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RE: 300B IT 1:1 845, A2 possible?

The problem lies with the chosen 300B operating point. At the 30mA standing current it's too low for the required swing. I will assume a 5K primary impedance for the IT in which case you only have about 120V one-way swing on the negative going cycle before the tube goes into cut-off. The typical 845 OP requires around 180V one-way swing at the grid for A1 operation.

If you're limited to the 30mA current for the 300B by the IT, it will be very difficult to impossible to push the 845 into A2 operation. By my (very rough) calculations you will need to set the driver tube closer to 400V Va-k at a plate current of 60mA. IOW you will need a very good interstage to pull it off.

If you're not averse to silicon, the easiest and cheapest way to accomplish A2 operation is something like Tubelab's power drive circuit. Can't speak from first hand experience on the sound quality that this circuit imparts, but from a technical point of view it will accomplish what you're after.


Edits: 12/28/21 12/28/21
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