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I have built the MOSFET amplitude modulator shown in the bottom right onto another class C oscillator tube circuit, and it does work. I am having trouble understanding why based on what I've read.

MOSFET amplifier

(source: danyk.cz)

It seems to be a common-source amplifier, but doesn't seem to have any Rd, or Rs, or even any power supply rail for that matter (the centre point of the tube filament should be at 0V, no?) Despite the fact that it seems to use a potential divider to provide bias for the gate.

I also rebuilt it on a tube circuit that has a separate heater and cathode, and rectified the heater supply to provide the MOSFET a power rail, and took the output access an Rd that I added. This makes the MOSFET run much cooler. (I assume this is by providing sufficient voltage for the divider to get the gate to a reasonable q-point.)

If my interpretation so far is correct, what is the consequence of not having an Rs between the the source pin and ground? Should there be one, ideally? Can I still add a bypass capacitor (I guess it's not a bypass capacitor since there's no Rs) between the source pin and ground to increase high frequency gain?

(PS - An afterthought, does it make more sense to think of the whole circuit as a cascode?)

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  • \$\begingroup\$ Look at the MOSFET as the cathode resistor in a self-biasing configuration (the grid is grounded to DC). Therefore the MOSFET is more like a voltage controlled resistance, modulating the cathode current. \$\endgroup\$
    – user16324
    Feb 11, 2017 at 20:04

3 Answers 3

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The anode-cathode current for the GU-81M is modulated by the FET. All the current from the valve flows through the FET and it's DC configuration (using the potentiometer) will set the operating point cathode voltage. The center tapped heater has no influence and the heater/cathode is not at zero volts.

Yes, this would be considered cascode operation as in this audio amplifier with two triodes:

enter image description here

In your case since the FET modulates the operating current (and therefore the C-G) of the pentode it changes the amplitude of the RF output.

If you look at the datasheet for your FET:

enter image description here

Excuse my mouse scribbling on top of the graph, but here you can see as the Vgs varies, the current through the valve will vary (and the cathode voltage).

I've no idea where you operating point for the GU-81m is so cannot work out the valve current, but you should be able to.

Note that your grids are at very low voltages, so as the cathode rises they are effectively negative biased. It's doubtful you'd see more than 120-150 V on the cathode so well with the ratings of the FET.

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  • \$\begingroup\$ From the tube datasheet here: g8wrb.org/data/Russian/GU-81M.pdf "Cathode: directly heated, carbonized thoriated tungsten". This makes the schematic look weird! \$\endgroup\$ Feb 11, 2017 at 20:08
  • \$\begingroup\$ Yup, I provided that link......but it does not tell me where your operating point is for your amplifier. \$\endgroup\$ Feb 11, 2017 at 20:10
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    \$\begingroup\$ Not my circuit! I'm not a fan of Tesla coils, mostly because the people I've met operating them were untrained in the hazards. I do like tube circuits, though. This caught my eye because I recently wrote about triodes and JFETs: medium.com/supplyframe-hardware/… \$\endgroup\$ Feb 11, 2017 at 20:26
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An afterthought, does it make more sense to think of the whole circuit as a cascode?

It's an example/spin-off from a cascode mixer. You alter the cathode voltage (with the MOSFET or JFET or BJT or a.n. other tube) and that modulates the amplitude of the signal on the anode. Here's a more conventional example using JFETs: -

enter image description here

The only difference with the pentode you are using is that it is self-oscillating with what looks like the lampbulb acting as a stabilizer.

But basically it's a cascode mixer.

what is the consequence of not having an Rs between the the source pin and ground? Should there be one, ideally?

The MOSFET is fairly stable due to the feedback of the 22 kohm and the 1 kohm so you don't really need a source resistor but adding one wouldn't make things worse but, it should only be about a kohm or so maximum.

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    \$\begingroup\$ If you do add a source resistor it should probably be only a few ohms - all the anode and grid current of the tube is supplied by the FET. There's enough grid current to heat the lamp sufficient to stabilize the oscillator. I would also think the circuit is prone to exceeding the 200V max Vds of the FET. \$\endgroup\$ Feb 11, 2017 at 19:29
  • \$\begingroup\$ @KevinWhite yes you are right, I didn't realize the GU-81M could run from such a high voltage supply. OP take note! \$\endgroup\$
    – Andy aka
    Feb 11, 2017 at 19:45
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There is a load for the mosfet - the whole anode circuit of the tube. The connection is made via the electron stream through the vacuum. The Tube is an oscillator that obviously generates a high voltage. The mosfet is an amplitude modulator. The mosfet is biased for plenty of current Id when the audio input is 0V.

A guess: The high voltage output generates a continuous plasma arc in a proper spark gap. The audio signal makes that arc to speak or sing.

ADDENDUM: In this circuit mosfet's load is an oscillator. The cascode amplifier interpretations are bad just in this case, because the vacuum tube has his own signal and runs highly non-linearly. The cascode mixer instead is a good simile. The nonlinearity makes the amplitude modulation possible. The oscillator takes all available current and the availability is modulated.

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  • \$\begingroup\$ There is no spark gap, it's simply a high voltage anode supply. \$\endgroup\$ Feb 11, 2017 at 20:16

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