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807 williamson amp with Hammond 1645
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6339
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Author:  alahti [ 14 Sep 2018, 03:56 ]
Post subject:  807 williamson amp with Hammond 1645

Hello,

I have years used williamson amp
Attachment:
amp.jpg
with my 8 ohm speakers. Now I have built a 16 ohm speakers and switched the secondary winding
Attachment:
Screenshot_2.png
to match it but getting terrible noise from my amp. What could be the problem in the connection? Some thoughts was that negative feedback should be connected to other output. I have connected to 16 ohm output.
Using 6SN7GT preamp tubes and 807 power tubes and HAMMOND 1645
Attachment:
Screenshot_1.png
output transformer.
http://www.hammondmfg.com/pdf/5c0051-52.pdf

Author:  mike567 [ 14 Sep 2018, 12:44 ]
Post subject:  Re: 807 williamson amp with Hammond 1645

Your negative feedback became positive. recheck the polarity of your secondary connections

Author:  alahti [ 17 Sep 2018, 23:50 ]
Post subject:  Re: 807 williamson amp with Hammond 1645

mike567 wrote:
Your negative feedback became positive. recheck the polarity of your secondary connections

It has 22K in negative feedback and I increased it to 56K, so now its all ok. Do anybody know how to choose correct one?
Attachment:
amp.jpg

Author:  Pim Scheffers [ 25 Dec 2018, 06:22 ]
Post subject:  Re: 807 williamson amp with Hammond 1645

When looking at a few articles I would say (sqrt(8) * 4400) = 12k.

Attachment:
original_williamson.PNG

Attachment:
ul_williamson.jpg


Williamson had a cathode bias resistor of 470 ohms on the first stage and used triodes in the output stage.
Reading the (UL-ing the Williamson) article I conclude you must double the feedback resistor to compensate for the UL stage (instead of triode) and your schematic has a 1k resistor in the cathode of the input stage so you have to double it again.

Williamson calculated a total gain of 1200 (yours will be slightly lower ~ 1100) in triode output.

So I calculate: sqrt(8) = 2.82 * 4400 = 12k resistor

Your instability may be fixed by placing a 10k grid stopper on the first tube and a small RC filter across the 68k resistor of the first tube (roughly 4.7k --> 220pf)
Just my take :D

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