Phase inverter fine tuning help. AX7 or AT7?

[Dingleberry]

Hi.

Hello everyone from blizzardy Finland.
This is my first post here, hope it fits this category.
I've built nice small amp from recycled parts I had lying around in my workshop.
I've always been curious about different long tail phase inverter designs, and I've read stuff about them, but still there is some questions that confuses me.

It's a 6v6 p-p cathode bias amp with no negative feedback.
Simple preamp: parallel connected 12ax7 with just volume and tone controls very similar to princeton. Because of the simple preamp the distortion is mostly generated in the p.i. and power amp sections, and only when cranked to max.
I wanted the amp to be mostly clean.

Phase inverter is long tailed pair type.
I've read that 12ax7 has more gain and less current and 12at7 has more current and less gain. What that means in practice?
What affects what?
Is it gain or current that drives the power amp tubes harder? Or both?
Will 12ax7 and "marshall" component values generate more distortion than 12at7 and "fender" component values or vice versa? Which will have more clean headroom?
Which will have more overall volume?

How changing the p.i. bias resistor afeects gain?

Any help welcome.
Thanks.

-Dingleberry

[Earl]

Hey Dingle! I'll take a stab at this - As far as the two different trodes as PIs go, there isn't a ton of difference between the two, really. The AT has a lower Plate Resistance, and can handle more Plate Power Dissipation.

Generally, if you want a clean amp, you taylor the gain stages through the amp to not overdrive any succeeding stages. You'd start at the tail end, and work back.

IOW, if your 6V6's (I'll assume Class A), has a Cathode Voltage of ~20V at idle, then you'd want the PI to make ~40V P-P at full volume. This will keep the output tubes from saturating. So, if the PI gain was ~20, then 2V input would be your limit there.

You could add a NFB loop, or you could convert it to Class AB with a fixed bias, or lower the gain(s) of the other stages.

I'm sure you'll get more suggestions on this.