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Old 7th April 2004, 05:34 PM   #1
Lico is offline Lico  United States
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Default triode=>pentode

I've been listening to this amp for a few months now and I want to change something. Nothing's wrong, but maybe I can make it better
How would I change this to run the 6v6 in pentode mode, or I guess it would be tetrode wouldn't it? I think I just connect the other grid to B+ through a resister, but how do I figure out the value?
Thanks,
Andy
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Old 7th April 2004, 05:45 PM   #2
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Move screen from plate to +250V. You can keep the series 100 ohm if you want.

Tim
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Old 7th April 2004, 06:31 PM   #3
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I agree with Tim, but remember that in tetrode mode the HT consumption of the 6V6 will be higher than in triode mode, so check that your power supply and output tranny are up to it.

Depending on how sensitive your speaker (or ears!) are to third harmonic transients you might like to try the effect of a small capacitor wired across the primary of the output tranny - 0.001uf or 0.002uf of at least 750v working would be suitable. Larger values will affect the treble resonse.
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Old 7th April 2004, 07:08 PM   #4
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Actually the screen and plate voltages are nearly exactly the same. Quiescently, the tube can't tell if its screen is on the plate or +V.

BTW, I wouldn't blame you if you think it sounds like crap...high Zo and distortion and all. By moving the screen connection you've removed the local NFB that keeps things in line. This is usually remedied with global NFB, but that's another can of worms that you can look for posts on...

Tim
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