by EWBrown » Thu Oct 04, 2007 12:17 pm
I just chose 35 mA as this is the normal cathode current for a 6BQ5 / EL84 / 6P14P, etc.
The LM317 can carry at least an amp, perhaps 1.5 amps, so 35 mA is just a sleepwalk for them, they won't need anyheatsinking.
The way an LM317 works, it likes to "see" 1.25VDC between the output and adjust pins, with "output" being the higher voltage level.
Using Ohm's Law, 1.25V / 35 mA comes out t 35.71428571428 ohms :o
Seeing that finding such a ridiculously precise resistor is totally unlikely, just use 36 ohms, which is a standard 5% resistor value. 1/4 watt is more than enough, and I've even used little bitty SMD resistors soldered directly to the LM's pins and it works. The "input" connects to the tube's cathode, and the "adjust pin" connects to ground / zero volts buss. Bypass from cathode to ground with 220-470 uF, 35VDC cap.
The LM317 has an upper input voltage limit, IIRC, it is around 35VDC, so this only works with tubes which operate with relatively low cathode bias voltages.
An alternate approach, if your junkbox has lots of 7805s but no LM317s:
Cathode to input, center pin to ground, load the output with resistance somewhere between 140 and 150 ohms @ 1/2 watt. (142.857 ohms being the theoretical perfect value, but I digress once again, Mister Spock). Bypass cap from cathode to ground. in the usual practice. . If you want to make the bias current "tunable", then use a 100 ohm resistor and a 100 ohm pot in series, as the LM7805 load. Then you can tweek anywhere between 25 and 50 mA.
There are higher current and voltage CCD devices available, I posted about them a few weeks ago... These can take up to 350 and 450VDC across them, so they should make nice CCD plate loads, as well.
I wouldn't run a nice vintage NOS Amperex or Mullard 6BQ5 / EL84 at 50 mA, but it's no "great crime" to treat one of the $2-$3 Russian 6P14Ps in such a "hostile" manner, for truly "hard" Class A SE or PP operation.
They're cheap and plentiful, so long tube life isn't such an economic issue under these conditions.
Oh, yeah, you'll need a HEFTY power trannie to get away with this...
/ed B in NH
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