freestompboxes.org

I'm trying to figure out how to go about switching a cathode follower stage in and out for A/B testing. I was wondering if this would work. Not sure how to keep that dead ended switch contact within a couple volts DC of the other contacts so the switch could operate safely and keeping the tube from drawing current I may give it more thought later and it will be one of these moments. I'd rather not have to go through the process of rewiring & rewiring for listening tests because by the time I've got the circuit set-up my ears will probably have forgotten what the original was like! Especially if the difference is subtle.

Thanks in advance!

I do not know the right way of doing it but offhand I would say put a 5-10k resistor in series with the cathode and your existing cathode resistor. Then when you switch it out connect your grid to the bottom of the 5-10k resistor. This will bias the cathode follower off or at least at a fraction of a mA.

you don't need the switch at the input of the cathode follower - this is how M.E.S.A did it in one of their amps - they called it a fat switch - a subtle but noticeable difference in body sound - give it a try
bajaman

First of all I'd like to thank printer2 and bajaman!

Bajaman, the version you described was actually what I was originally thinking (not even knowing MESA did this), but this would allow grid current to continue to flow into the cathode follower (~.4mA). As an input signal is coming in, the bias of the CF is shifting causing a sag in the upswing of the waveform due to more current flowing into the grid. Can you think of a way to de-couple the 2 stages without going above a normal switches Vdc differential ratings (normal being a switch that doesn't cost an arm and a leg)?

Printer2, I like where your going with this.

I'm actually going to try something along printer2's thoughts but the voltage differential may go past the switches rating when turned off. I'm not sure how the new bias will affect the voltage above the cathode resistor.
If this doesn't work I'll go with bajaman's version.

I'll be getting parts in a couple weeks for other sections of the circuit, and I'll check back here before I dig in to see if anyone else has any ideas or thoughts. I'll do some tests and some readings in the following weeks and if I can remember, I'll come back and post my results!