Mosfet cathode follower

[R.G.]

Ok right on. Why is that? Is this not going to bias the following stage cooler?

Yep, what Steve said. You might be able to get by with a voltage a little negative, but realistically you'd get into less potential trouble if you made it at least as far negative as it can go positive.

You don't really need to turn the tube off more than it goes when it's completely not conducting, as it can't conduct less than zero. But figuring out exactly where that is gets tricky. Makes more sense to just give yourself some room.

Of course, in a cathode biased setup, that eliminates some of the attractiveness of not needing a minus bias supply.

Probably -40V to -50V would cover all the cases even allowing for increase in the cathode bias under max signal.

[imaradiostar]

Please correct me if I'm wrong but Lowell was originally talking about grid drive in a cathode biased single ended amp. The conversation since then seems to relate to a push-pull amp. I don't know if grid drive is worth trying on a single ended amp or not- what would you call that operating class?

I suppose it would distort a little prettier?

jamie

[Steve Conner]

Class AA2?

I think my assertion holds true for a cathode biased single-ended amp too. You have to be able to swing the grid below zero to turn the tube all the way off.

[R.G.]

Please correct me if I'm wrong but Lowell was originally talking about grid drive in a cathode biased single ended amp. The conversation since then seems to relate to a push-pull amp. I don't know if grid drive is worth trying on a single ended amp or not- what would you call that operating class?

I suppose it would distort a little prettier?

Good point. Yes, I realize now he said "champ".

Tubes in general distort more softly if you can drive the grid from a lower impedance source to make the transition into grid conduction more softly. You can make a 12AX7 clip with a laser-flat edge if you drive the grid from a high impedance source. I've seen triode circuits biasing the grid from a 470K resistor to B+. This was referred to as "running in clamp".

Whether this is nicer sounding distortion or not depends on other things as well, but it's possible.

Class AA2?
I think my assertion holds true for a cathode biased single-ended amp too. You have to be able to swing the grid below zero to turn the tube all the way off.

I like it! Oh, wait! I think Randall Smith may have patented that.

Yes, your comment applies for cathode biased single ended too. You need a negative supply to be able to turn the tube all the way off.

But... hmmm... single ended, small amp, mushy distortion good... maybe the soft, mushy not-quite-turned-off would sound good. Worth a try now that I think about it.
What do you think?

[lowell]

TH quick similar inquiry. What value source resistor should 1 use for the mosfet follower between triode preamp stages?

[imaradiostar]

How much current do you want to dissipate in the mosfet?

If the previous triode's plate is at 200 volts and the supply voltage is 300, you're going to drop roughly 100 across the source follower. With a standard 100k cathode follower style arrangement that means 300 volts / (100,000 ohms resistor+50,000 ohms mosfet)= .002 amps. This means the mosfet would dissipate about .2 watts. (.002 amps times 100 volts=.2 watts)

Scale it up and down depending on how much heat you want to make and how low you want the output z to be. If I understand the things I've read correctly I doubt there is a tonal advantage to a lower R since it isn't likely to draw grid current like a triode.

jamie

[lowell]

Ok where do you get 50k ohms for the mosfet? Also, the more current through the mosfet means a lower output impedance?

[tubeswell]

Also, the more current through the mosfet means a lower output impedance?

I would be interested in this as well. As far as a tube is concerned, the output impedance is a byproduct of the internal/dynamic plate resistance in series with the plate (load) resistor (and, IIUC if unbypassed, the cathode resistor) from what I can gather. So I could see how the drain resistor and source resistor could form part of the output impedance for a mosfet (because the a/c load is affected by them - is it not?), but I don't understand what the mosfet equivalent is of 'plate resistance' (- is that because there is no such equivalent in a mosfet?)

[defaced]

I'm a little late to the party, but RG gives an excellent description of how to approach the MOSFET follower design in this thread - third post down. I think it answers both of your questions.
http://music-electronics-forum.com/t16854/

[imaradiostar]

Ok where do you get 50k ohms for the mosfet? Also, the more current through the mosfet means a lower output impedance?

Maybe I'm wrong to think this but the mosfet gate and source are going to be more or less at the same potential- within a few volts anyway, based on how a mosfet works. If the supply voltage is 300 and the previous triode's plate is at 200 volts and is directly coupled to the gate of the mosfet, there will be about 100 volts dropped across the mosfet and 200 across the source resistor. If the source resistor is 100k ohms, ohms law tells us the mosfet has to be about 50k ohms at rest. I was just using ohm's law and didn't explain myself.

How does this relate to the source resistor? If the mosfet is trying to hold the source within a few volts of the gate so the current it (the mosfet) draws will be based on how hard the source resistor is "pulling down" on the source. A vox style 56k cathode resistor will draw more current than a 100k marshall style cathode resistor. If the mosfet could hack it, you could use a 1k source resistor and make some major heat. Of course I can't think of a practical reason to ever do that. Any 47k-100k value should be fine for driving a tone stack.

The output impedance is the source resistor in parallel with 1/gm where gm is the transconductance of the mosfet. For all practical circuits we tend to build, this will mostly look like the 1 over the transconductance of the mosfet or lower than we need, which is useful. RG's article has lots of good info on useful ways to use a mosfet.

jamie