the grand return. 3d21A triode pp

the thermionic watercooler

the grand return. 3d21A triode pp

Postby sorenj07 » Tue Dec 15, 2015 10:15 pm

cleaning out the parts bin after about 8 years out of things. got a nice quad of 3D21A's in ST bottles that have been now through 3 moves or so and hopefully aren't Kaputt. same with a sleeve of 6bz5's that seem to be pretty hardy little 6DJ8 lookalikes. i had a few other tubes and options but these seem to be the best bet. i am in the states for vacation but am now based in Berlin so bringing larger amounts of transformers back in luggage or shipping them becomes a huge hassle. i have some no-name 6.6K PP OPT's sitting in Berlin, unfortunately no UL taps, but i figured this would be an OK use for them.

brushing off my load line skills and using some handy online calculators, it seems like i can get together a decent 2-stage amp that will more or less reach full volume with a standard 2 volt input. i'm a little fuzzy about how to draw load lines for CCS so i just drew the slope for the 6BZ7's based on the 33K load resistors, which is above the rule of thumb for 3 times the plate resistance that i remember from somewhere. same with the anode voltage. let me know if there are any glaring mistakes here.

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my thinking is that if i build some compact monoblocks with a pair of nice potted 8H 200MA chokes i'll fly back over with, i can later drop in some more macho tubes if i start itching for more power. or go to pentode mode (no UL taps here unfortunately). but my speakers are some Klipsch RF-52 at 96dB efficiency so high power is not crucial for me. ... 100000.pdf

i guess my main questions are whether i'm completely wasting my time with these obscure tubes. they're pretty but for only 15 watts' worth of plate dissipation for a whopping 6.3V 1.7A heater, it's hardly much bang for the buck. 6.8 amps for power stage heaters is more than most single power transformers can handle, for example, so that's another reason to go monoblocks.

anyone have any opinions about linearity? seems like a fair amount of 2nd harmonic distortion, as with the driver, that will hopefully get cancelled out by the PP action.

power supply will be garden variety. 500VAC, choke input to 450, minus 20 or so with a 5AR4, then allowing say 20V of sag to reach a nice round 400V. and if it's running a tiny bit high i guess it won't be a crisis.

feels good to be back!
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