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some Mk3.3 changes

Posted by Ralph on April 19, 2016 at 09:35:51:
The little circuit board is a 2-stage CCS circuit, replacing the 6SN7 2-stage circuit of older amps. It has better performance (although the older CCS performance was excellent); because the MOSFETs have so much more gain, they really can perform better in a CCS circuit. The circuit has the added benefit of really preventing any chance of filament/cathode arc-over in the voltage amplifier circuit although that really wasn't a serious problem with 6SN7s in general.

This freed up a tube socket, which is in this case paralleled with the original 'V1' tube sections, to allow the bottom portion of the differential cascode to better drive the top portion. This yields lower distortion and slightly more headroom out of the voltage amplifier.

The other device, the bridge with the caps on it, is for blocking any DC component that may occur on the AC power line due to electric heaters and the like.