Mosfet drivers prototyped

[Gingertube]

Yes - remember that MOSFETs are an "enhancement mode" device meaning that you need to apply bias to turn them ON, a positive voltage at the gate with respect to the source (for an N Channel MOSFET).
Quite unlike a tube which is a "depletion mode" device meaning you need to apply bias to turn them OFF, a negative voltage at the grid with respect to the cathode.

So if your mosfet gate is less negative (i.e. more positive) than the source then things are OK.

Cheers,
Ian

[exclamationmark]

Are you absolutely positively sure the pinout is correct? One of the things that really irked me about mosfets is the drain and source are right next to each other in most cases, with the gate off to the side somewhere. I've hooked up many mosfets thinking they were transistors (ie, gate in the middle), and the schematic symbol always causes some confusion...

[Tage]

Yes I know the PITA of realizing that I've designed something with pinout for a mosfet only to realize that no Mosfets have pinouts like transistors. I learned that one with pedals...

[J M Fahey]

Maybe that's true in the pedal world, dunno, but I've been making MosFet power amps for ages and one Blessing I found is that TO220, TO218 and TO247 MosFets *do* have the same pinout than their Bipolar brethren, so much so that it allowed me to gracefully recycle my age old PCB designs into modern ones , just by increasing bias and adding gate stopper resistors.
In fact, any old amp with, say, 2N3773 (classic powerhorse I used a lot) which comes back for repair, leaves the bench with shiny new plastic devices, which to boot can be mounted in the same heatsinks and PCBs.

[Tage]

yeah power transistors are often the same with the base on one side and the emitter and collector together. but with smaller transistors the base is usualy in the center...

[Tage]

Sorry to drag this oldish thread up but I have a related question, really the original one stated more clearly...

Waht voltage will my Drains rest at with -65V at the gate? should I read my Positive voltage at my drain pins or is do they "swing" up to that voltage with an applied AC signal on it's up going swing?

I ask this because no matter what I do with my circuit it doesn't change, there is no shorts and no leaking caps or bad Mosfets. I'm really at a loss for why my drain voltage is virtually the same as my gate voltage, unless it is supposed to be until an AC signal is applied.

[Gingertube]

Tage,
The Drain Volts should not move with signal, C7 from the schemo on post #1 should keep it steady.
The source should sit at about -68 to -69 Volts such that the gate is +3 to +4V positive with respect to the source. The audio signal applied at the gate will appear at the source (coz it is a source follower) and sit on the approx -68 Volts.
For best performance I would suggest that the drain should sit at a minimum of say +25V. Adjust (decrease) the dropping resistor feeding C7 if it is below this. The steady state idle current will introduce a voltage drop across that resistor feeding C7. If the resistor is too big then you get too much voltage drop in theat +ve rail. The idea here is that when the signal at the source swings maximum positive (to about 0V) you want 25V minimum left across the Mosfet (drain to source) so that Crss (reverse capacitance from drain to gate) is reasonably constant and not being modulated by the audio signal - otherwise you can get some high frequency distortion introduced to the output signal.
Hope this answers your question.
Cheers,
Ian

[Tage]

Yeah that is what I figured, maybe I have that resistor value feeding C7 set too high, it just seems very strange that I get negative voltage at my drain, even though my source and gate voltages are proper. I'm gonna reduce that resistor value tomorrow night and see if I can get positive voltage there, I think I've just been dropping too much voltage once the Mosfets are drawing current.

Thanks again Ian.

Tage

[Gingertube]

That resistor is labelled as R16 on your schemo and is shown as 68K. The 2 source followers are going to try to run at about 2mA each giving 4mA total.
R16 will try to drop 272 Volts at 4mA current draw. That is almost certainly your problem, try a 33K or less till you get around +25 to +40 'ish on the drain.
Cheers,
Ian

[lowell]

Did you ever get this running? How did it work out? What power tubes are you using? PI? Curious as I've been wanting to do this myself.

[Tage]

Well I did get my driver circuit all good, now with a proper +60V rail for the drains, who knew one resister caused all my grief. I am still finding time to get the rest of my chassis wired up but as soon as I plug in I'll report back with how it performs.