How important is the quality of a triode used as a cathode follower

[Miles Prower]

Originally posted by Johan Potgieter
To broaden my horizons ... I presume we are still basically talking of hi-fi amplifiers, so where does class AB2 really come in? As you gather I have never used them as domestic amplifiers; I would accept that as theater units they have a place. On the other hand, class AB can still give several hundred watt. A GEC class AB2 amplifier of 700W is giving a maximum of 5% distortion. Is this acceptable for the guitar scene - otherwise?

Regards.

From the 807 STC:

Vpp= 600Vdc
Vsgsg= 300Vdc
Vgk= -30Vdc
Pi= 100mW (G-2-G)
Rl= 6K4 (P-2-P)
THD= 3.5% (before NFB!)
Po= 80W

That is pretty good, I'd say. Especially when you consider that the THD before feedback is the same as the minimum given for the 6V6, and you know its reputation for audio goodness.

I was considering doing an 80W, Class AB2, 807 design. However, I opted for the even better Class AB1 operation since I really don't need 80W, and the 30W I get from the design I have is quite loud enough. (That design states that the Po is 26.5W, but since it's equipped with grid drivers (6SN7 cathode followers for DC coupling and current drive for slew rate considerations) it will go far enough into Class AB2 to saturate the OPT cores at their rated max of 30W. This extra bit of headroom means that this amp'll "go to 11".

With an 807, there really isn't any reason not to Class AB2 it if you need the power. Of course, Class AB2 can also be used with 807's big brother, the 814, if you really need lotsawatts. And, of course, you have no choice in the matter if you're using RF triode PAs like the 811A, 812A, 809, &c.

And don't forget that sandy amps use Class AB2 all the time.

[Johan Potgieter]

Thanks folks.

Yes, I would say that 3,5% distortion is acceptable at 80W (before NFB). So the drive can keep it under control. Would I use it .... I will at least keep this input in mind!

Originally posted by Miles Prower
And don't forget that sandy amps use Class AB2 all the time.

I deserved that! (Apart from the fact ....... Oh OK!)

[Tweeker]

I have seen data where a high quality mosfet is tested against a high GM tube. The slight edge went to the mosfet. In this testing the load was a fixed resistance. I find that the best place to use a mosfet follower is to drive the grid of a power output tube. Especially a tube that draws grid current. In this situation the load is not constant and go from megohms to ohms when the grid transitions positive. If you are going to use a CF, use one that has a high peak current capability. The prevoiusly mentioned vertical output tubes are best here.

Its not practical with filamentary tubes, but could one not get a more constant (too low) load by driving the tube in grounded grid?

[EC8010]

Yes, and it has been done, but the compound output device becomes a cascode and has a high output impedance, which is less than ideal for the output transformer.

[cerrem]

I have had good results usinga a 6V6 as a follower to drive output tubes in AB2....
I keep it wired as a pentode.....
From a distance it looks like a 6SN7 driver valve...since it is similair size....

Chris

[ray_moth]

It occurs to me that a small signal pentode (e.g. 6AU6) might make a dandy cf in pentode mode, for direct coupling to an EL34. The EL34 could be in either triode or pentode mode. A 6AU6 triode-strapped might be pretty good too, with its gm of ~4800 umhos and mu of 36. Anyone tried this?