Tubes?

[Earl]

Jason,

I left tubes behind some decades ago, but if I was to use tubes in the TPU (which I have no intention of doing), I would use the attached schematic.

Since the circuit inductance of wiring, leads, components, etc determines the maximum speed, I do not believe that a tube circuit offers any speed advantages.  I could never build a tube circuit that would switch as fast as a modern FET.  Even if the tube switched a hundred times faster than the FET, the entire tube circuit would switch slower, in my opinion.

The only advantage I see is that if there is a voltage spike at turn-off of say 3kV, this would destroy all transistors, but only cause a plate to grid arc, which most likely would destroy neither tube nor FET in this circuit.

Earl

[Jdo300]

Thanks Earl!! I'll try it out asap .

[Jdo300]

Hi Earl @ others,

I finally had a chance to try out Earl's suggested setup and have posted a few screenshots and a scope shot below. The setup kinda-sorta worked but it appeared that the 12J5GT triode was not switching on all the way. I used a 0.1uF cap for the capacitor (just a first guess) and a 34V zener diode on the MOSFET (IRF840). I drove the MOSFET gate at 15V with a mic4427 driver IC. For the test, I used a frequency of 1kHz with a pulse width of just under 8 ?s.

I'm not sure what parameters I can change to get the tube to turn all the way on and off. In the scope shot, the top trace is the voltage measured between the anode of the tube and the circuit ground. You can see a small negative dip in the DC level (which was 300V) when the input signal on the fet goes high (bottom trace). The coil that I used was a simple toroid choke that looked like it was somewhere around 20-30 mH (no actual measurement of inductance taken). What am I doing wrong?

God Bless,
Jason O

[Earl]

Hi Earl @ others,

I finally had a chance to try out Earl's suggested setup and have posted a few screenshots and a scope shot below. The setup kinda-sorta worked but it appeared that the 12J5GT triode was not switching on all the way. I used a 0.1uF cap for the capacitor (just a first guess) and a 34V zener diode on the MOSFET (IRF840). I drove the MOSFET gate at 15V with a mic4427 driver IC. For the test, I used a frequency of 1kHz with a pulse width of just under 8 ?s.

I'm not sure what parameters I can change to get the tube to turn all the way on and off. In the scope shot, the top trace is the voltage measured between the anode of the tube and the circuit ground. You can see a small negative dip in the DC level (which was 300V) when the input signal on the fet goes high (bottom trace). The coil that I used was a simple toroid choke that looked like it was somewhere around 20-30 mH (no actual measurement of inductance taken). What am I doing wrong?

God Bless,
Jason O

Jason,

you are doing several things wrong.  The coil has much too much inductance and especially much too much inter-turn capacity.  There should be only one layer around the toroid circumference.   In a triode switching circuit, a lot depends on the plate load, and I see a lot of capacity in that choke.  This is not audio, but RF.

Please post screen shots of cathode voltage.  Did you add a zener pull-up resistor to make sure at zero plate current that the zener still has full voltage?

What is plate current when grid to cathode is equal to zener voltage?

Your leads are very long, remember I said all lead lengths should tend towards ZERO mm.
The FET driver, zener, FET, ceramic capacitor plate-coil by-pass, and ground point should be right at base of tube.  The circuit can not be a "bread-board" and at the same time have lead lengths approaching zero.

Regards, Earl

[Jdo300]

Hi Earl,

Yes, I did miss the pull up resistor on the setup. Just to make sure I have this straight, the pull-up resistor should be tied between the zener diode and the +300V plate supply? Should I use a huge value like, say, 1MOhm for this?

I realize that I probably had a far from ideal setup for this test, but I didn't want to invest time in making a PCB or finding a way to solder the chips directly to my vacuum tube before doing at least some preliminary testing on it. In the final version, I would hope to make a small PCB that would fit right under the tube itself and just 'plug' the tube right into the PCB.

I also meant to ask you earlier about the capacitor. What size range cap is appropriate for this arrangement? I just wily nilly picked the size that I had (0.1uF), but I honestly don't understand how this circuit actually functions. Could you explain it in detail to me? I would probably help me not make stupid choices in picking out the appropriate parts and things.

Thank you again for all your help!

God Bless,
Jason O