POWER UP AND INITIAL OPERATION
Well, power up this morning went very well indeed! I found a couple of things to correct (missed a B+ connection and my input muting switch was wired incorrectly), but after than she came up like a dream on the Variac.
The amp responded very well to the controls and after roughing in the screen and EFB range voltages, I biased and balanced the new output tubes (Sovtek EL84M's--I'm a big fan of these; matched set from Mr. McShane) at 25 mA cathode current per tube and tuned the screen voltage per the formula in Dave's schematic. With the tubes drawing the recommended quiescent cathode current, I measured the voltages all around. In general, my high-end B+ voltages are about 20-25 V lower than Dave's amp, likely because I'm using a 700 VCT power transformer (201 mA) instead of the Dave's 720VCT 250mA X101ST P.T. The rectifier is a Genelex Gold Lion U77/GZ34 (a McShane recommendation), a Westinghouse (Japan) 6FQ7, and a garden-variety GE 12AX7 (for now).
With the line voltage set at 120 VAC, here's what I got (with Dave G's values in parentheses):
B+ VOLTAGES: B+ "A": 422VDC (447); B+ "B" 322VDC (339); B+ "315": 303VDC (315), B+ "225" supply: 213VDC (225)
BIAS SYSTEM: 6.3V C.T. point: 41VDC (43); "C": 2.3VDC (~2.1); "D": 13.3VDC (~15)
FILAMENTS: At P.T. Windings: 6.34 VAC (6.3); At 6FQ7: 6.28 VAC (6.3); At 12AX7 (Parallel): 6.3VAC (6.3); 5AR4: 5.15VAC (5), no tube. The 12AX7 and 6FQ7 are at the end of their respective heater strings.
EL84M VOLTAGES: Plate Voltages: 422VDC (445); Screen Voltages: 322VDC (339); Cathodes: 13.55 to 13.6 VDC (~15.25); Grids: 1.45-1.6 VDC (~1.4)
With these cathode and grid voltages, bias was right at -12 to -12.1 VDC (-13.85) across the board. In my experience, the Sovtek's are a bit cold-blooded and have to be biased pretty hot (more positive) to get the desired cathode current and this was the case here, accounting for some of the voltage differences with Dave's original set up. The fact that the EFB range control and the bias controls together can handle this end of the bias voltage range (with some room to spare) is good news indeed!
At 25 mA per tube quiescent, allowing for 3 mA for the screen current, we are running the output tubes at about 9.3W quiescent plate dissipation, about 66% of the 14W max rating for the EL84M's. Some of the magic of EFB (tm)!
The plate voltages for the 12AX7 were about 97VDC (95), the cathodes were 0.56VDC (0.65). For the 6FQ7, the plates were both at 200V (215V), the grids were at 97VDC (as expected, being direct coupled to the plates of the 12AX7), and cathodes at 100VDC (100).
I played in the amplifier for over two hours and it ran very cool. The P.T. was just warm.
OK, what about sound? Realizing that my bench speakers are expendable low-budget units, the sound was excellent with levels of detail that I've not been hearing through those speakers lately. Can't wait to get it on the 'real speakers' for a listen! With no inputs hooked up, there is a pretty good hum, but this circuit has no switching to automatically ground open inputs, but that's where my muting switch comes in handy! Dead quiet when muted and with an input plugged in and no music playing.
I put the bottom plate on it (without holes, but there seems to be very little heat build up in the chassis now, but I'll keep an eye on it) and took a couple of pics with the tubes installed and glowing away. I'm going to listen for a bit before putting it back on the bench for performance measuring in the next couple days. Very happy with the build so far!