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RE: A question about the role of tube?

Posted by John Swenson (A) on October 25, 2007 at 18:18:38

In Reply to: A question about the role of tube? posted by ychungmd on October 24, 2007 at 17:54:06:

The pentode is primarily there to dissipate the heat and handle the voltage.

You can certainly do it with just the MOSFETS, just realize that the voltage rating of the MOSFET has to handle the total voltage excursion of the output, not just the idle voltage. You will probably wind up with fairly high current and low impedance of the OPT.

Lets say you use Mike's 800 ohm OPT (EXO-800?) if you run at 300mA idle current, thats 600mA peak to peak, which is 480V swing on the OPT. Half that is 240V + 50 to make sure the CCS is running at all times, thats 290V B+. Plus the 240V = 530V peak. That comes out to 36watts output and 87watts dissipated by the CCS, which I think is a little bit pushing things. For this service you want a fairly low current, but high power dissipation device, say rated for 2-5 A with at least a 100 watt dissipation. And of course a humongous heatsink.

Oops thats way past the power rating for the EXO-800, so lets say 200mA, thats 320V swing, 16watts out, 42 watts dissipation, 370V max voltage, much nicer. Still a pretty big heatsink though. Thats a 200mA 210V B+, so finding the right power transformer etc is going to be interesting. And then you need the right plate choke etc.

It can be done.

The "sound" with the tube is almost identical to one without the tube so don't don't worry about that. And you don't need filament supplies etc. But you don't get nice glowing things.

John S.

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