Strange Starved Plate Cathode Follower

[Bill Mountain]

So I have been playing around with starved plate tube circuits and one of the things I've been experimenting with is using a diode to bias the cathode in common cathode gain stages.  I thought I would try it with a cathode follower as well.  I used an MXR Micro Amp to boost my signal going into the cathode follower.   I needed 3 LED's to get even close to the volume I could get with a more basic cathode follower (like the one in Marshall heads).  It was still usable but not necessarily more desirable.  For fun I decided to start messing around with different feedback ideas to see how it would affect the sound.  After a while I started placing caps from the cathode to the grid.  At first it was nothing special but then I put a 10uf electrolytic (with the negative end at the grid) and the gain skyrocketed.  I won't say it sounded better than a normal cathode follower but it did sound different.  I tried some 1uf, 4.7uf, and 22uf caps and each one sounded a little different.



My first guess was that the signal in fact bypassed the tube entirely through the 10uf cap but with some quick listening tests it did sound different with this set up versus just bypassing the tube entirely.  I think I'm going to stick with a more traditional cathode follower but I thought this was a fun idea worth exploring.  Maybe some of the tube heads in here could explain what is happening (if anything).

[Earthscum]

Try running it like you have, minus the tube. Boost -> cap -> 3LED's to ground -> cap

Now take a resistor from the C-D-C junction and run it to positive to forward bias the LED's. If my suspicions are correct, your tube is acting like a sort of reactive resistor. You might try running a JFET in place of the tube (maybe even just use the fet like a resistor and run the gate tied through a 1M to ground).

Just tossing ideas for ya. It's interesting, for sure.

[iccaros]

Merlin has some stuff on his site. The diode acts like a resistor, but I believe it has little to no effect on tone. Local feedback should give less gain, as feedback is to control gain to a point, but you are taking the non inverting side, so this is positive feedback which adds gain, but can cause stability issues if not handled correctly

from http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_014.html

One way of achieving the extra gain without adding extra stages is to use positive feedback. Care must be taken to see that the beneficial effects of the negative feedback - reduced distortion - are not cancelled in the process. The secret is to use positive feedback to boost the gain in a part of the amplifier that has very little distortion. Then increased negative feedback can be used over the whole amplifier to reduce distortion elsewhere.