More info from our engineer:
You can note to the one dude that I over-rated the transformer to handle 5.6A @ 6.3VAC. It is not ideal to overload it with 6550s, but it is not as dramatic as he made it out to be. Honestly, I forgot when I was thinking about it.
In regards to the primary difference not being a big deal, I disagree. We auditioned different primary values in the initial stages of design right around 20% and found there were performance and tonal differences. 2200 sounded and performed better, which is why we are using it.
Plate voltage is 445VDC and this amp is fixed bias, not cathode.
Thanks for posting the pics and getting back to us with answers. We've learned a few things:
a. the 6550 conversion wouldn't be THAT hard on the PT because it's pretty-well over rated for 6L6 application, but it's still 800 mA shy of an adequate rating for use with 6550.
b. Primary Z differences of 20% really are not all that dramatic in terms of a 6L6 operating characteristic. Knowledgeable amp designers commonly choose far more discrepant operating conditions in order to either increase or decrease power output and distortion characteristics.
The following chart normalizes the Ra-a loads we've been discussing to a plate characteristic chart that's referenced for 2-tube operation. To apply the 4-tube numbers to this chart, the numbers have to be doubled from 1850 and 2200 to 3700 and 4400 respectively.
This chart gives you a rough idea of how the two different primary Z operating points will compare. The blue line corresponds to Ra-a = 1850, while the red line corresponds to Ra-a = 2200.
You can see that the power output isn't significantly different, but the amount of THD is increased from 8% to about 9.4% by choosing the higher operating point. That's not much of a difference to write home about. I doubt that anyone here could tell the difference in a blinded test.
c. The plate voltages of 455 VDC aren't all that high. In other words, should be totally safe/reliable for use with modern day 6L6GC. But the question that remains: Is that 455 VDC plate voltage high enough to provide tonal equivalence to the original v4-B that ran the B+ at 545 to 590 VDC? That question will ultimately be answered by the individual player who gets a chance to A/B the amps. From experience I know that I can easily hear a 100V difference in B+.
All in all, it looks like a really nice package.
