Unity Gain Tube Buffer

[AKSA]

Hi RunEight,

Thank you muchly; this looks pretty good!

Tim,

If fed to the more sensitive 6SL7, probably running around -1V of bias, I'd be crossing three tubes, not two, and the poles would start to get tricky to manage.

I'd probably use something like a 6CG7 (nine pin 6SN7) with a mu around 20. Two of these, each with Av of 10, would give the OLG of 100 I'm after, but the question is, would feedback to the cathode of the first of these two triodes permit 80Vpp into that triode's grid?

If yes, then I'd precede the two triodes with a tube input stage to add the sonic overlay. Of course, any sonic overlay of triodes two and three would be utterly lost in the NFB; alternatively we could give triodes two and three a gain of say 5, and the input triode a gain, with no feedback, of say 5 again, giving us our gain of 26dB or so.

I'm sorry I've not been quick on this; it's a tricky design brief.

Cheers,

Hugh

[Sch3mat1c]

How's this cheezily-drawn (note excess or absence of coupling caps, you can figure out the biasing and stuff) idear. The sand hanging off the plate looks weird but think of that tube as a CF with the loads in the plate. NFB is wrapped around both tubes and the SSatanic part as needed. No signal loss between 1st K and output so it will indeed be a follower, voltage being limited to +V capability (uh, headroom) and wattage of the cathode resistor that output V appears across.

Tim

[runeight]

Well, I guess I am confused!

Tim, if the BJT here represents a more complex output stage (or even if not), then isn't this a fairly conventional global feedback loop into the cathode of the first stage?

Or if preceeded by a gain stage, then a hybrid unity gain power buffer with local NFB?

In which case, the way to do it is very well understood and the cascaded stage can be put together a number of different ways depending, as you say, on fiddling with coupling caps, etc.

OTOH, I really did think we were talking about an isolated, local NFB unity gain buffer that could swing 100vpp independently of what preceeds or follow it.

Hugh, which problem are we trying to solve?

[Sch3mat1c]

Look closely at the NFB. It *is* a follower! The 1st cathode must track the output voltage, thus signal input must be Vin + Vo, where Vin is the actual drive the 1st tube sees.

Indeed it is a common NFB arrangement. But most such arrangements leave some residual gain for line-level inputs; that's what your 6SL7 is for.

Tim

[AKSA]

Hi Runeight,

OTOH, I really did think we were talking about an isolated, local NFB unity gain buffer that could swing 100vpp independently of what preceeds or follow it.

Yes, that's precisely it!

Goal: To create a tube buffer to drive a SS PP Class AB output stage with sufficient NFB to smooth the SS non-linearities and crossover disjunction.

Why unity gain? Because I'm trying to:

1. Maximize OLG from just two medium mu triodes,
2. Absorb all this gain in the Triode1/Triode2/SS OP stage so as to reduce distortion to virtually unmeasurable levels.
3. Add on an SRPP front end (high detail, good drive, excellent overload), OUTSIDE the NFB loop, so as to add the sonic overlay of a triode.

The reason I wanted to use tubes in this buffer is that every attempt to use SS as voltage amplification is flawed sonically because of the very high, poor quality parasitics of nearly every transistor I've ever used in common emitter. This always mandates lag compensation, which is not required with a tube buffer. It is arguable that tubes might be best, in fact, because it should be possible to do this with mosfets/fets very neatly too.

I like your 6pi45 circuit, Runeight, looks very clever. I suspect it could be as easily done with a bipolar or mosfet diff pair, and this would bring voltages down considerably, which could be an advantage as it will eliminate the current sink.

Tim, your circuit is a conventional, cascaded circuit block and I'm well familiar with it. Your addition of an emitter follower buffer at the end is probably not required in this application as the drive to a double emitter follower is high impedance and can be effected capacitively straight from the plate. My doubts revolve around its overload capacity; I'm sure it may not take 80Vpp at the input.

Since the circuit is hybrid, there is no reason not to use the whole gamut of devices; triodes, pentodes, jfets, mosfets, bipolars. It could be quite an exotic mix!

Thank again guys. This is very interesting!

Cheers,

Hugh

[SY]

Now let me get this straight- you DO want to include the power emitter followers within the buffer feedback loop?

[AKSA]

Yes, SY, absolutely.

This to eliminate the non-linearities, particularly crossover, but also to compensate the wide variation in Vbe as the transistors swing into heavy conduction.

There will be phase shift here; inevitable. However, with fast transistors, the phase shift in emitter followers is very small, and it's possible, given the use of lower frequency tubes, that lag compensation need not be used.

Cheers,

Hugh

[fdegrove]

Hi,

This to eliminate the non-linearities, particularly crossover, but also to compensate the wide variation in Vbe as the transistors swing into heavy conduction.

If you precede the buffer block by an SRPP (inverting polarity), would it then be possible to take feedback, as a global loop, from the output of the power stage to the cathode of the bottom triode of the SRPP?

Just an idea floating in my head...

Cheers,

[AKSA]

Hi Frank,

Thank you for your input! You wrote:

If you precede the buffer block by an SRPP (inverting polarity), would it then be possible to take feedback, as a global loop, from the output of the power stage to the cathode of the bottom triode of the SRPP?

Not quite. The feedback point must be at the cathode of the first of the two stage buffer for correct phase. At the grid of the 1st tube of the buffer, assume inphase; second grid antiphase, input and output from output stage, inphase again. The feedback is taken from the output, inphase, and must be plumbed into the cathode of the first tube; not any stage BEFORE that tube.

Actually, I want to keep the very first stage, an SRPP, out of the feedback loop entirely. Why? I do not want the tube sonics submerged in high levels of feedback - the sonic kiss of death.....!

Cheers,

Hugh

[fdegrove]

Hi Hugh,

Actually, I want to keep the very first stage, an SRPP, out of the feedback loop entirely. Why? I do not want the tube sonics submerged in high levels of feedback - the sonic kiss of death.....!

Yes, I see what you want to do...

There was a mistake in my train of thought...
I completely overlooked the phase behaviour of the buffer block...
Silly me.

I do not want the tube sonics submerged in high levels of feedback - the sonic kiss of death.....!

Read you loud and clear.