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Ideal operating point of 6sj7


2005-08-16 9:43 am

I need some advise. I like to know on the ideal operating point of 6sj7 tube (plate, screen and bias voltage) in pentode connection. I have have seen the WE 310A schematic in sound practices by Joe Roberts but no operating voltages were given. According to Joe, a 6j7 and the 6sj7 will also work in that circuit. I will be using the 6sj7 to drive a 2a3.

I have used a 6SJ7 for the input stage in a guitar amp. Never tested the actual distortion, but it sounded clean. Plate voltage about 150, depending on the tube. Current about 3 - 4 mA, set by a cathode resistor of 820 ohms. Also used 1K ohms. Screen resistor was 47K ohms. Total supply voltage 250 volts.

These conditions were determined by adjusting the cathode resistor for an average of 150 volts plate voltage after trying about 20 tubes. Most of these were the older metal tubes. Screen resistor was 47K ohms.

I have used these in triode mode most of the time. They want more current to sound good in triode mode. Something like 10 mA.
If you like to use a 2A3, I think it is because you trust in triodes.
It is the case to prefer triodes design. For example the Loftin-White by Asano: http://members.aol.com/IKSchiffer/2a3amp/loftin.html

Btw, if you want to use the Pentode, simply use the curves, having in mind the chosen working point of 2A3(for example: 2k5ohm load, bias -43,5V, alias it need a drive swing of 87V).


Paid Member
2003-06-12 7:04 pm
Maine USA
> I like to know on the ideal operating point of 6sj7

There is no ideal operating point.

For a good operating point, use the RCA resistance-coupled amplifier tables.

Oh, crud. Even RC-22 does not have R-C data for 6SJ7.

Going to older data: driving self-bias 2A3 with Rg=500K, try a 100K resistor in the 6SJ7's plate.

However: for outputs over 30V (which will be needed to drive 2A3 hard), IM will be much lower with 6SJ7 working Triode instead of Pentode. At 60V, pentode makes 25% IMD, triode 12% IMD. You get a lot less gain as Triode, forcing another stage, but 25% IMD is too much.

6J7 is significantly more linear than 6SJ7.

For either tube as Pentode (pentodes in general), for large output (as for whacking a 2A3), use Rp about 1/5th of next stage Rg, use Rg2 about 5 times Rp, then fiddle the cathode resistor to set the plate voltage between 0.5 and 0.7 of supply voltage. Exact plate voltage ratio may be critical for "best results", but is NOT easy to calculate even in theory, and in practice has to be ear-judged for most pleasing nonlinearity (including cancellation of nonlinearity between driver and output tube).

Although ancient and low-gain, a 6J5 triode is an excellent and linear driver for a 2A3.