Matador
Well-known member
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.
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.
