FET amp modelling, the next step?

[gez]

High Voltage MOSFETs.  I wonder how well they'd perform as class A output stages?  Cheap enough to find out...

http://www.rapidonline.com/productinfo.aspx?kw=47-0484&tier1=Electronic+Components&tier2=Discrete+Semiconductors&tier3=MOSFETS&tier4=SuperMESH+high+voltage++power+MOSFETs&moduleno=72375

[R.G.]

You might want to read:
http://geofex.com/Article_Folders/sstremolo/sstremolo.htm
http://geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
ftp://www.zetex.com/data_books/thru.hole/zvn0545a.pdf
http://www.passlabs.com/ - Zen amplifier
http://sound.westhost.com/project36.htm
http://www.blueguitar.org/new/articles/other/hi_v_ss.pdf

MOSFETs good for followers and anywhere where the high capacitance from gate to source will not give you a problem. They do not bias or distort like tubes. But that leaves some places. The JFET/MOSFET cascode is a good candidate for 12AX7 replacement, and possibly for power tube replacement with a high Vgsoff JFET.  More work is needed on this last - see the blueguitar article.

[gez]

Thank you RG, amps aren't really my thing so I look forward to learning something!

[Roland]

They definetly are good for power amp stages. Some pretty famous designs have found use by guitarists. The legend amp that Billy Gibbons used had a mosfet power amp. The H&H power amps van halen used to amplify his already distorted tone were mosfet.

[Thomas P.]

Aren't there depletion mode MOSFET's?

I learned about it but as you can imagine physicians are teached very theoretically, so I don't now if there are "consumer" devices?

Regards,
tomboy

[Mark Hammer]

I'm still waiting for people to implement transformers on the output stage and provide "true" presence controls.

[billings]

There is a lot of room for improvement in simulation of the output stage apart from the effect of the output transformer.  E.g. the even order distortion of the JFET isn't what you get out of the push pull pentodes.

Would you necessarily need a whole transformer?  Could a choke provide the desired effect?

[R.G.]

I've been through this technology a bit. Here's a random summary.

The characteristic curves of a pentode and a power MOSFET look similar. Pentode curves are spaced by a 3/2 power law and MOSFETs are spaced by a square law. But they are similar, in that the device has a so-called triode region at low voltages that changes to a flat current  as voltage across the device increases.

It is seductive to think that there must be some way to hook up a MOSFET to drive an output transformer like a pentode does. I've speculated on that a lot, using things like a JFET or triode to drive the MOSFET and the MOSFET to play pentode. A simple first order setup uses a  high Vgsoff JFET in cascode with a power MOSFET. This gives you depletion mode biasing as well. A low-mu triode can do a similar thing, with perhaps a 12AU7 cascoded with an IRF820, for instance.

I've never tried that into an OT yet. However, it should not be too hard to get an octal relay base, strip off the relay, and hack onto it a 6L6-faker. It might be possible to use a low power pentode like a 6JD8 or EF86 and use that to diddle the MOSFET and get a real vacuum tube transconductance amplified in current in the MOSFET.

OT's are part of the tube amp sound, and this might help. Now that Workhorse amps are starting to show up in the USA, I may be able to find a "victim" for some of this kind of experimentation. We'll see. I think you'll need the whole transformer. A choke by itself is just a reactance. An OT is much more. It's a combiner and an impedance converter, as well as a canceller for certain distortions.

Depletion mode MOSFETs exist, but they are quite low power, not made in the high power needed to fake a power pentode. All of today's power MOSFETs are enhancement mode.

It's funny. I've researched tube amps for a long time, and the harder you look for where the magic is hidden, the farther away it seems to be. I've bored right down to the bare metal on every part of a tube amp, and by the time I completely understood every part, the magic bits were not hidden there, like trying to understand a forest by knowing each tree more and more intimately. At some point, you lose the forest in knowing the tree. But back way out and look wider and the forest reappears.

[amz-fx]

High Voltage MOSFETs.  I wonder how well they'd perform as class A output stages?  Cheap enough to find out...

Gez,  I did some experimentation with this using some depletion mode mosfets to make a high voltage mini-booster. There is a schematic of one of those in my mini-booster newsletter (for anyone who has a copy)...  there are some advantages; headroom being the most obvious.

regards, Jack

[gez]

Thanks for all the replies.  When I posted this thread I was thinking along the lines of Mark, ie using an OT (feasible given the high drain source voltages these can take).

I really appreciate your input RG, that's a cool idea about cobbling something together to drop in as a replacement.

Jack, I'll be sure to check your article.  Thanks.