18 Watt Lite iib phasing issue help?

[ncusack]

Hello,

I have built an 18 watt lite iib with master volume and the amp sounds wonderful except when things get dimed. When everything is up to its max the tone still sounds good but there is a slight sort of phasor effect that happens. I'm just wondering if other people have had issues like this elsewhere and what some of the possible causes might be?

Cheers,

Neill

[OldHouseScott]

That's not uncommon with EL84 amps, and from various posts I've read here and other places, seems to stem from grid conduction on the EL84s causing the coupling caps to charge and shift the bias, resulting in crossover distortion in the output.

Here's a link to the Paul Ruby mod that describes using zener diodes to reduce the cap charging effects:
18-watter buzz info

Here's a discussion on another forum about both the Paul Ruby mod and another mod, using a zener across the cathode resistor of the power tubes to limit bias shift there as well:
Zener diode mods

You can also do a search on "EL84 swirl" and probably find some related discussion.