Wow, 8W of audio from a 12AX7. I assume the distortion is pretty high at full output. 8W, 8K p-p, that's pushing the specs with a 12AX7, because of cathode emission and plate dissipation. And with zero bias, you would need to set up a phase splitter that that can provide grid current. I guess it's possible, but I'd try a 12AU7 or preferably a 12BH7 at 300V with -20V grid bias in class B1.
Cathode emission 6SN7 filament: 6.3V 0.6A, 3.78W filament power, 70mA cathode emission per tube. 90 mA would even be pushing that a little bit. 12AX7 filament: 12.6V 0.15A for 1.89W filament power, expect lower cathode emission. 90 mA peak is pushing that. 8W from 8K p-p, not even considering output transformer efficiency, needs almost 90mA peak. 6SN7 dissipation spec: 5W per plate, but 7.5 w total when both plates are hot. 12AX7 dissipation spec: 1.2W per plate. Whoa. Maybe this figure is low, anticipating a shield around the tube that would hold in the heat. Plate Dissipation 8W RMS, that's 16W peak. 8K p-p, that's 2K plate to B+, that means: 16W peak, 2K, 178V peak to zero 178V peak, 2K, 89.4mA peak per plate. 178 V peak, plate at least +250V, maybe +300V, to give that much swing. 250V plate and -2.2V grid bias would have 1mA resting currrent per plate, for 0.25W resting dissipation per plate. Minimum instantaneous plate voltage at the audio peak would be +72V. 300V plate and -2.6V grid bias would have 1mA resting current per plate, for 0.3W resting dissipation per plate, probably OK. Minimum instantaneous plate voltage at the audio peak would be 122V, so dissipation would be significantly higher with audio than it would be with 250V at -2.2V bias, but the 90mA peak plate current would be easier to achieve. 8W out, class B, at this low voltage means at least 5W dissipation, 2.5W per plate, for a continuous sine wave at full power. That's pushing it. Maybe it's OK if the 12AX7 was rated very conservatively, or if the duty cycle is low enough. Zero bias? 250V plate, zero bias, 7mA resting current, 1.75W per plate resting dissipation per plate. 300V plate, zero bias, 9.2mA resting current, 2.8W per plate resting dissipation. Can the 1.2W per plate dissipation spec be this conservative? Maybe with a high grid leak resistance it would self-bias a bit, and work. Bacon, WA3WDR ----- Original Message ----- From: "Patrick Jankowiak" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Saturday, October 01, 2005 10:15 AM Subject: Re: [AMRadio] GG 813 Linear in GE Ham News -12AT7 12AX7 > don't laugh too hard, In the ARRL handbook it claims 8 watts audio from an > 12AX7 in zero bias class B push pull into 8000 ohms. I have never > understood that one, if you look at the currents stated in the book, they > seem pretty high for the tube. > > PJ > > > __________________________________________________ ____________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami >
