How would a cathode follower output section sound for guitar?

[Austin]

Plenty of people have them in the hifi scene and I am sure everybody who plays guitar and builds tube amps and reads enough has wondered the same thing. What would it sound like pushed to the max compared to the grounded cathode topology? Would it be better for a for a clean amp like a PA amp, or might the sound be too sterile even for that? Is the cathode follower output section the most solid state sounding way of using a tube? Or does it have tube sweetness too but because of the huge drive signal required and the extra cost that requires has just never been implemented in a comercial design? I mean guitar amps are usually not built short on gain.. Except maybe a Fender Blues Deluxe or something..

[Alan0354]

Do you mean using cathode follower and skip the transformer? If so, you don't need much voltage gain, but the output impedance of the tube might not be low enough. According to 6L6GC, the gm is 5200umho, cathode impedance is 1/gm=192 ohm. If you want to drive an OT, you might have a problem with heater to cathode voltage. I looked at the 6L6GC, it's only 200V.

[Austin]

Do you mean using cathode follower and skip the transformer? If so, you don't need much voltage gain, but the output impedance of the tube might not be low enough. According to 6L6GC, the gm is 5200umho, cathode impedance is 1/gm=192 ohm. If you want to drive an OT, you might have a problem with heater to cathode voltage. I looked at the 6L6GC, it's only 200V.

I mean using an output transformer attached to the cathode instead of the plate, for a pp output stage the ground through the center tap and the b+ attached directly to the plate but otherwise the same. I would love to skip the output transformer someday, one thing at a time here...

[Alan0354]

You have to find a lower turn ratio OT so you don't have to swing a lot of voltage.

[Steve Conner]

Too much speaker damping, too much feedback cleaning up the tube's characteristics. It would sound pretty dull.

[printer2]

I asked the same question.

mosfet source follower output stage. - diyAudio

[Steve Conner]

You might have asked the same question, but you asked it in a different context: hi-fi. Negative feedback and high damping factor are desirable there, but in guitar amplification they make for a somewhat boring sound.

[R.G.]

It would probably be helpful to look up the articles on the design of the McIntosh output stage, which is half plate-coupled and half cathode-coupled. There is a lot of comparison blather in there before they present the "Oh! Just use half of each!" conclusion.

[Austin]

You have to find a lower turn ratio OT so you don't have to swing a lot of voltage.

I thought so too.

cathode-follower output stage part-2

Here he says: "Cathode-follower power amplifier design: Step 1
Here’s the usual cathode-follower-power-amplifier design thought process: since—as compared to a grounded-cathode amplifier—the cathode follower offers a much lower output impedance (Zo), the output transformer’s winding ratio can be equally reduced, allowing the use of 500-ohm primary instead of the 5,000-ohm primary needed for a grounded-cathode amplifier. Right? Absolutely dead wrong! It is as wrong as believing that holding a magnifying glass over a gold coin will not only make the coin appear larger, but weigh more as well. Just as the coin’s actual size and weight remain unaltered, the triode’s transconductance (gm), amplification factor (mu), and plate resistance (rp) remain unaltered, as nothing inside the tube’s envelope has changed."

And when he explains it like that it makes sense somehow..

One thing you could do that is kinda like cheating is you can use the dc resistance of the transformers primary as the cathode bias resistor which I thought was a neat trick.
Anyway it might be fun to test it out since according to the link I should use the same transformer impedance that I already use for a plate loaded setup that I already have.
So if 35:1 turn ratio equals 9800 ohms into 8 ohms then to get 20 volts swing for 50 watts I would need the driver to swing more than 35x20=750 volts divided by two for pp equals 350 volts for each side. Wow, can that be that right? I don't know how you would ever get that much. Maybe choke loaded el34 driver tubes with lots of volts on them.

[printer2]

You might have asked the same question, but you asked it in a different context: hi-fi. Negative feedback and high damping factor are desirable there, but in guitar amplification they make for a somewhat boring sound.

Well the first guy posted a schematic with no NFB, another followed his lead and also went no NFB (the linked thread), another that said,

I built a SE tube + MOSFET follower parafeed amp with a fixed ratio of feedback to the tube' plate so it can interact with the load in a scaled way, as if the load was scaled up relative to the tube's plate resistance. It acts exactly like a SE amp using a power tube.

One said the amp sounded very tube like, whether that is due to the transformer remains to be seen. From my limited exposure to the hifi tube world, I have come across some that go the low dampening factor, low NFB route. I am not quite ready to write this one off yet.

[J M Fahey]

It will lose the tube "punch".
Will sound quite solid state-y.

[Alan0354]

I thought so too.

cathode-follower output stage part-2

Here he says: "Cathode-follower power amplifier design: Step 1
Here’s the usual cathode-follower-power-amplifier design thought process: since—as compared to a grounded-cathode amplifier—the cathode follower offers a much lower output impedance (Zo), the output transformer’s winding ratio can be equally reduced, allowing the use of 500-ohm primary instead of the 5,000-ohm primary needed for a grounded-cathode amplifier. Right? Absolutely dead wrong! It is as wrong as believing that holding a magnifying glass over a gold coin will not only make the coin appear larger, but weigh more as well. Just as the coin’s actual size and weight remain unaltered, the triode’s transconductance (gm), amplification factor (mu), and plate resistance (rp) remain unaltered, as nothing inside the tube’s envelope has changed."

And when he explains it like that it makes sense somehow..

One thing you could do that is kinda like cheating is you can use the dc resistance of the transformers primary as the cathode bias resistor which I thought was a neat trick.
Anyway it might be fun to test it out since according to the link I should use the same transformer impedance that I already use for a plate loaded setup that I already have.
So if 35:1 turn ratio equals 9800 ohms into 8 ohms then to get 20 volts swing for 50 watts I would need the driver to swing more than 35x20=750 volts divided by two for pp equals 350 volts for each side. Wow, can that be that right? I don't know how you would ever get that much. Maybe choke loaded el34 driver tubes with lots of volts on them.

I am not familiar with tubes to comment on the sound. I read through 3/4 of the article and it does not make sense.
1) What is rp anything to do with the cathode output impedance. the output impedance of the common plate stage( cathode follower) is 1/gm according to material I read which is the same as BJT. The guy kept talking about rp that is in the plate circuit. For cathode follower, plate is shorted to B+ which ideally is 0 ohm, so the plate load is rp//0ohm=0 ohm!!! There is no gain in the plate end!!! Why is he keep talking about load line. As I said, I am still studying about tubes, BUT if it were BJT or FETs, if you are in common collector or drain, the whole thing on the collector/drain is irrelevant as long as the voltage is high enough for the transistor to function. I don't see any different with tubes here.
2) The cathode impedance is 1/gm, that is the output impedance of the tube. Nothing is said about matching Zout to speaker!!! And what is rp to do with output impedance in common plate stage ( cathode follower)?
3) Of cause if you try to use a 12AX7 cathode follower to drive 600 ohm, it will sound awful if it is beyond a few tens of volts It is just common sense if you run the tube at 1mA current, you can only drive 0.6V on the negative swing before the tube turn off( no more current!!!). Of cause it sounds bad if you clip the low side!!! This is true for BJT and FETs. This is a single end circuit, it can only sink the amount that is conducting in the tube at quiescent state.....1mA for most 12AX7 circuit!!! That has absolutely nothing to do with cathode follower cannot drive low impedance. You find a tube that run 50mA and drive 600 ohm, you can drive to -30V!!! If the cathode follower is low distortion, you should be able to get +/-30V swing with low distortion..........If the other condition permitted.

Then he was ranting on how people's miss conception about matching rp to speaker in normal power amp. I don't think I read a single thing about match rp!!! You use transformer to step up the impedance from the speaker to get the load to obtain max power from the tube.

This is what I think looking at it as general electronics. Let me know if there is something unique about tubes that I don't know. I am not disagree about not sounding good, I just disagree with the guy's reasoning.

[printer2]

It will lose the tube "punch".
Will sound quite solid state-y.

So a tube will sound solid state-y if it is not driving a transformer?

[Steve Conner]

Two things you are missing:

Even if no NFB is used at the circuit level, a cathode follower has internal negative feedback. The gain of the tube is "degenerated" down from whatever it is in common cathode mode, to a little less than 1. The output impedance falls (to 1/gm as Alan points out) and the amount and order of distortion harmonics is also changed. Both effects are just the same as if you ran the tube in common cathode and brought the gain down to 1 with external NFB. That is way more NFB than we would use in a guitar amp.

The small-signal output impedance might be 1/gm, but the optimum load impedance for maximum power output is a large-signal consideration, it is still the same as if the tube were used in the normal way.

The McIntosh topology (and probably the Circlotron too) is a kind of compromise between common-cathode and cathode follower.

[Steve Conner]

Two things you are missing:

Even if no NFB is used at the circuit level, a cathode follower has internal negative feedback. The gain of the tube is "degenerated" down from whatever it is in common cathode mode, to a little less than 1. The output impedance falls (to 1/gm as Alan points out) and the amount and order of distortion harmonics is also changed. Both effects are just the same as if you ran the tube in common cathode and brought the gain down to 1 with external NFB. That is way more NFB than we would use in a guitar amp.

The small-signal output impedance might be 1/gm, but the optimum load impedance for maximum power output is a large-signal consideration, it is still the same as if the tube were used in the normal way.

The McIntosh topology (and probably the Circlotron too) is a kind of compromise between common-cathode and cathode follower.