I want to share some recent experiences... My friend helped me to add the jumper from Pin1 to Pin8 allowing use of EL34s.
He remarked that the filaments appeared soft and we measured them. 5.8vac, and he thought the EL34s were drawing too much current. 6l6 are good for 1amp per tube plus 600mv for the 6SL7? So with 1 6.3 winding you're right at the edge? The EL34s use 1.5amps? Needing ~3.5amps?
Im using a XPWR105-120 which has 2 6.3 @ 2.5 amp windings. The current hookup only had me use 1... the yellow wires, with the brown 6.3 winding cap’d.
Using the EL34s, appeared to overrun the winding... too much current draw. So, we made sure the windings were in phase and combined them... Im not sure if this is paralleling or in series... The voltage did not go up, but it doubled the available current (amps).
2nd issues. The filaments still appeared (to his eye) to be dimmer than they should have been. So we measured the input voltage. With ~120v into the CL-90 the power transformer was only getting ~114v. Using a Varac @ 122v in, and the CL-90 still in place we had ~6.3. We placed a jumper across the CL-90... I know, thus killing its benefit, but allowing for a simple reversal. With 120v in and no CL-90, Im getting ~6.2 on the filaments.
Now the amps has the right amp of current available to the filaments to run either 6L6GCs or EL34.
Im a complete novice... but all this did seem to help make the wave form even soother.
Now for wattage....
The Tungsol 6L6GC Str RMS was 3.1 watts before mods @ 1000hz
The Tungsol 6L6GC Str RMS was 3.4 watts after mods @ 1000hz
The Wing’d C EL34s were 4.5 watts with mods @ 1000hz
All the wave forms were really clean, put till clipping.