Like Woody, I have been thinking about ways to put a higher current driver
in front of an output tube pair, to minimize distortion due to grid
current due to the output tubes distorting like crazy. Woody's Boom amp
does this, and I bet it sounds fantastic.
My first attempt was to design a long tail pair phase inverter with a
higher current tube like a 6sn7. Unfortunately, this is tricky because
the current involved keeps the tail resistor from being large enough to do
any good.
You could tie the tail resistor to a negative voltage source, but part of
the beauty of these cathode biased amps is that you don't need a negative
source.
So what the Boom does is to use a conventional phase inverter, and add a
stage between it and the output tubes that is a high current driver stage.
I like this solution, but it makes me a little nervous to have so many
stages of gain (and sources of distortion) without summing the difference
in a transformer or something. Plus, the way the gain works out, you sort
of have an extra triode. But that isn't a big deal because it lets you
parallel it or use it as a cathode follower.
I think a neat solution might be to create a phase inverter with a 6sn7 or
12au7, but replace the tail resistor with a choke. It is exactly what we
want - low DCR but high AC impedance. A $20 Hammond should do the trick.
I think you can get something around 500 ohms and 17H or so.
This should provide enough impedance to "anchor" the center of the
cathodes and allow the phase inverter to work well, but low enough DCR to
not drop huge voltages.
What do you guys think?
Phil
