Marshall JVM205 Cathode Follower Death, Sprial Filaments and Internet Mendacity

[g1]

Not wanting to argue with the author, but I don't see follies as being the same as heresies.

http://www.geofex.com/Article_Folder...osfetfolly.htm

[nickb]

As one more extension to the MOSFET Heresies, I could point out that MOSFETs do not have heaters, and they make a dandy-fine DC follower.

See post here and articles on MOSFET Heresies at geofex.com.

..except that, at least in this case, the heater was not the problem. Besides, where's the joy in gathering around a cheerless, non glowing, stone-hearted MOSFET on cold Winter's day?

I fear we digress. That never happens around here....

[Gregg]

In a Marshall type amp the CF is not just a "follower". As very well known it distorts in a certain way which is part of "that" tone. An FX loop send stage is another story.
MOSFETs are nice to play with however they don't distort the same way as a tube CF. In order to make a MOSFET distort like a tube CF it needs some additional tweaking. I think I posted this article somewhere but here it is again:

http://media.amtelectronics.com/tver...mt-warm-stone/

Another one (in Russian) with plenty of scope pics:

http://milas.spb.ru/~kmg/irf.html

[R.G.]

In a Marshall type amp the CF is not just a "follower". As very well known it distorts in a certain way which is part of "that" tone. An FX loop send stage is another story.

I have never personally done A-B testing by instrument or ear on a CF and a SF in a Marshall type of amp to know this either way. I'd like to read up on this a bit. Can you give me some references on this point so I can go educate myself? If it's well know, I'd like to go know it.

MOSFETs [...] don't distort the same way as a tube CF. In order to make a MOSFET distort like a tube CF it needs some additional tweaking.

Yeah, MOSFET followers are probably cleaner, as they have much more gain to convert to feedback to linearize themselves. I suspect that if there is an audible difference between CF and SF as a tone stack driver, the difference will be lack of distortion, as opposed to different distortion, unless the signal swings are pretty darn big at that point.

As an amplifier, triodes are probably cleaner without feedback than MOSFETs without feedback, but then followers are always run at 100% negative feedback.

I've read some of the "make a MOSFET distort like a triode", but haven't tested it myself. I'm pretty sure that you would have to tinker with a MOSFET to get convincing emulation of triodes. The fact that triodes haven't already been replaced by MOSFETs argues that the emulation isn't yet "good enough".