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Old 27th December 2015, 08:47 AM   #21
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Stan White wrote about the flaws of the Williamson amplifier here, which was a re-hash of his POWRTRON article published in Audio in 1953. Some of the claims were later refuted by Patrick Turner on R.A.T.

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Old 27th December 2015, 09:30 AM   #22
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Turner's comments are interesting, but he is utterly wrong about the cathodyne phase splitter, as was White. As I experimentally demonstrated, Turner's "fix" causes a perfectly balanced circuit to unbalance badly.
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Old 27th December 2015, 09:48 AM   #23
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Turner made a clarification on the CPI's balance later in the thread by saying:

"The Miller capacitance seen by the anode or the cathode is virtually the same to certain HF, and while the load stays the same value, including the capacitance component, the balance of the CPI also stays substantially the same. However, in practice, the stray capacitance seen by the anode or the cathode is often different, therefore there is a tendency for the anode's response to sag before the cathode response, and as I have been saying for years, to make the sag in the balanced response for both outputs above 20 kHz, all you need to do is to add a little capacitor across the cathode resistor. I mentioned 15pF in my last post; but it may be too large - the actual value has to be determined experimentally."

So I guess to be generous and not beat on a dead-horse further, we could interpret Turner's comment as - if all else being equal, then the cathode compensating capacitor would not be needed at all, which mates well with your experimental results.
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Old 27th December 2015, 09:55 AM   #24
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I will bet a paycheck that Turner made the same mistake as many other people did when measuring the cathodyne- only using a single probe. Do it properly (probes on cathode and plate simultaneously) and the "imbalance" disappears.
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Old 27th December 2015, 10:07 AM   #25
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Perhaps, even though I could not find a reference on how he tested the CPI despite his voluminous writing, and it is also hard to imagine the stray capacitances being that different in well-constructed amplifiers, then again, I have not built enough amplifiers like Turner to contradict his finding.

But the point about using two identical probes when measuring CPI has been drilled into my head thanks to your revealing article, so nowdays, any claim about CPI imbalance is automatically viewed with suspicion.

p.s. thank you (and the other Mods) for getting rid of coinmaster, what a pest...

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Old 11th February 2016, 07:52 AM   #26
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I just put up a doc that tries to collate design details for the Williamson amp, and make a listing of the changes applied to the circuit over the decades.

I didn't envisage it would end up over 10 pages long, as the aim was not to wax lyrical over particular pros/cons or changes, but rather to note down the known information and keep comments curt and to the point.

I'd be happy to correct any errors and add any missed items, as I had to scrounge around my limited technical library and the internet for appropriate references.

http://dalmura.com.au/projects/Willi...ign%20info.pdf
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Old 11th February 2016, 08:30 AM   #27
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Quote:
Originally Posted by trobbins View Post
I just put up a doc that tries to collate design details for the Williamson amp, and make a listing of the changes applied to the circuit over the decades.
A great summary of the Williamson! I only skimmed through it since I am traveling at the moment... but I do have a question on the step network calculation - on p.2, A' for the 6SN7 is shown as 14.3, while on p.5, A' is shown as 10, shouldn't it be 14.3-3 = 11.3? Ditto for the HF re A' ~ 6.5...

Also the calculated frequency response (HF ~52kHz) of the Partridge OPT, seemed to differ quite a bit from the factory spec and measured examples, which is closer to 100kHz.

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Old 11th February 2016, 11:46 AM   #28
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Sorry for ambiguity Jazbo. I'll try and tighten up the meaning and symbols used.

The first stage V1 internal resistance, with closed loop operation, is I think going to be influenced by the unbypassed cathode resistance. So the voltage gain of that stage is going to be as per the mid-band stage gain A' ~ x14.3 (23dB).

The mid-band gain is for R3=47k, and there is no other circuitry loading on the anode.

At some frequency, the step network would reduce the effective plate load R3' down to 19k impedance, for which the gain will fall by 3dB, from x14 to x10 (20dB), which is at about 25kHz.

At frequencies beyond about 185kHz, the stage gain will plateau to x6.5 (16dB), making the total gain step about -7dB. The references indicate what that level of step means with respect to phase change and roll-off within the region of the step. Williamson doesn't go in to detail about the step performance, and Cooper (1950) indicates it is more than 9dB - I'll have another look at Coopers assumptions when I get a chance.

The WWFB datasheets indicate a measured leakage inductance and shunt capacitance levels, which I calculated out a range of simple LC resonance at between 47-58kHz, and then took a middle position as being indicative. Williamson indicated his sample gave a resonance at 60kHz, but unsure if that was diy or WWFB prototype. I have one of the WWFB/0.95, so at some stage I was going to try and recreate the factory test conditions.

http://dalmura.com.au/projects/Partr...datasheets.pdf

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Old 11th February 2016, 02:18 PM   #29
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Quote:
Originally Posted by trobbins View Post
At some frequency, the step network would reduce the effective plate load R3' down to 19k impedance, for which the gain will fall by 3dB, from x14 to x10 (20dB), which is at about 25kHz.
I see where I went wrong now, it's not "14.3-3", I mixed up the multiples with the decibels!

Looking forward to see more data on the Patridge WWFB's, even though they are basically unobtainium these days...
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Old 12th February 2016, 06:27 AM   #30
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I am in a quandary about the leakage inductance and shunt capacitance related parameters that Partridge used for their WWFB and CFB output transformer specs, and how they relate to the equivalent circuit models offered up to assist with estimating high frequency performance in a class A PP output stage.

In particular, Partridge refer to a half-primary leakage inductance, which I was planning to use along with the half-primary related shunt capacitance, but they also refer to a “leakage inductance measured as a series element in the primary”. The concern is that the CFB datasheet indicates the WWFB has 900mH of half-primary leakage inductance, whereas the WWFB datasheet makes no reference to that parameter.

I just updated the on-line design doc, outlining that quandary in Section 4(a), as well as cleaning up a few bits and pieces.

Last edited by trobbins; 12th February 2016 at 06:30 AM.
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