Mosfet Follies Question...

chris_d · January 22, 2009, 02:27:18 PM

Hallo. I figured i would ask this here as RG Keen pops in now and then.

I am messing (out more accurately, about to mess) with some changes to a homebrew amp i built. Among the things i am thinking about is adding a cathode follower stage but without adding a triode to the mix. This led me to the Mosfet Follies.

So circuit-wise i am just making sure i understand what i need to do here a bit better. Basically i am just going with a Vox AC30-ish CF-driven tonestack.

If i am reading correctly, i can do this merely with the addition of an IRF820 and a 220R resistor, and no other changes to the circuit?

So off the plate of the last gainstage, i would run through the new 220R resistor into the gate of the IRF820, and hook the drain to the power rail, and the source to the same 56k cathode resistor that a tube would use?

Is there any reason to use a value other than a "normal" 1/2 watt for that 220R resistor?

Is there anything else i should take note of?

It almost seems like it is too easy? Am i missing something?

Thanks for any thoughts.

-chris

R.G. · January 22, 2009, 03:18:59 PM

It does seem too easy, doesn't it?

But it does work. Yep, hook the source to the same 56K resistor to ground a cathode follower would use. The 220R gate stopper needs to be as close to the gate of the MOSFET as you can get it. The drain is at B+, so be very, very careful that you don't touch it accidentally. Ever.

But yes, if you wire it up correctly, it works.

chris_d · January 22, 2009, 03:26:14 PM

Quote from: R.G. on January 22, 2009, 03:18:59 PM
It does seem too easy, doesn't it?

But it does work. Yep, hook the source to the same 56K resistor to ground a cathode follower would use. The 220R gate stopper needs to be as close to the gate of the MOSFET as you can get it. The drain is at B+, so be very, very careful that you don't touch it accidentally. Ever.

But yes, if you wire it up correctly, it works.

Fantastic! Thanks so much for the original article and the reply!

-chris

DougH · January 22, 2009, 08:35:43 PM

And I left out the 220R (

) and it still works great:

http://www.aronnelson.com/gallery/main.php/v/DougH/amp/chime/Chime+_n+Grind.GIF.html?g2_imageViewsIndex=1

I've done this on two amps and it works great. Put an insulator under it and mount it to the chasses via the drain tab (use insulating washers too). It works like a charm.

R.G. · January 22, 2009, 09:10:43 PM

The only reason the 220R is in there is that power MOSFETs in the source-follower mode can oscillate at VHF frequencies, way above what you can see on a 20MHz scope

.

A tube amp won't notice it, but the MOSFET itself can eat a lot of power at VHF, and do ugly things to communications. You won't even know it's on.

DougH · January 22, 2009, 10:06:42 PM

Well I may go ahead and add them then, R.G. Been a while since I read your article. Thanks!

petemoore · January 22, 2009, 10:33:26 PM

Looks like a darn good place to put a Mosfet to work !

R.G. · January 22, 2009, 11:53:31 PM

By the way, the IRF820 is an awfully big device. It works OK at this, but the gate capacitance is big.

Zetex and Supertex both make TO-92 MOSFETs rated to 500V that would probably be better as long as the B+ never gets over the rated Vds even with all tubes out, and the dissipation (volts x amps) is under the rated power dissipation.

See:
http://www.zetex.com/3.0/pdf/ZVN0545A.pdf
http://www.supertex.com/pdf/datasheets/VN2450.pdf
Mouser has them, about $1.00 to $1.50 last time I looked.

brett · January 23, 2009, 07:03:15 AM

A buck for the MOSFET and 2c for the resistors.
Nice.

chris_d · February 06, 2009, 04:19:42 PM

Just a quick update and a big thanks!

I just built an amp yesterday with this for a cathode follower, and it sure does work!

I ended up using an IRFBE30 for the thing, as my local store didn't have any of the IRF820 or Zetex devices, and i just happened to have a couple of the BE30s lying around that i pulled out of some device (don't remember what!) a long time ago. The capacitance is much higher, and the on, rise, off, fall times are all larger than the IRF 820, but it certainly works anyhow!

Basically it wound up being an AC30 preamp into a pair of cathode bias 6V6GTs, and it is a LOT of fun to play with. I also put a DPDT switch in it to switch between the Fender and Vox tonestack mid resistor location styles. A very useful switch i am and finding as well, adds an interesting flexibility to the voicing.

Anyhow, Thanks a huge amount RG for this article! To get two gainstages, a cathode follower, and a long-tail pair P.I. out of two 12AX7s is truly a handy feat of efficiency. To do it without needing to create a power supply for an IC-based solid state solution, is pure brilliance, IMO!

Thanks again!

-chris

R.G. · February 06, 2009, 06:38:05 PM

Hey, Chris, glad it worked for you!

chris_d · February 06, 2009, 07:58:59 PM

Quote from: R.G. on February 06, 2009, 06:38:05 PM
Hey, Chris, glad it worked for you!

I have just one (or two) remaining question(s) about it. I am pretty happy with it as is, but am wondering if there is any good reason for me to put a "better" mosfet in there, like the IRF820, or Zetex one? Am i likely to be suffering from any detrimental effect of the higher capacitances or slower on/off times of the IRFBE30?

Also, as a last minute substitution of necessity(or was it ordering absentmindedness?) i had to put a 68k on the cathode/source spot to ground instead of the 56k. Should i bother going back in to rectify that value substitution?

Thanks again!

-chris

R.G. · February 06, 2009, 10:48:04 PM

Quote from: chris_d on February 06, 2009, 07:58:59 PM
I have just one (or two) remaining question(s) about it. I am pretty happy with it as is, but am wondering if there is any good reason for me to put a "better" mosfet in there, like the IRF820, or Zetex one? Am i likely to be suffering from any detrimental effect of the higher capacitances or slower on/off times of the IRFBE30?

Also, as a last minute substitution of necessity(or was it ordering absentmindedness?) i had to put a 68k on the cathode/source spot to ground instead of the 56k. Should i bother going back in to rectify that value substitution?

Your ears would know if something was grossly wrong.

Smaller capacitive loading on a plate is always good; a 12AX7 has a nominal plate impedance around 66K, so you start getting audio-band rolloffs with loading over 120pF or so. However, just like an emitter follower makes the emitter resistor look bigger than it is by feedback, the source follower makes the gate capacitance look smaller than it is by the feedback action. I never came to a good conclusion about a cutoff on Ciss in this application. So I'd say that unless you are unhappy with the sound, leave it. If you like the effect, next time try a "proper" one and see if you like that too.

chris_d · February 06, 2009, 11:01:17 PM

Quote from: R.G. on February 06, 2009, 10:48:04 PM
Your ears would know if something was grossly wrong.

Smaller capacitive loading on a plate is always good; a 12AX7 has a nominal plate impedance around 66K, so you start getting audio-band rolloffs with loading over 120pF or so. However, just like an emitter follower makes the emitter resistor look bigger than it is by feedback, the source follower makes the gate capacitance look smaller than it is by the feedback action. I never came to a good conclusion about a cutoff on Ciss in this application. So I'd say that unless you are unhappy with the sound, leave it. If you like the effect, next time try a "proper" one and see if you like that too.

Cool, that is roughly what i was assuming, insofar as if it were *really* wrong i would probably know it to hear it already. Still, the input capacitance for the IRFBE30 is listed as quite high compared to the IRF820 even, at 1300pf, so i will definitely aim to build the next one with the "better" choice and hear what kind of a difference it makes.

But no, so far i am actually way more pleased with this build than i was expecting. Really sounds best cranked though! Next up, some alternate master volume experiments!

-chris

R.G. · February 06, 2009, 11:20:34 PM

If you really get into this, you may want to look at running your output stage in Class AB2. Using MOSFET followers to drive the grids of the power tubes is a little trickier, and you'll probably have to do some power supply work to generate +/-75 to 100V for the followers. But several people have built these and liked them very much. It softens the transition into clipping a lot.

chris_d · February 06, 2009, 11:29:32 PM

Quote from: R.G. on February 06, 2009, 11:20:34 PM
If you really get into this, you may want to look at running your output stage in Class AB2. Using MOSFET followers to drive the grids of the power tubes is a little trickier, and you'll probably have to do some power supply work to generate +/-75 to 100V for the followers. But several people have built these and liked them very much. It softens the transition into clipping a lot.

I will definitely look into that when i start kicking around ideas for the next build. Though i must say that a large part of what i like about the mosfets in this cathode follower position is the degree to which it simplifies things, and allows really a lot of mileage to be had from a very simple topology and tube collection. Even my perhaps imperfect version with the "wrong" mosfet has me thinking i may never use a tube for a cathode follower again... well unless it is one that is *meant* to clip a whole bunch.

This is a fine solution, IMO. Its ease of implementation and simplicity are fantastic.

-chris

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