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Has anyone ever got this to work? I can get the AC at the power tube grids to balance out, but the only do it at one setting, as I go up/down on the volume they go out of balance.
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well, regardless of whether they go out of balance or not, how does it SOUND??
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Sheepishly, I have to admit to not getting as far as plugging in a speaker...I have tried amps with various methods of unbalancing the PP arrangement, but not particularly keen on it. I have tried regular, cathode fed self split & had good balance & a relativey wide sweep of useable volume. Probably being a bit irrational about this unbalancing...you're right listening tests to follow (when the neighbours allow).
At the point that things do even out, I am seeing significantly more power with the screen fed, fixed bias set up, 36W for a pair of 5881.Comment
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Well, we gave it a workout yesterday....it "works/functions", a little splatty sounding when pushed, but I think that achieving a better balance via the screen grid resistor value will help (using 3.3K at the mo', will go a little larger). Imbalance as it is, is "livable with", not as bad as amps specifically designed to skew the PI balance. Output & dynamics are greatly improved over cathode baised, self split, fed by the cathode resistor (which is obviously no great surprise). The application I'm anticipating for this is as a harp amp, it's looking promising, lots of volume before feedback compared to the other fixed bias, PI driven, amps we compared to, giggable for sure already, but I think that there's better to come with a bit more fine tuning, perhaps 90% there?Comment
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Screen grid resistors upped to 4.7K each (535v at plates, 2v dropped accross screen grid R's at idle), better balance, still some mild see-sawing in AC from one 5881 grid to the other (+/-) 20-25%, but balance is improved over a greater sweep of volume, up 5W to 41W (not clean RMS rating) at balanced AC. Further listening tests to follow, but turning out easier than anticipated!Comment
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