DC coupled cathode follower - Design Q's

[exclamationmark]

As I understand it, a DC coupled cathode follower basically works on the premise of a CF 'stealing' current from the preceding stage because it's biased so hot, which then leads to compression/distortion as it tries to maintain it's equilibrium state. From my understanding, in order to get the desired effect of the DC coupled CF, all that's actually needed is to make sure the bias point of the CF is outside or near the Vgk=0 line?

For example, the application I intend to use it for is a tone-stack driver in my low voltage pre-amp (mainly for the tonal reasons though... output impedance is only like 5k without the CF). The B+ is only around 48v (from an old laptop supply run through a CLC filter) and the tubes are 12au7's. I've attached the load lines plus the roughly intended bias points. Is there anything I'm missing here in regards to "designing" (ripping off) one of these for low voltage use?

[Amp Kat]

I don't think so and feel free to post a schemo if you want. Lots of critiques to help and nobody is going to rob you on it I think. Obviously you have some type of gain maybe op-amp circuit before the follower as you won't get any gain from the follower and may be only adding noise and you'll likely need a recovery stage after the tone stack to make up for the loss in the stack and the CF but you should be fine.

[exclamationmark]

I should probably mention I'm planning to use the 'bootstrapped' variant featured on Merlin's webpage (my poor 12au7's need all the gain they can get!). Schematic attached!

[Merlinb]

You will want larger resistors on the anode of the first triode, otherwise your poor CF is trying to drive less than 5k, and it will struggle!

[exclamationmark]

You will want larger resistors on the anode of the first triode, otherwise your poor CF is trying to drive less than 5k, and it will struggle!

I'm not quite sure I understand this. Is this in reference to the positive feedback loop driving the split anode resistor? I'm assuming that if I increase the plate resistor to 50k (25K + 25K) the CF should see a load a of 12.5k, and I would literally just adjust the bias resistor of the normal stage and keep the CF load the same?

[MikeM]

I am curious about this.. I assume that one'd run a coupling cap after this stage?

Also Mr Merlin!! When would you recommend your "bootstrapping" over the regular dc follower?

[Amp Kat]

Hey Merlin what do you think about this bootstrapper for OP may be a tad better would offer a little better gain and drop the output impedance although not running the higher voltage could be a problem but same concept.

The Tube CAD Journal: Out of Control: Solid-state variation

[Merlinb]

I'm not quite sure I understand this. Is this in reference to the positive feedback loop driving the split anode resistor? I'm assuming that if I increase the plate resistor to 50k (25K + 25K) the CF should see a load a of 12.5k, and I would literally just adjust the bias resistor of the normal stage and keep the CF load the same?

The CF has to drive the parallel combination of:
(top-most resistor) || (lower resistor + tube)
The second term itself depends on whether the bias resistor is bypassed or not. Either way, with 25k+25k the load will be more than 12.5k. I guess around 20k, which is not TOO bad for a 12AU7. It has to drive the tone stack too of course (or whatever comes afterwards).

When would you recommend your "bootstrapping" over the regular dc follower?

Most of the time! But it's personal preference. I like the ability to remove the cathode bypass cap and still get gain equal to mu, and the crunchy sound you can get out of this topology, with a little tweaking.

Hey Merlin what do you think about this bootstrapper for OP may be a tad better would offer a little better gain

Maybe. I tend to stick to the usual suspects (ECC81/2/3) for guitar work (not ECC88 / 6DJ8). It's always an option though, especially at such low voltage.