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RE: One of the problems with this design

Posted by JKT on February 11, 2017 at 10:44:28:
Let me give an example to clarify what I was saying. Suppose the driver tube (2nd stage) required a grid bias voltage of -6V at its operating point. As a first approximation this would limit the incoming signal to 6V peak to peak. If we're talking about a diff. pair, as an example let's say that the grids are offset by 3V. Then this condition would cut that 6V envelope in half and incoming signals greater than 3V p-p would distort. This is what I meant when I wrote that it would reduce headroom.

An imbalance in plate currents in the 1st stage diff. amp will result in different plate voltages (depending on the DCR of the plate loads) for the pair. Since the 1st and 2nd stages are direct coupled this difference will be expressed on the grids of the second stage.

If this proves to be the problem you will need to replace the input tube with one that has matched sections as measured in circuit.