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Old 10th September 2012, 03:53 PM   #1
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Location: Graham, TX
Default Biasing Improved Williamson

I was wanting to know the best way to measure cathode current,would it be possible to insert a small value cathode resistor,and measure bias current,much like other Dynaco curcuits?
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Old 10th September 2012, 05:40 PM   #2
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Join Date: Aug 2008
I assembled this amplifier, its very good but quite hard to stabilize.
Having debugged it, I would suggest to do the following:

1) Install separate BIAS pots.
2) Install 10 Ohm cathode resistors in order to measure idle current.
3) Lift fake center tap of filament winding (2 x 100 Ohm resistors) to about 30V.
4) Install trim pot instead of fixed NFB resistor.

I've got over 60W with pair of 6550/KT88/KT120 (tested all 3 types of tubes), THD at 1KHz was something 0.15% - 0.25% for power up to 50W.

Pictures of breadboard PCB build attached.
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Old 10th September 2012, 10:04 PM   #3
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Nice juicy fat toroid in pic: who wound it ?
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Old 11th September 2012, 07:11 AM   #4
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There are many factories in Eastern Europe and Germany which can wind toroids up to your specification. Toroid on picture is audio output type, you have to submit technical specification incl. winding layout to get one.
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