I realize this is a 10 year old thread, but I thought I'd say thanks and mention why I say thanks. I recently built a number of somewhat novel 5 watt EL84 amps using a Matchless inspired topology, i.e., a paralleled twin triode in V1 and an EF86 pentode for V2 before the tone stack and then the EL84. I had encountered a nasty, ugly sounding intermodulation distortion that was very hard to shake, as soon as I turned the gain up, there it would be rattling and buzzing in the bottom end. Anyway, to get to the point, I tried the tail resistor on the cathode bias resistor/bypass pair on the EL84 and it seems to have worked wonders, so this literally saved the design because I was not happy with it the way it was and now I think it's turned out pretty well. I'm pretty sure it will be a feature of any SE amps I build from here on because it does the trick and allows a SE amp to sound much like a P-P amp without the nasty intermod distortion., so thanks for the info!
This discussion is mentioned earlier in this old topic. You add a 15% or so of cathode resistance (in series to the cathode/cap pair) tail resistor to the cathode resistor/bypass capacitor pair of the power tube on the ground side to ground. It adds enough negative feedback to remove nasty intermodulation distortion in single ended amplifiers. Intermod distortion is very common in SE amps because it does not have the cancelling effect of P-P OTs. It actually works. I did it to 3 amplifiers because it helps greatly.
im fixing SE amps with cathode feedback winding from output transformers.. it works the best.
but trying to understand this technique with resistor.. but i'm not getting this in text mode how it is described.
you add unbypassed resistor in cathode circuit? you are just bending the curves to more linear yes.. but you are loosing gain and increasing output resistance
I'm finding this to be a very interesting and useful topic, especially regarding 120hz intermodulation. Might this find it's way next to the lead dress info in the FAQ's?
