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previous: Chris Harden Ray,Thanks for givi... -- 6/5/2000 9:55 AM View Thread

Re: Building a 'clean' PI

6/5/2000 2:44 PM
Ray Ivers
Re: Building a 'clean' PI
TubeCAD is available from for $49.00, and TubeCAD and Audio Gadgets (a handy program for miscellaneous amp-related calculations of all kinds, from RC filters to power supplies) together are available for $79.00. I can't say enough good things about TubeCAD; it's not perfect, but I can't imagine designing without it now. I've been hassling John Broskie to write a push-pull output stage TubeCAD add-in, but he says he just ain't got the time right now (evidently the pentode modeling's a bear).  
Here are the 12AU7 numbers for 300 and 550 volts:  
300 V: Plate 27K, cathode 560 ohms, 10.2 mA  
-105/+134 P-P AC, 795 mW plate dissipation  
550 V: Plate 47K, cathode 1500 ohms, 10 mA  
-220/+224 P-P AC, 1.5 watts PD  
The Ampeg B15S (Groove Tubes book) uses an 820K/1M voltage divider for a 12AU7 driver with 47K plate resistors, 1.5K cathode, 405 V B+. I think I would probably use 1M and 910K resistors with an additional 250K variable resistance in series with the 910K for AC balance adjustment; that way, you have all sorts of balance/imbalance options at your disposal. I should think you could use the second grid (I don't think I would call it a 'spare') as another input; the common unbypassed cathode resistor should insure the signal would be seen equally at both PI plates. I don't think I can recommend this, though; even with a 12AX7 driver the gain just isn't there for anything other than a super-hot effects return.  

Chris Harden Sounds like a 12AU7 would be OK. W... -- 6/5/2000 7:27 PM