I Have no info on trans. but it runs very cool with one 6l6GC wondering would it handle two 6l6s or should I go with 6v6s?
Plug in the pair of 6L6's, and use a meter with clip-on leads to check for 6.3vac at your heater pins. For simplicity, connect one lead per side of the heater wiring (this will also avoid the "why am I getting 3.15vac" question when you measure from one pin to ground). Switch the amp on.
If you read substantially-near 6.3vac and don't let out any magic smoke, you're probably done.
If you just plug in 6L6's, you may hear a slightly cleaner sound and/or a different break-up from using 6V6's.
My opinion is much of this is due to the differences in what bias voltage/idle current the 6L6's will land at, plus some contribution from the slightly-different shape of the 6L6's characteristic curves. I'm betting the 6L6's will idle at a higher current, which you will observe as a higher voltage across the cathode resistor, perhaps something like going from 19v across the cathode resistor to 28v.
That higher bias voltage also means a bigger signal required to drive the 6L6 to full clean output power. You fully drive an output tube when the incoming signal peak equals the bias voltage. So that might be like going from a 19v peak signal to fully-drive 6V6's to a 28v peak signal to fully-drive 6L6's. Since the driving signal voltage needed to be bigger, that probably equates with you having to turn the volume control higher. I think many misinterpret this as "more headroom" because "the amp stays clean to a higher volume knob setting." I'm not sure many/any of them used a SPL meter to see if the limit of clean output with 6L6's actually got more speaker-volume into the room.
Power = Voltage * Current. If you want the amp to kick out more actual power, you need to either raise the supply voltage, or lower the output transformer (OT) primary impedance (which then allows more current for the same voltage swing), or both. Fender often kept supply voltage similar across amp models and halved the primary impedance to double the current. The OT core would need to be bigger to pass the additional power, down to the same low frequency, at the same amount of distortion. However, sometimes the power handling ability of the OT is
purposely restricted for sonic effect, which may also explain why some modern clones sound big & bold, but maybe also "too clean" or "too hi-fi". Unfortunately, we often don't know core weight, as well as the stack size to get a good handle on what a given OT will sound like.
And the power transformer would need to be able to supply this additional current. We don't know the capabilities of your power transformer, but it seems sizeable. You might clip meter leads from the high voltage center tap to one side of the high voltage winding, and measure a.c. volts at idle and while you're cranking the amp flat out. If the voltage drops
significantly, it could indicate that the amp is attempting to draw more current than the transformer can deliver.
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