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Output penthode with MOSFET driver

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emosms

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Joined 2010
#1
Just researching now. Anyone tried this?
- http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

mosfol2.gif

A power MOSFET driver for an output tube.

Moreover - an Op-amp driving the MOSFET.
Op-amp as a preamp (for a SE amp) and as a phase inverter (for a PP amp).
 
#4
The DC voltage of gate (if source follower DC coupled to previous stage) is almost the same as power tube's grid voltage: few ten (even near to 70-80V) negative (!) Volts.
The swing of driver (in this case opamp) output peek-to-peek equal (or large) than double of the power tube's bias voltage, even 140-160Vpp.

Is any regular opamp fulfil its?
 
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#6
I suspected so.. (high gate voltage).
The point was not that much to have the benefits of the driver, but actually to spare a tube.

I have 2 tubes in the preamp, then go to 'line level' with a MOSFET follower/ buffer (from the same link above.
Then there is a solid state EQ and send-return, then the power section.

This is a layout for a hybrid guitar amp (chip power amp) but I wanted to design the chassis so that it can be extended to a tube power section, still keeping the same size.
The op-amp part manages trivial stuff like EQ, send return and dry-wet mix, instead of extra unnecessary tubes.

One option is to use triode + pentode power tube, the other was to find a solution (the MOSFET driver + something...) and make a use of some 6p14p/ el84 at hand.

Now the problem is how to drive the ...power driver 😄😄😄, or just abandon that route.

Or any other idea how to drive a power penthode with a simple solid state solution.
 
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#7
I see... Well then you really don't need that much voltage swing, direct coupling of the OP amp isn't necessary as I understand it, only negative bias supply for the power tube might be handy... But still I'd rather stick another cascoded 12ax7 or some signal pentode to do the job, maybe that should be sufficient to drive el84 SEP...? Or use some combined signal/power tube like pcl86/14gw8?
 
#9
The link suggests the fet should not be used in a gain stage.
- http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

But... that gain stage is not supposed to add overdrive. I know I am missing all the sonic stuff one can get by overdriving phase inverter, e.t.c., but anyway.

If another MOSFET can drive the voltage to the MOSFET driver, haven't seen tried schematic for it.
I've built the MOSFET follower/ buffer from the link 2-3 times and it is ok.

Another issue is the power dissipation.
The MOSFET as a buffer output can work even without a heatsink.
If the voltage driver before the power driver is fed by the HT+ and needs to drop some voltage across, I don't know...

Actually I can't design such a stage and almost anything, except simple op-amp stuff and copy paste working circuit snippets from here and there 😄
 
#11
emosms said:
The link suggests the fet should not be used in a gain stage.
- http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
Click to expand...
That page is written for beginners with limited circuit knowledge, and it is aimed at guitar not hi-fi. The LND150 is quite popular as a substitute for a valve gain stage.
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm

emosms said:
Actually I can't design such a stage and almost anything, except simple op-amp stuff and copy paste working circuit snippets from here and there 😄
Click to expand...
You can buy high-voltage opamps, but they are normally limited to about 100Vpp output.
 
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#12
Does it have to be exactly lnd150?
Seems it cannot be irf820/830 (enhancement mode)
And if it can be set almost like a tube, so I guess it could make a cathodyne pase inverter.

I thrashed the local second hand market and might get some pcl86, having also some ECL86 and 6p14p (el84 analog).

The point is to be able to run the 6p14p with mosfets and have an amp with 2 preamp and two power tube sockets.
It would be a subproject on its own, on a separate board that could be swapped with the pcl/ecl board
 
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#13
LND150 - any need to choose SOT-89 (1.6W) over TO-92 (740mW)?

Not into the calcs yet, but if it is used as:
  • preamp stage to elevate the gain from op-amp line levels
  • cathodyne phase inverter (or longtail)
  • power tube driver

Possible layouts (I can think of):
- Option 1/2: lnd150 pre -> 1(2)x lnd150 pi (cathodyne/longtail) -> 2x penthodes

Extra thoughts:
  • is there any need for a MOSFET driver after the phase inverter, or the MOSFET PI is pretty ok to drive the tubes
  • could the phase inverter be irf820 (some saturation mode transistor)
 
#15
I wrote it - to substitute a double triode (the gain stage and phase inverter).
Then I can make amp with the following layout:
  • 2 preamp tubes (gain stages only)
  • 2 power tubes, case the tubes are penthodes only (6p24p/ EL84)
  • 2 power tubes, case ecl/pcl86 and no mosfets, the tubes have triodes

Another option:
- op-amp balanced output (phase inverter) -> 2x lnd150 gain stage -> 2x output penthode.

This one seems the neatest to me IF lnd150 can drive the tubes alone, preferably to-92 package.
At first glance there is not much current going there, but I haven't looked through it in detail.
 
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