Paul Ruby mod to Rocket and Liverpool amps?

[Mark]

I have noticed that many of the amp designs at the 18 watt forum utilise a diode network to minimise crossover distortion with EL-84 valves.

It seems a popular mod to minimise buzziness in distortion, that seems like a good idea. Has anyone tried it in a Liverpool or Rocket amp?

It doesn’t seem like a bad idea to me.

Any thoughts?

[Fischerman]

I recently did it to a 2xEL84 amp. When cranked, that amp’s PI got super lopsided and the cathode voltage would swing way high, like double the idle. I put the back-to-back 16v zener/1n4007 across the bias splitters and a 15v/5W across the cathode resistor/cap. These were the lowest values I had on hand. It certainly cleaned it up on the ‘scope and it did sound a tiny bit less buzzy (it wasn’t bad before). But it wasn’t a huge difference. I need to try slightly lower voltage zeners like 12v/13v.

[Mark]

Thanks for your observations and sharing your experience.

[2tone]

I remember seeing this on one of the 9 Trainwrecks I owned. Most didnt have the diode mod. The Express bias settings and even brand of El34's would affect crossover distortion..