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Old 28th February 2010, 06:04 PM   #1
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Default pentode mode distortion 6LR8? George?

I have been planning on a SE 6LR8 UL amp for my main FR drivers (5 1/4 Rat Shack Foster OEM) but am thinking that a higher Zout might be advantageous in this instance. The obvious move then would be to try pentode mode without NFB. My concern is high order HD products of course. Has anyone measured such a circuit? If my concerns are justified are there any clever solutions that would leave the transconductance amp characteristics alone while taming the high order HD? Would I need to go to the trouble of regulated supplies to tame the distortion?
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Old 1st March 2010, 12:34 AM   #2
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An unbypassed cathode resistor will linearize a pentode and also increase the plate resistance. If the plate resistance gets too high that way, you can also add some Schade feedback resistance from plate to grid. That will lower plate resistance and linearize the pentode further.
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Old 1st March 2010, 10:15 AM   #3
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Ah, good idea. I hadn't thought of that. By partial bypass one could even tune the Zout.
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Old 1st March 2010, 06:27 PM   #4
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Well, the bypassed part of the cathode resistor will not linearize. I would start with the required auto-bias resistance, then if more is needed for linearity, tap off for the grid bias there and then add more cath. resistance below until the linearity is where you want it. Then if the plate Zo has gone too high, try a bit (high R resistor and cap) of Schade feedback (plate to grid) to get it back down to where you want it.
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