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RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?

Posted by Eli Duttman (A) on April 25, 2013 at 22:06:45

In Reply to: RE: Driving 6Y6GTs with El Cheapo 12AT7 front end? posted by jyourison on April 25, 2013 at 19:03:17:

" I have a lot of Amperex 6DJ8s. Might they be up to the task? Frugality would seem to warrant using up some of my tubes that I already have, but I know your penchant for good engineering."

I have no idea about how tough your Amperex ECC88s are. That really doesn't matter, as they are worth MONEY. Sell some them off, for all that the traffic will bear. :> D

The Russian 6H23П-EB (6n23p-ev) is a very tough hombre, with OK sonics. The Russian variant "laughs" at the conditions Mr. Joppa indicated makes many 6DJ8 family specimens fail. Tubes labeled ElectroHarmonix (EH) 6922 are, actually, 6H23П-EB.

"May I get some clarification just to be sure I'm following you? Are you describing each section of a 6922 (one section per channel) to be ANODE-loaded with a CCS and LED biased? Then the anode at the tube goes straight into the gate of a MOSFET IRFBC20 with no capacitor coupling? Perhaps a stopper resistor?"

100% correct, especially the gate stopper!

"Is this the kind of splitter you are describing?"

Yes, the graphic describes a "concertina", AKA split load, phase splitting setup. "Cathodyne" doesn't quite "make it", when the object of the exercise is a MOSFET. ;> )

"How do I figure a good value to use for the source and drain load resistors on the splitter? I found a source talking about tube cathodynes, but not much on the values for MOSFETs."

It's not too different than working with a tube. In this DC coupled setup, the gate is (of necessity) at the triode's anode voltage. Assume the FET's source is at the same voltage, even though it actually is slightly lower. Pick an arbitrary drain current value. The IRFBC20 I suggested will loaf at 20 mA., while providing huge transconductance and a LOW O/P impedance. Good old Ohm's Law is what you use to crunch the numbers. Make the total source resistance variable and somewhat larger than the drain resistance. Then, you can bring the amplitudes of the 2 phases into "perfect" congruence.


Eli D.

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