Another PI question...

[EFK]

I've been looking at schematics and it seems like an awful lot of 2 X 6L6 type amps use a 'classic' bassman/jtm45 type PI set up, i.e shared 470/10 tail cathode resistors and 82/100 on the plates. .02 cap on one side and .1 on the other and these seem to generally run @ 250-270 plate voltage.

In comparison, my PI is running about 250-260 on the plates but has a shared cathode resistor of 1.2k with a 47k tail. Plate resistors are balanced, both being 100k and the caps on inputs are both the same at .02uF. Now on this there is no negative feedback but nevertheless I'm curious: (1) why is mine basically "balanced" whereas most are unbalanced, and (2) what is the effect of the resistor choices - can a generality be made? I.e., the 1.2/47 being 'cleaner' than the 470/10 or perhaps hotter and faster to distort? I realize a lot is preamp circuit dependent (this is very straightforward, btw: input -> 1/2 12ax7 -> volume -> 1/2 12ax7 -> tone stack -> PI...) but I'm just wondering if any generalities can be made with this limited information? Thanks for your time!

[Merlinb]

my PI is running about 250-260 on the plates but has a shared cathode resistor of 1.2k with a 47k tail. Plate resistors are balanced, both being 100k and the caps on inputs are both the same at .02uF.

With a typical LTP using an ECC83, if the tail resistor is less than 16k then the "first triode" will have more gain than the second, so it is usual to mis-match the anode resistors to compensate for this, hence the 82k/100k.
Your LTP has a big 47k tail, so it uses equal anode resistors, since this gives better balance than 82k/100k would.

Larger tail resistance means a bit less output swing is available (though it doesn't usually make a LOAD of difference). So depending on what power valves you have, it may not be capable of such high levels of overdrive. (Actually there's a bit more to it than than that, but it will suffice now).
The more tail resistance there is, the bigger the bias resistor needs to be to get a given bias voltage. That's why yours uses a 1.2k.

Finally, does your amp inject two different channels into each grid of the LTP? That would explain the 0.02 caps on each grid. A big 0.1uF cap is normally used when a grid is not used at all (grounded at AC).

[EFK]

That does explain a LOT. Thank you. And yes, it is 2 channels. Much clearer now.