EWBrown wrote:Direct driven (without the resistor) and with a 32 ohm output, a half-watt should be sufficient to cause near ear-damaging SPLs. So, be careful...
Yeah, this is my biggest concern. I'm probably going to have a big, clearly-labelled switch for headphone use. I like the idea of so many watts of headroom though, it should help the frequency response.
EWBrown wrote:The OPT can be anything from 6.6K to 10K P-P, B+ is about 300VDC.
I'd rather not bother paralleling tubes, it means more plate current, and a bigger PSU, and possible instability with unmatched tubes. I don't need the power. The toroid at 17.3K a-a should be fine.
EWBrown wrote:I'm sure that a 7AU7 will run just fine with 6.3VAC on the filament, and the 6GU7 or 8GU7 is functionally equivalent to 12BH7, just with filament connected to pins 4 and 5 only.
/ed B in NH
Yeah, I figured that most US AC runs the 6.3 at closer to 6.6-6.8V, and that's well within 10% of 7.2.
Last night, I realized that I can use a couple back-to-back 6A 6.3V filament transformers I have lying around, with a voltage doubler feeding an LCRC filtered B+ at ~293V. They're nice enclosed models, the bells of which which I will probably strip of varnish and paint a fiery red-orange. I'll probably post a schematic later today.
Two questions: What current set resistor would I use to a 7AU7 in LTP, 6mA between the two? B+ is 280V dropped to ~100V with two 62K resistors, if that helps, to bias at -4V quiescent. The swing remains pretty linear (I know 12AU7's don't have a great reputation for this) as long as you stay under -2V or so, which should be fine for digital sources (-3V to -5V swing). |
Thanks!