Direct-coupled cathode/source follower driver in PP?

[Wavebourn]

Originally posted by gingertube

Waveborn has obviously been reading Allen Wright. He is showing a MOSFET version of Allens SLCF.

No. Wavebourn has obviously a diploma of Engineer Designer and Technologists of Radio Electronics Equipment from famous TIASUR.

I just loaded a source follower on a CCS and bootstrapped it with one more source follower so the main SF sees nearly constant SD voltage and current. As the result, distortions caused by variations of transconductance and capacitances are minimal.

Obviously Allen Wright had similar purpose in mind...

Also, if I remember right, George (Tubelab.com) used something similar. Sometimes very similar tunes may be made of 12 notes available for composers. 12 notes. How many elements are available for electronics designers?

[SY]

Allen will be the first to tell you that it's not original to him. There's a pretty similar circuit in Horowitz and Hill. In tube form, it certainly was pretty common in oscilloscopes back in the '50s and '60s.

[ray_moth]

SY, I should hace said that a pentode OP stage has a much harder time recovering from blocking than a triode stage because of the much greater amount of NFB used in the former case. The triode will clip but only momentarily.

I suppose what I really want to know is if there is any reason not to use direct coupled cf/sf drivers, apart from economy.

[SY]

No reason at all, beyond superstition.

[Yvesm]

Originally posted by ray_moth
. . .
I suppose what I really want to know is if there is any reason not to use direct coupled cf/sf drivers, apart from economy.

Build one, don't forget to insure that screens supply does not sag (easier if you do not use UL ) and you'll know the reason to use !

Listen to something with heavy percussions

Yves.

[Wavebourn]

Originally posted by ray_moth
SY, I should hace said that a pentode OP stage has a much harder time recovering from blocking than a triode stage because of the much greater amount of NFB used in the former case. The triode will clip but only momentarily.

Screen grid is a good sensor of overload: I usually put Vactrol there to sense current peaks by it's LED, and reduce amplification factor by it's opto-resistor. Also, bright red LED's grab my attention to too much input level. Triodes don't offer such a convenience, but I drive them with 1'st grid current, and a source follower does that fine.

[Bandersnatch]

hey-Hey!!!,
I have gotten good results with using a cascode pair of MOSFET's instead of a single every time I have needed such a thing. I even tried them as grounded source amplifiers. It seems to deliver the near-constant capacitance of a pentode without the need( and trouble ) of g2 current.
cheers,
Douglas

[oldeurope]

Hello ray_moth

Originally #1 posted by ray_moth
If not using transformer coupling, ... to supply sufficient drive current to overcome miller capacitance of triodes, or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?

A transformer does not solve these problems and Miller capacitance does not cause any problem in a PP design.
This is one advantage of push pull against single ended.

Originally #1 posted by ray_moth
... a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. It can be used to prevent blocking distortion ...

You'll get the blocking distortion at the grid of the cathode follower.
Thus it is not preventing blocking distortion.

Originally #1 posted by ray_moth
If not using transformer coupling, a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. ... or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?

There are a lot of good reasons not to go to AB2 mode, especially for triodes.
If you can live with the crossover distortion from AB1 to AB2 the follower is a good solution.

Kind regards,
Darius

[ray_moth]

Hi Darius,

A transformer does not solve these problems and Miller capacitance does not cause any problem in a PP design.

I don't follow that. Triodes have significant Miller AFAIK, whether they're SE or PP. It's pretty well negligible for pentodes, though.

You'll get the blocking distortion at the grid of the cathode follower.

That can happen, true, but it's easy to avoid.

There are a lot of good reasons not to go to AB2 mode, especially for triodes.

I had the opposite impression. Ithought that that triodes had more to offer by going into AB2, in terms of extra power, than pentodes.

In any case, I wouldn't want to run in AB2 for any length of time, but it does seem to offer a bit of useful headroom for transients. Better than ugly clipping, anyway.

[oldeurope]

Originally posted by ray_moth
Hi Darius,

...
That can happen, true, but it's easy to avoid.
...

How?

Kind regards,
Darius