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RE: Looking for simple fully balanced 6BQ5 schematic

I recently built a fully differential amplifier based on the EL95. Both tubes are designed to be easy to drive so a 12AT7 wired as a differential amplifier should drive it nicely.

I recommend an OPT with UL taps.

You can run a 12AT7 with 100K plate load resistors. If you run a good quality constant current source you will be able to drive the 12AT7 single-ended or balanced with no appreciable difference in performance.

I would cathode bias the power tubes using a common cathode resistor, so that the operation of the power tubes is also differential. A constant current source for them would help.

If you are planning to drive the 12AT7 directly, you'll want to run an OPT that employs 4,8 and 16 Ohm taps so that the 4 Ohm tap can be grounded (and not the Common). In this way the impedance to ground from the Common and 16 Ohm taps will be the same so you can run a feedback resistor from each back to each grid of the 12AT7. If you have 4.75K resistors as a series resistance to each grid of that tube, you'll be able to have reasonable feedback resistance values. If this looks familiar it might be because the approach is similar to that of an opamp.

I would run a B- for the 12AT7 of equal value to the B+. This means your CCS will have to be able to handle the voltage drop, but this will also increase its effectiveness. I recommend a 2-stage device over a single stage- otherwise you'll leave performance on the table.

How much power are you planning- are you OK with running the power tubes class A?

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