Tube based octave effect?

[Austin]

I was thinking about how I could use tubes for an octave effect in my amp using some sort of small signal rectifier type tubes, or even a 12ax7 strapped as a diode. Would that work okay? I would full wave rectify my signal and then somehow have a volume knob to blend it into the clean signal, is that about right? Then just build it into my amp like reverb or anything else. Thanks, Austin

[loudthud]

Use a long tail phase inverter only cascode each output into another dual triode. Tie the plates of the upper tubes together at a common plate resistor. For best balance use an active current source or a really long tail for the bottom triodes.

[R.G.]

This comes up every so often, as does a tube-based [insert whatever you like here]. The problem is to come up with what particular virtues using tubes provides.

The standard problem with all rectifier style octave generators is that you have to somehow either get rid of or make irrelevant the forward voltage of the diodes doing the rectification. A tube rectifier makes this many times worse than a solid state diode or transistor setup.

There is one variant that might be made to work. See the MOS Doubler and Mu Doubler at geofex.com. In this setup, opposing amplifier elements are fed out-of-phase signals which cancel at source/emitter (and presumably could be at the cathode) and combined at the drain/collector (and possibly plate) so that the signal components cancel but the even-harmonic distortion reinforces. Works fine with JFETs and MOSFETs, and is the stock octave used in the Univox Superfuzz. However, vacuum triodes are the most linear known amplifying devices with no feedback. That means that unless you deliberately misbias them to run in some kind of clipping, the distortion products are very small, and so the output of a tube-based distortion selector like this, you get much less octave output.

I guess if you had 50-60V of signal, even 1% distortion comes up with 0.5V of octave, but it makes matching and cancellation a big deal indeed.