The following design is meant for DIY use only and was originally written in 2001 or thereabouts. No commercial use of any of this schematic is allowed without permission from Wavelength Audio, ltd.. No reprinting of this information is allowed without permission from Enjoy The Music.com.
Ok enough BS let's get to it. Why the AV8? Because it takes a lot less gain up front to get the tube to it's final output. This means a simpler circuit with much better parts can be used for lot less dough.
Always start at the rear and work forward. The AV8 is capable of 20W dissipation, we cranked this back a little to assure the use of the mesh tubes. Using 50ma as the DC component of the output we see from the curves below.
That at 360V on the plate, 50ma of current, that the grid sits at 38V. Divide the 38V by 50ma and get just right around 750 ohms. This is the self bias resistor. The bypass capacitor calculations can be found in many of the Sound Practices articles. We need between 47uF (8.5Hz -3dB) and 80uf (5Hz -3dB) of capacitance at 50V or better. We need a stable 5V supply for the filament. The circuit provided will make the amplifier very quiet and no need for a hum balancing pot is required.
The grid is attached by a choke. This is a new idea in some respects the idea is that the DC component of the choke is very small. But the AC properties are very high.
I usually design the amp so that they are low gain considering the amount of gain that preamps have these days it only makes sense.
Ok for the driver/gain section I chose the 6922 because it has high transconductance and that is what we need to preserve the waveform. I biased the 6922 for 90V's, 5-6ma and 2V's of bias. The standard SRPP is indicated in the drawing. To drop the voltage from the B+ down to 180V's we use a resistor and bypass it with a good cap (SCR 3.3uF/630V).
The capacitor coupling the 6922 to the AV8 is critical use a good Hovland or Paper and Oil. Try different brands till you find the sound you are looking for.
Power supply, the 5Y3 is available and easy to find. Using the PGP 8.1 string the two 2.5V windings as indicated (S=start, F=finish). The B+ will be right at 400V's the 5Y3 will provide good, slow ramp in voltage to stabilize the power triode and caps appropriately. It will also kill all the RFI and BS on the mains that has been amplified by the high voltage winding.
I wired this up about 8 months ago and got a real nice 5W's out of the unit. About 1.25V's in will take it too full wattage. Hum was less than 0.5mv, with the 47uF bypass cap in the AV8 I got a frequency response at 1W of 18Hz to 40kHz (-3dB).
All the magnetics can be had at www.magnequest.com
AVVT AV8 tubes are available at www.jacmusic.com
Feel free to make changes, sorry I don't have time to comment on them. Others will be interested and most of all have fun!