... In addition, you probably want a separate and higher anode voltage for the preamp. ...
When you lower the supply voltage to the output tubes, the bias voltage needed goes down. ... With 1/3 the output requirement, high preamp supply voltage is not needed for drive signal to the output tubes. ...
Let's talk about it the opposite way, thinking about how to increase the power of a 1934 push-pull Class A amp.
In our Class A amp, output tube peak plate current is 2x idle current, for an
average plate current = idle current (assume current rises/falls the same amount).
To increase power output, we want to increase plate current and will get part of the way there by lowering the load impedance. If peak plate current goes up, so does the average plate current, and the average plate dissipation. We need to offset this effect by idling the tube cooler (and perhaps allowing it to shut off part of the cycle) so we'll use a larger bias voltage.
If we were able to apply a higher screen voltage, peak plate current (and power output) would rise further, though again we need to make the bias more-negative to keep the plate from overheating during the signal cycle. Plate voltage will also rise, because choosing a less-low load impedance will make it easier to avoid overheating the plate.
Where are we?
- Increased output tube plate/screen voltage
- Increased peak plate current
- Need a large bias voltage to keep the whole thing from melting down in use
In making our Class AB amp, we didn't increase the output tube's transconductance (ability to swing plate current for a given input voltage); in fact, we made it a bit worse by idling cooler. This means our big-big plate current output will need a big-big grid signal voltage to create.
The larger drive signal to feed the output tubes has to come from our phase inverter, and we might be motivated to not give away the game by using a well-balanced inverter circuit to get the maximum clean output power (as we've worked hard everywhere else to boost output power). We need a good 50v+ peak from each output, and will need to drop another 80v across a tail resistor (or derive 2 outputs plus leave enough plate voltage for a split-load).
Good thing we have ~400vdc we can count on after the boosting the output tube plate & screen voltages.
Hopefully it's clear the output section design dictates what will/won't work in the phase inverter & preamp. And if we're really pressed, skip 6L6/6V6 and move over to EL84/EL34. These tubes have twice the transconductance of the American types, and further ease the driving requirements.
