MPF102 JFET Op Point Help Needed

Tweakers' Asylum

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Those of you active on Tube DIY as well as this "board" may have seen the ongoing $100 School amp project story. Orren Kickliter, the project coordinator, has been having problems in getting a 1U4 to work as a voltage amp. Orren and I have been able to work out a PP O/P stage that uses 2X 3S4s fed by a MPF102 JFET phase splitter.
Since the circuit is already a SS/tube hybrid, yet another FET just might help on the voltage amplification front. Using a FET for the voltage amp definitely simplifies power switching, as 1 4PST/channel will get the job done.
The phase splitter was easy; 10 KOhm load resistors on the source and drain combined with a 2.2 MOhm gate resistor. Frankly, I don't know enough to QUICKLY work out a MPF102 voltage gain stage. The B+ rail is 45 V. and, I suppose, a RC decoupling filter on the B+ is in order. Global NFB from the O/P trafo secondary to the voltage amp's source will be employed (it's a mini Mullard circuit). As an expedient, the NFB resistor pair will be in a 10:1 ratio.
TIA for any assistance you can provide in selecting values for the decoupling filter, drain load resistor, and source bias resistor.

Eli D.

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Topic - MPF102 JFET Op Point Help Needed - Eli Duttman 09:41:22 03/22/04 (0)

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