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Old 12th February 2016, 07:05 AM   #31
jazbo8 is offline jazbo8
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I have read that the CFB has a worse HF performance than the WWFB, can't remember where now... perhaps it was in the Naval Research report? Where are my files when I need them?!
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Old 12th February 2016, 07:14 AM   #32
trobbins is offline trobbins  Australia
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Yes the Naval report showed poor CFB response, with no tangible grasp as to why. The report indicates they spent time checking what feedback circuit changes could influence the particular transformer they had swapped in, so it seems strange for such a different response performance.

That report certainly 'went for broke' on the WWFB, reporting output response to 1MHz !!

The stock Williamson amp they used for OT comparisons was subtly different to the original circuit, but I couldn't identify anything that should interact with the OT to such a degree.

http://www.dtic.mil/dtic/tr/fulltext/u2/011593.pdf
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Old 12th February 2016, 11:24 AM   #33
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Quote:
Originally Posted by trobbins View Post
The stock Williamson amp they used for OT comparisons was subtly different to the original circuit, but I couldn't identify anything that should interact with the OT to such a degree.
That is indeed very strange... Also I could not find another test/report that could collaborate with Navy Research's finding.
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Old 18th September 2019, 07:06 PM   #34
PB2 is offline PB2  United States
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Quote:
Originally Posted by Bas Horneman View Post
Did find something with the title you mentioned. However the author is different.

http://www.mif.pg.gda.pl/homepages/j...il/willimp.pdf
My dad built the W3M Heathkit Williamson amp in the 1950s and by the 1960s when I was a kid the amp motorboated badly. A friend of the family also built a similar Heathkit and his did it also. I worked with my dad mostly watching as he tried to fix it, he'd swap tubes and coupling caps but it would always come back. I was certain that there was something marginal with the design. I also measured the output power in UL mode and only got about 12W before clipping. The article linked above by Wright explains that when the design was brought to the US similar tubes were substituted without adjusting the operating points and by rebiasing them he is able to get much more power. The US Heathkit version used 6SN7 drivers and 5881 outputs. He also points out that more power supply decoupling was needed to eliminate the motorboating. Mystery solved, I've not tested the mods but Wright seems to have debugged the main issues with the US design.
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Old 18th September 2019, 08:52 PM   #35
trobbins is offline trobbins  Australia
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The article by Wright has a number of innacuracies and unsupported assertions - I would suggest caution rather than blanket acceptance.
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Old 18th September 2019, 10:24 PM   #36
peteki is offline peteki  Australia
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For any prospective builders May I suggest you read about the original design by DTN. Motor boating can occur and the original operating point of the driver 6SN7 leads to overload near full output; my comments are prompted by personal experience some decades ago. There are solutions and its worth perusing the (copious) literature.
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Old 19th September 2019, 03:53 AM   #37
trobbins is offline trobbins  Australia
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peteki, the original operating point of the driver 6SN7 has each anode sitting about 160V which allows sufficient swing for KT66's.

I tested Wright's proposed bias change on the driver stage using a 860 ohm common cathode resistance, and noted that 2nd and 3rd harmonics at 25Vrms output signal level on each anode were worse than when using DTN's original 390 ohm.
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Old 19th September 2019, 04:22 AM   #38
grovergardner is offline grovergardner  United States
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The original W-3M was indeed a disaster, as Dave Gillespie documented in his "From the Frying Pan Into the Fire" thread over at Audiokarma. It was quickly replaced by the W-3AM which was a far more stable design.

Having spent a year building a reconstruction of the original "Musician's Amplifier," it's possible to achieve decent stability without going through hoops, provided the output transformer is up to the job, which the Peerless S-265-Q certainly is. When you take shortcuts like Heathkit did you run into trouble.

I have tried those Wright mods and frankly, they sound awful. They drain the amp of it's character and musicality. The only sensible (and IMO musically sound) changes I've made to the MA circuit are to 1) increase the value of the first pair of coupling caps and reduce the value of the second pair, to 1uF and .1uF respectively, to reduce the chances of LF instability, 2) add a stock Williamson shelf network across the first stage plate resistor to improve HF stability, and 3) increase the driver stage cathode resistor to 560 or even 680 ohms. These were all sensible improvements made over the course of the early '50s by Keroes, Hafler and others. There were other changes as well, but those are more dependent on the specific components and tubes used.

If you still have that W-3M and want to get it working properly and turn it into an enjoyable piece, head over to Audiokarma and read Dave's "A New Beginning" thread.

Last edited by grovergardner; 19th September 2019 at 04:26 AM.
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