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In Reply to: Build ideas for Allen Wright PP-1C Push Pull Power Amp posted by Burney on April 15, 2007 at 06:39:37:
A couple of quick points.
1) Those 330K Ohm Rg1 resistors on the output tube grids is something you'd need to watch. They may need to be lowered in order to use other tubes (cetainly would have to be lower for 6550). If you try to go fixed bias then they are even too high for EL34 (Max Rg1 values are always x 2 or x 2.5 lower for fixed bias than for cathode bias). I suggest you stay away from fixed bias in this case.
2) Gain can easily be adjusted. The gain of this circuit is set by those 470 Ohm resistors in the diffamp cathodes. Lower resistance means less degeneration of gm which in turn means higher gain. (gain of a cascode is approximately equal to the gm of the lower tube x RL at the top tube anode).
Cheers,
Ian
Follow Ups:
Thanks Gingertube
I am a happy builder of your Baby Huey amp.
1.Yes im aware of the rg1 limitations. Just wondering if going FB given any advantage
2. What could be the trade off in jacking up the gain?
Thanks Ian.
I am afraid my speakers are not too good at the moment. i will surely try and avoid NFB but its just an option that i like to retain.What should me the right way to apply some feedback? Is it a good idea to tap feedback from the anode of output pair? And just where should it be injected?
Burney,
Jacking up the gain will have some impact on the linearity and balance of the diff amp. I'd really doubt if you'd notice it. In fact you may find that the slightly increased, mostly second harmonic, distortion is something you like.
Unless you are seriously woried about Output Impedance I would not be doing it just to apply some global feedback. That would introduce other problems and you may then need stabilising componets (zobel networks across the anode loads).
Cheers,
Ian
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