previous: R.G. q{I still think each output tube sh... -- 1/14/2000 4:29 PM

Re: MOSFET source follower driver - help!

1/16/2000 11:57 AM

lion

Re: MOSFET source follower driver - help! Thanks a bunch for the answers!     I’m going to town sometime next week to get the MOSFETs and a few other things I need – meanwhile I might as well get things right with a schem/layout ect.     OK, let’s see if I got it right so far.     I’ll have to change the original bias circuit in order to   1) get –100V for the source supply   2) and still have the same bias voltage range at the end of the string   furthermore the voltages to the MOSFET sources should be individually adjustable - trimpots?     Source resistors 33K. Gate damping resistors 100 to 220 ohms.     I can derive the drain voltage in one of two ways   1) direct from B+ 430V   2) or by adding a positive string (resistor, diode,cap ) at the bias supply to get +100V     With the MOSFETS in place   1) the V- adjustment to the MOSFET sources will set output tubes bias (-32V).   2) the original bias trimpots will now be setting the standing bias of the MOSFETs gates – wich should be 3V higher than the source voltage (-35V).   Which also means that possible future changes in the bias adjustment (for tone) also involves adjusting the standing bias of the MOSFETs gates???   Hmm – that doesn’t make biasadjust easier!!!     And while we are at it (just for future reference) all of this would hold true for a 12AX7 also – except that the plate/cathode voltage has to be kept within max. rating.     RG – I really appreciate (and need!) the support – and I hope none of you will loose patience till I’m done with this project.   Thanks again all.     I’ll post a new schematics as soon as possible.     Regards   Lion.

 

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R.G. q{I’ll have to change the original ... -- 1/16/2000 6:29 PM