The Big Three: a nice sounding space charge tube circuit.

[brad]

I'm going to start working on an overdrive based on this tube now that uses a non-tube boost stage in front of it.  I've only ever used opamps in the same circuit as tubes, but would a Mosfet or Jfet boost sound better?  If so, can I use something like a Fetzer Valve by just adjusting the bias so it works at 12v? 

[brett]

Hi
To me, JFETs seem sweeter than op-amps as boosters.  Particularly if they are doing a fair bit of boost.  The bias change between 9V and 12V should be fairly small.

Check out the Stratoblaster at generalguitargadgets - it's a very simple, but excellent JFET boost.
cheers

[comfortably_numb]

It seems to me a little silly to use a JFET emulation of a tube input stage to drive a tube into distortion.  Just use the ACTUAL fender front end and replace the FET with a tube.  I imagine you can continue using the 12AJ6 with only minor changes in the surrounding components.

Just my 2 cents.

[brett]

Hi
Is the JFET so silly?  I think not.  It has nice boost characteristics, and nobody is going to tell the difference.  To implement the Fender pre-amp you'd need a B+ of 200+V.  That's a lotta trouble for nothing.

I wonder whether you could simplify this design by using cathode bias resistors of 470 to 1k, and eliminate the middle tube?  My calculations indicate that 1k would give about 0.7V at the cathode, and 470 would give 0.35V, which are close to your values for tubes 1 and 3.

Maybe that would make it "the big two" ?

[syndromet]

How about using cathode bias as mentioned over here, and using the second tube as an additional gain-stage?

[brad]

I've had a vacuum tube-filled afternoon and experimented with both adding more stages and using regular cathode resistors.  These tubes seem extremely forgiving when it comes to biasing because they work with anything from a few hundred ohms to a few Kohms without any audible difference at all.  I've even found some old literature that says no cathode resistor is the way to go.  When it comes to adding more stages, I found that 3 were required to create overdrive.  It didn't sound so great on my bass...although it did sound better the higher I went up the neck...so perhaps it might be better on guitar.  A better idea than adding more and more stages would just be to use a more powerful booster right at the start.

I liked the idea of using a third tube in The Big Three simply because it's esoteric ...and I needed to add another tube so I could use the name    Please don't take my experiments as the final word.  I urge you to try these tubes out for yourselves and see what's possible with them!  No doubt there are many better, theoretically correct, circuits that are possible.