Adding one more tube - PT enough?

[Mark Hammer]

I have an old Imperial/Cobra amp that I picked up cheap and would like to use as the basis for another amp. This started out life as a roughly 20W head, using a pair of 6V6GT power tubes, a pair of 12AX7s, and a 5Y3 rectifier.

The power level is fine by me. I picked up a matched set of JJ 6V6s that'll suit me fine, so I have no urge to alter the power stage side of things. My musings have more to do with the front end.

I've looked at a number of schematics for "classics", and have found a number of contenders for potential rebuild that involve just one more 12A_7 tube, whether to provide bias tremolo, or reverb, or simply a second channel.
Now, I could confine myself to the dual triode-pair tube configuration noted above, and make whatever that permits, but the range of possibilities available with just one more triode pair is awful tempting.

So, my question is, how would I know if the power transformer is up to the task of supporting a third preamp tube? And, assuming it is, are there any other things to take note of before machining another hole and installing it?

[MWJB]

Chances are that the PT will take the extra 300mA heater current and 2-3mA of B+ of a 12A?7.

You might want to add another stage of filtering between PI 7 preamp. PI has it's own filter cap, other 2 12A?7 can share a cap.

[Enzo]

I'm with MWJB here.

I doubt the couple mas of plate current will bother anything, and the heaters are usually the main concern adding tubes to a PT. But the simple test if to clip another 12AX7 into the heater circuit and see if you get a drop in heater voltage. DOn;t have to wire the rest of the tube, two clip leads should do it.