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RE: Power amplifier

Posted by Eli Duttman (A) on October 29, 2007 at 17:31:01

In Reply to: RE: Power amplifier posted by Zefiro on October 29, 2007 at 12:18:20:

Now things make more sense.

Unless SUPERIOR quality O/P trafos are used, stability can be a MAJOR issue in the Williamson circuit. Please keep in mind that 6SN7 sections and GLORIOUS Partridge "iron" were used in the original.

All triode and no loop NFB can work well. However, I think you may be selling yourself short by excluding loop NFB and UL "finals". Your Chinese O/P trafos just might benefit from a little linearization. The Mullard topology (schematic attached) has fewer issues with phase shifts than Williamson topology has. So, the risk of instability is reduced. There is way too much gain in Mullard's complement. You will do quite well with 1 section of a 12BH7 in the voltage gain position and a 12AT7 in the LTP position. Wire 1 channel to use system 1 of the 12BH7 and the 2nd channel to use system 2. All the sockets are occupied and the total heater draw of the amp will be unchanged. When the voltage gain tubes show wear, simply exchange them between the 2 channels.

Oh yeah, you don't have to use Mullard's cathode bias scheme. "Fixed" bias WORKS in that style of circuit.

Eli D.

Edits: 10/29/07

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