6AU6 DC coupled to mosfet follower questions

[JWK]

I'm trying to figure out the parameters for coupling a mosfet follower to a 6AU6 which will be the input stage.

I'm looking at the RCA Receiving Tube Manual in the section "Resistance-Coupled Amplifiers". For a 300v B+ supply to the plate, it gives rp-220k, rg2-530k, rk-780. The cap bypass for the cathode is 13.2uF and the cap bypass for the screen is .077uF.

What I don't understand is that it gives an output voltage of 53 and a voltage gain of 200. I'm guessing this means that the input sensitivity is such that the tube can't take more than about .26v input before the grid starts conducting current. So yes, you might get a gain of 200, you just can't put too much into it, compared to the typical 12AX7 (which if I understand correctly, is about one volt). Is this correct?

How do you calculate what the voltage of the tube will be (where the mosfet follower will be biased)? You will need to know that so you can make the plate supply voltage for the mosfet about double, yes?

As a side note - if my guesses are correct regarding the input sensitivity, would it be worth the trouble to make a follower with say, a 10k pot before the input stage as a sort of sensitivity control? Pritchard uses this concept on his amps. I like the idea, but I would be concerned about extra noise. Would that add more hiss to the circuit? I know some people here use a 6AU6 as an input and probably just use their guitar's volume knob. I'm just trying to make it usable for as many different guitars as possible. I also like mostly clean preamp stages.

Thanks for any help.

[tubeswell]

An IRF820 will be a suitable straight substitute for a triode, if you put a 12V zener between the source and the gate (banded-end pointing to the gate) and a 100-200R resistor in series with the 6Au6 plate and the mosfet gate, and use a 220k resistor for the source load and AC couple it to the next thing/stage/tone stack.

[JWK]

Yes, I should have mentioned I plan on using an IRF820, as per the RG mosfet follies. My problem is figuring out how to make a transparent stage to provide a low Z, higher current drive for a bass cut circuit and volume pot. I wanted to direct couple to avoid another cap, but I don't want the follower to affect the 6AU6 at all. I also want to make sure the follower will be able to handle all the signal swing coming off the 6AU6.

[R.G.]

I'm looking at the RCA Receiving Tube Manual in the section "Resistance-Coupled Amplifiers". For a 300v B+ supply to the plate, it gives rp-220k, rg2-530k, rk-780. The cap bypass for the cathode is 13.2uF and the cap bypass for the screen is .077uF.

What I don't understand is that it gives an output voltage of 53 and a voltage gain of 200. I'm guessing this means that the input sensitivity is such that the tube can't take more than about .26v input before the grid starts conducting current. So yes, you might get a gain of 200, you just can't put too much into it, compared to the typical 12AX7 (which if I understand correctly, is about one volt). Is this correct?

Hmmm. I don't know. A quick glance at the RCA tube manual's plate curves for the 6AU6 shows it with about a 1ma plate current (quinting - it's hard to tell from the place in the curve), and a Vgk of about -2V under those conditions. So you'd have to go up more than 0.26V to get to grid current. I suspect it's from plate current limiting at the "saturation" knee instead.

How do you calculate what the voltage of the tube will be (where the mosfet follower will be biased)? You will need to know that so you can make the plate supply voltage for the mosfet about double, yes?

No. The MOSFET can "saturate" closer to the current axis than the tube can, so it will go lower in voltage than the plate can. The MOSFET needs about 3-5V to turn on, so it turns off about 4V under the B+, while the plate could go up another bit into full cutoff. If you jigger the DC voltage bias point down another 4V, it won't be the MOSFET cutting off. You can tinker the DC point by messing with either the cathode resistor or the screen voltage.

As a side note - if my guesses are correct regarding the input sensitivity, would it be worth the trouble to make a follower with say, a 10k pot before the input stage as a sort of sensitivity control? Pritchard uses this concept on his amps. I like the idea, but I would be concerned about extra noise. Would that add more hiss to the circuit? I know some people here use a 6AU6 as an input and probably just use their guitar's volume knob. I'm just trying to make it usable for as many different guitars as possible. I also like mostly clean preamp stages.

It's in general a bad idea to put a follower as your first signal stage from a noise standpoint. However, how much noise is there depends on a huge number of things. This is probably one of those "try-it-and-see" things. It will add more hiss. Whether this is objectionable depends on more than the circuit.

You may be able to dink with the gain by doing Rp/Rk/Rg2 not equal to the ones in the RC amplifier charts. The problem you're facing is using a first stage with a huge gain, then wanting it to be both low noise and clean for guitars with a huge range of input levels. Some humbuckers put out more than a volt.

[tubeswell]

Yes, I should have mentioned I plan on using an IRF820, as per the RG mosfet follies. My problem is figuring out how to make a transparent stage to provide a low Z, higher current drive for a bass cut circuit and volume pot. I wanted to direct couple to avoid another cap, but I don't want the follower to affect the 6AU6 at all. I also want to make sure the follower will be able to handle all the signal swing coming off the 6AU6.

That's what I understood - see schematic

[J M Fahey]

Yes, I understood the same, and add: the advantage of putting a "something" follower would be to have slightly more gain available (not needed here) and/or be able to drive a complex tone control network or use a much lower volume pot value, if such a thing is needed.
But in this case, being a simple volume pot and a simple bass cut circuit, maybe it's not necessary.
jm2c