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Posted by Naz on July 14, 2006 at 17:05:50:
Pieter, but the results for the two amps were predictably different. The gain and BW of the 300B amp reduced although not significantly because the driver was a conventional grounded cathode stage. By reducing the idle current the tube works in the less linear and lower gain region of the curve. Rp also increases and, depending on the circuit, that could affect BW.
The 845 was my own design using a CCS 6H8P driving a 6H30P White CF driving the grid of the 845. Because the WCF has a little under unity gain and does not invert I varied the idle current through the 6H8P since this is is the stage that could cause the cancelling effects. The gain varied even less significantly but the BW remained the same since the CCS is buffered by the WCF (which BTW also makes the CCS work better).
Rgds,
Naz