Controlling tube gain with logic signal

[Matador]

I've seen digital control of tube gain accomplished with Vactrols, such as in Soldano and Mesa Boogie preamps, where a LDR is used to place elements in parallel (like adding a cap to a cathode resistor, or adding a cap in parallel across a series resistor to make a filter), even in high voltage portions of a circuit. However Vactrols are becoming incredibly expensive ($5-$10 a piece), and vary greatly from unit to unit.

Has anyone experimented with using low cost, high voltage MOSFET's to accomplish something similar? For example, the STN1NK60Z NMOS has a 600V breakdown voltage, a 1V threshold, and has a PMOS equivalent, that run only 30 cents each in bags of 100.

Something like this:



VP and VK are incoming, logic signals (3.3V), that add a plate resistor and cathode resistor respectively. The NMOS MOSFET above is spec'd at 1.2 ohms with 3.3V VGS drive, so when 'on' should place R7 in parallel with R5 when VK is on. Such a scheme could also be used to add a cathode bypass capacitor, or LED, etc. I don't think Q3 needs to be rated for high voltage, as the drain shouldn't ever see another much above 1V.

Q2 needs to be level-shifted, and will only turn on with VGS at 3.3V below VB+, so when Q1 is turned on, it will pull ~1mA through R2 and R3 (if VB+ is about 250V), and will turn on with the drop across R2 thus adding R4 in parallel with R6. With Q1 off, the gate is pulled to VB+ thus turning Q2 off.

 

[ruffrecords]

Both logic signals will cause a change in dc conditions of the tube so may well result in a thump in the output. Maybe ac couple them instead?

Do you just want to switch gain values or vary them smoothly?

Cheers

Ian

 

[Matador]

Do you just want to switch gain values or vary them smoothly?

Just to switch them. I'm thinking of adding another one (NMOS) on the output in parallel with R8 (as a mute), so I can turn that one on first, adjust the others, then turn it off (mute-adjust-unmute).

I tried this on LTSpice, switching R7 for a 10uF bypass capacitor, and see the gain jump at the point where Q3 is switched on which "adds in" the bypass cap (after settling):



The natural question is: why do this? I'm trying to make a test fixture, where I can measure a tube under 4 different biasing conditions, to test suspect tubes to make sure they bias as expected, and I can listen for self-noise by tapping the output at the plate. I already made such a fixture, but it requires I use trim pots and adjust each plate and cathode resistor for each of the conditions, and it takes a long time per tube (plus adjusting a plate resistor when the tube is on is a miserable scratchy mess of noise).

If I use a cheap rotary encoder and a PIC to switch in the circuit elements by just turning a knob it would greatly speed things up and allow me to truly A/B different biasing schemes while listening.