative solution back in 1952 with the Quad II output stage.
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Old 3rd May 2021, 9:16 pm   #4
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Another option is to include a resistor in the screen feed, this has certainly been done before to reduce screen voltage/current.
A zener or voltage stabilizer would play havoc with the AC signal.
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Old 3rd May 2021, 9:43 pm   #5
qualityten
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

As in the Mullard 5-10 and 5-20 UL circuits?
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Old 3rd May 2021, 9:48 pm   #6
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Indeed so... the 5-20 had, I believe, a value of 1K connected to the screen's.
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Old 3rd May 2021, 10:36 pm   #7
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by PYE 405 View Post
... I have a feeling there could be trouble with unwanted phase shifts introduced with capacitive coupling ...
Presumably the capacitor would have to be large enough that it would not discharge or recharge significantly even during the lowest-frequency audio cycles. In that case it would behave like a constant voltage source and the phase shifts would be negligible. I'd be more concerned about losing the main feature of ultralinear operation i.e. that it can lower the distortion to a level close to that produced by a triode. True triode operation comes literally from having the screen and anode at the same voltage i.e. wiring them together. If we arrange a large DC (i.e. fixed) voltage difference between them will the AC behaviour of the device still look like that of a triode ?

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Old 3rd May 2021, 10:50 pm   #8
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

The possibility of having the anode and screen at different voltages was covered in the original Hafler and Keroes ultralinear article, and in the associate patent. It was done by having a separate transformer winding for the screen grid.

Click image for larger version

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In fact, it looks as if this were the primary approach to ultralinear, with the use of a transformer tap for the screen grid viewed as being a simplified approach that incurred the need for a common HT voltage.

The original Audio Engineering article and the patent are attached.

Cheers,
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File Type: pdf Hafler & Keroes Audio Engineering 195111.pdf (1.09 MB, 12 views)
File Type: pdf US2710312.pdf (568.3 KB, 12 views)
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Old 3rd May 2021, 11:03 pm   #9
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Blumlein also covered the possibility of using different anode and screen voltages in his original distributed loading patent, albeit by different means.

Click image for larger version

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Blumlein’s work precede that of Hafler and Keroes, although the latter appear not to have acknowledged it.


Cheers,
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File Type: pdf GB496883A.pdf (460.8 KB, 15 views)
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Old 3rd May 2021, 11:42 pm   #10
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by PYE 405 View Post
As you probably well know, Peter Walker had an alternative solution back in 1952 with the Quad II output stage.
First use of the Quad distributed loading circuit was in the Acoustical M31 and MB31 PA amplifiers of 1945:

Click image for larger version

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Although it was noted that a patent had been applied for, so far, I have been unable to establish that a patent was actually issued.

The first “hi-fi” use was in the Acoustical QA12 amplifier of 1947:

Click image for larger version

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Quad claimed that its circuit allowed the use of different anode and screen voltages without the complication of separate transformer windings. That claim was made in the third page of the attached article.


Cheers,
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File Type: pdf Amplifiers and Superlatives WW 195209.pdf (190.1 KB, 16 views)
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Old 4th May 2021, 2:29 am   #11
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

I agree with the idea of higher plate voltages making a "better" amplifier. In a recent discussion on 807's (another thread) we "all" agreed that the voltage rating of the 807 was officially upgraded to include the origional williamson design, where it was used as a triode strapped tetrode. I have been using 807's and now some of the variants ( CV428/5B/254 ) for many years and have found them to be like the origional 807's. Essentially indestructable.
( Read between the lines there, and do do the math)
My own attempt at plagiarizing Joseph Marshalls Goldenears amplifier will be my test!. The amp is relatively complex and expensive enough without throwing extra power supplies or screen windings into the argument.
Larger plate voltages also means more expensive filtering components, regardless of what type of filter you require or design. Running 600 volts DC onto the plates would need at least 750 volt rated capacitors, be they electrolytics or PIO's. Better insulation at least for ground shorts means wider thicker insulation for chokes, and for that matter even the HT windings on the power transformer, especially if you use vacuum rectification. At the end of the day it is like everything else ever designed!
1. Today, as we know the number one factor is profit. Expensive doesnt sell well ( mostly) so profit is down.
2> Longevity, or how much warranty do I provide, especially in todays world of "plug'n'play". There are a few other transformer winders on the forum that will attest that making very nice output transformers wont win the lottery, OR let you sleep well!. A really well designed transformer for a specific, exact job can take quite a few hours and the "software" thats blowing around at present is not really best design, more like best practice, taking into account point one. Finding components like onion paper is IDEAL thicknesses is getting very difficult if not impossible. Whatever happened to 7" thou laminations in SiGO steel stampings? 14" thou is available everywhere for cheap, but doesnt make "very good" output transformers. About the best they can achieve is 100KhZ WITH feedback.

I am going to run eight CV428/5B/254 in my amplifier and I will post my build and the end result..

I do have a quick question though G6, WHY do you need at least 75 watts to begin with? A single pair of 807's can do that easily. Forget the slight red plating, thats perfectly normal for 807's.

Just my take.

Regards
Joe
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Old 4th May 2021, 7:26 am   #12
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by Synchrodyne View Post
The first “hi-fi” use was in the Acoustical QA12 amplifier of 1947:

Attachment 233215
Quad claimed that its circuit allowed the use of different anode and screen voltages without the complication of separate transformer windings. That claim was made in the third page of the attached article.
That's a wonderful diagram, Synchrodyne, and really gives the game away on Walker's sectional transformer winding scheme.

David
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Old 4th May 2021, 7:57 am   #13
G6Tanuki
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by joebog1 View Post
I do have a quick question though G6, WHY do you need at least 75 watts to begin with? A single pair of 807's can do that easily. Forget the slight red plating, thats perfectly normal for 807's.

Just my take.

Regards
Joe
It'd be to anode-modulate a single 813 RF amplifier.

I'm weighing-up the merits and demerits of different modulator designs; a triode-connected [grids strapped together] pair of 807s in zero-bias Class-B as recommended by RCA is the other alternative under consideration.

I have a nice big Woden multi-ratio transformer which would allow 'ultra-linear' wiring, and that led to my strange idea. Ultra-linear operation would have the advantage of giving greater tolerance-of-error with the impedance-ratios available.
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Old 4th May 2021, 8:11 am   #14
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Bailey and Radford made some interesting improvements at the phase splitter stage. Hi Fi News ca 1961 had details I think. See also http://www.audiomisc.co.uk/HFN/Radfo.../circuits.html
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Old 4th May 2021, 9:04 am   #15
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

I thought the thread starter would encourage interesting input and it certainly has !
I will continue to read with keen interest and just to add more to the mix, how about the Circlotron amplifier? https://www.angelfire.com/electronic...irclotron.html
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Old 4th May 2021, 9:32 am   #16
Robert Gribnau
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

There are ofcourse output transformers with dedicated seperate windings for ultra-linear operation, like the ones on this page having "SSCR" in their designation: https://www.mennovanderveen.nl/cms/i...ten/specialist
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Old 4th May 2021, 9:47 am   #17
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Oh dear Andrew !!! The circlotron is fraught with problems!!. Wind a power transformer for a start!!.
I have made one only. I had to wind it on gate laminations, which have twice the length of normal laminations for the centre and outside legs. What that acheives is twice the winding length for the bobbin.
Regulation is of SUPER importance, because if one supply sags by even a small amount the output bottles will melt almost instantly. My attempt was just a puny pair of ( wonderful ) 6V6's. Its a pity that TRUE 8 ohms speakers are not and never will be available.

To G6, well, as modulators it might make sense. The modulator generally runs 100% modulation. so distortion with your real class B doesnt matter. I mostly do audio Hi-Fi, especially in my dotage, so class B is out. AB1 is nice because output power versus distortion and efficiency is quite good. NICE ultra linear transformers work well and can achieve good efficiency. As I am not seeking to modulate the other side of the world, I stay with UL designs.

Joe
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Old 4th May 2021, 10:05 am   #18
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Be a little careful of Menno's designs. I bought his book when it was released and "played" with a few ideas he presented. Toroidal output transformers are ten times harder to design than EI types. Yes, you get better coupling, but worse interwinding capacitance. Fix the capacitance and the coupling goes out the door. Toroids are very nice things, originally a spin off of double C core transformers. Double C core still require bobbins that you can section the windings fairly easily. Almost impossible with a toroid as one winding is basically stacked on top of the last, ( or next ). I tried winding a few simple ones, but ran into big problems. Yes I am not Menno Van Der Veen, but my EI designs beat the frequency response and efficiency of a toroid on any given day. Mind you they are more than twice the size. I run VERY conservative flux densities with my EI designs which is almost not a problem with toroids, nor is coupling that big a factor with toroids. BUT!!! interwinding capacity is a major drawback because vertical sectioning on a toroid reduces coupling by a huge percentage.

If you can't follow my discussion, please read up on your transformer theory.

With respect

Joe
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Old 4th May 2021, 11:25 am   #19
PYE 405
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Hi Joe, yes indeed the circlotron is a scary design and best avoided. I threw it in just for interest, but UL is the proven option adopted by most HiFi designers of yore.
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Old 4th May 2021, 11:44 am   #20
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

It depends why you want UL, after all.

Resistive/capacitive coupling to UL taps starts to negate one of the benefits of UL, that is, that the screen current contributes to power output in addition to anode current.

I do like the separate windings for the screens - with care, and provided the voltage separation is not too great, it should be possible to wind bifilar with the appropriate portion of anode winding.

Quote:
Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by Synchrodyne View Post
The first “hi-fi” use was in the Acoustical QA12 amplifier of 1947:

Attachment 233215
Quad claimed that its circuit allowed the use of different anode and screen voltages without the complication of separate transformer windings. That claim was made in the third page of the attached article.
That's a wonderful diagram, Synchrodyne, and really gives the game away on Walker's sectional transformer winding scheme.
It's the sort of diagram that makes my eyes go funny! Great for understanding the transformer construction, but not so obvious how the circuit works!
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