Mu Follower with a fixed bias upper triode

[Helmholtz]

Since the amplification comes from the lower triode (graphically in the schematic) there IS amplification of the ripple.

No, ripple is attenuated (rejected) in both cases.

To compare gain you need to feed the input of the circuit, i.e. the lower grid.

What do you mean with "fully" bypassed? Larger cathode cap?

[Chuck H]

What do you mean with "fully" bypassed? Larger cathode cap?

Yes

[Chuck H]

What NFB?

How much does gain change and what frequency did you use to test the gain?

Not looking at it right now but the reduction is about 10% for fixed bias. Tens frequency was 700Hz.

[Helmholtz]

Not looking at it right now but the reduction is about 10% for fixed bias. Tens frequency was 700Hz.

THe very low value 680p coupling cap forms a HP filter together with the bias resistors.
With original resistor values 470k/1M the corner frequency is around 700Hz, i.e. gain is down by 30% at that frequency.
With cathode bias the corner frequency should be much lower because of the bootstrapped 220k grid resistor.

So at low frequencies gain will be lower with fixed bias.

[Chuck H]

THe very low value 680p coupling cap forms a HP filter together with the bias resistors.
With original resistor values 470k/1M the corner frequency is around 700Hz, i.e. gain is down by 30% at that frequency.
With cathode bias the corner frequency should be much lower because of the bootstrapped 220k grid resistor.

So at low frequencies gain will be lower with fixed bias.

Ah, but I wasn't using those values because I don't have that tube model. And as discussed earlier in the thread I increased the coupling cap value. My bias resistors are 820k/1000k and the coupling cap is 10n.

[Helmholtz]

Ah, but I wasn't using those values because I don't have that tube model. And as discussed earlier in the thread I increased the coupling cap value. My bias resistors are 820k/1000k and the coupling cap is 10n.

Without seeing your circuit/values I might waste time with false assumptions.

[Helmholtz]

The fixed bias circuit is not too different from a solid state circuit discussed here: https://www.ssguitar.com/index.php?topic=3373.0 (you probably need to join the forum to see the attachments) The advantage is, it works without a tweek if the JFETs are matched.

I'm familiar with that Fet circuit. I has lots of gain and provides smooth and symmetrical compression/overdrive. So makes a nice soft distortion cell.
The circuit description (including concerns) given by member "Roly" is excellent.

Now it's actually a different circuit from what we're discussing here.
The circuit is a µ-amplifier or half-µ stage. The upper Fet doesn't receive an input signal (gate and source are AC shorted) and only acts as a high impedance (CCS) load.

While with the µ-follower here the upper triode is used as a CF which makes for a much lower output impedance.

[Helmholtz]

Will the 6111 work with only 50V across it ?

Should work, but the low voltage limits available output swing/headroom.

The 6111 is a low µ triode (µ=20), so gain of the µ-follower will be a bit below 20, i.e. about one third of a bypassed ECC83 gain stage.

[Chuck H]

Ok. This is a forum "add reverb" project that was kicking around in my LTSpice stuff and I just arbitrarily stuffed the mu follower into it for the purposes of test for this thread. It seemed to work fine but if you see any flaw for the implementation please let me know. These schematics represent the partial cathode bypass for the lower triode. Full bypass was plugged in for test but not saved.

[SoulFetish]

Chuck, I think I might have a 6111 model if you want it.

[SoulFetish]

Should work, but the low voltage limits available output swing/headroom.

The 6111 is a low µ triode (µ=20), so gain of the µ-follower will be a bit below 20, i.e. about one third of a bypassed ECC83 gain stage.

right, since the circuit example was taken from an input stage. Headroom wasn’t a big concern.

[Chuck H]

Chuck, I think I might have a 6111 model if you want it.

That's very nice of you. But I haven't had much luck importing models for LTSpice. Something (or things) always seem to fail the process. I'm kind of a techno tard and my VCR blinks 12:00.

[Helmholtz]

right, since the circuit example was taken from an input stage. Headroom wasn’t a big concern.

As you typically don't want the input stage to distort, sufficient headroom is most important here.

If you feed a 1Vpp PU signal and the gain is 20, the output must be able to swing a clean 20Vpp output.
Maybe not a problem here, because gain is low. But should be considered.

[SoulFetish]

As you typically don't want the input stage to distort, sufficient headroom is most important here.

If you feed a 1Vpp PU signal and the gain is 20, the output must be able to swing a clean 20Vpp output.
Maybe not a problem here, because gain is low. But should be considered.

I understand what you're saying. My point was that this circuit, as drawn, is not dealing with large signal voltages. Like you mentioned, with a gain of 20, there's plenty of swing available at the output.
I'm pretty sure that I center biased the input of the bottom triode, and you'd clip the input before you hit the rails.