Johnny said@BHK what is the max heater current that the BHK Pre can supply? I have found a wonderful sounding pair of Russian 6n6p (6h6n) tubes that are essentially 12bh7’s,… plot wise they work as the bias point falls around -2v (@5ma),… Heater current is ~700ma (per tube) though,… pretty much double a 12au7,… I don’t want to over tax/stress the heater supply if it’s not up to it…
One other question,… when using a cc source/drain on a tube, doesn’t that flatten out the load line from an operation standpoint? realizing that the bias point will still be the same and you arrive at that point in the same way (Pr\V),… the slope flattens out some because of the constant current being delivered, correct?.. thus operating the tube in a more linear area between the grid lines?
Anywho,… if you confirm that the Pre heater supply is up to snuff for the higher ma draw, then I would highly recommend the 6n6p as a tube to try,… you can get a nice matched pair for ~$40 so not crazy expensive either. Please note that you do not want the 6n6p-I as it is for impulse use and not suitable,…
Best!
I will check out the tube heater regulator to see what kind of current it can supply before the regulator drops out or, in my opinion, the regulator heat sink gets too hot. This opens up the possibility that I had been thinking about of a considerable number of tubes that could be used with the BHK circuitry like 12BH7, 6N30, 6CG7 5687, and more. Of course circuit gains will vary but in general, gains would most likely be lower than with the 6922 but that in itself is OK. One thing to note though, high gain triodes like 12AX7 won’t work because they can’t support 4 or 5 mA with out drawing grid current with a positive grid bias voltage. Construct the load line for the BHK tube circuit conditions (120 V plate voltage supply and 10 K plate load resistors) on a set of 12AX7 plate characteristics and you will see what I mean.
To your other question, yes a constant current load in the plate circuit of a tube will make the load line horizontal and more linear. However in the case of a differential amplifier configuration like the BHK circuits, a constant current source in the cathode circuit doesn’t have this effect but does cause the plate currents to be constant at the desired value i.e. 4 or 5 mA IF the tube can support it, see above.
BHK