EL84 anti-fizz trick?

[solderstain]

Once upon a time, I heard about some trick done with a Zener diode and a resistor in series, that are then put in parallel with the 220k resistor to ground on each signal lead going to the EL84s. (The 220k resistor value is just an example, based on something most are familiar with - AC30 and the TW Rocket - R17 & 18 on the Rocket schemo).

Anyone here done that? How do you select the resistor value? How do you select the Zener value? How is the Zener oriented?

Does is actually work or is it snake-oil?

Or is my memory bad? (A distinct possibility... )

[Alexo]

It works miraculously IF your problems are caused by that which the trick fixes.

You take a zener diode rated for 1 or 2 volts above your cathode voltage (assuming you have a cathode-biased amp) put it pointing AWAY from the grid headlong into a normal diode, both diode bands on the inside of the junction, if that makes sense, then the regular diode goes - end to ground.

So if grid conduction leads your grid to develop a voltage more negative than your cathode voltage is positive, the breakdown voltage in the zener is crossed and it opens up, the reverse regular diode also opens up, grounding out your grid current (and turning the tube back on) but closes when presented with a positive voltage.

Make sense? The whole thing keeps your grid from going into excessive negative territory, cold-biasing the tube into cutoff in a bad way. This phenomenon leads to a lot of crossover distortion, the frequency of which changes with coupling cap discharge, leading to a "squishy mosquito" tone.

Happens with 6V6's too!

[---] ---- -Z>l----l<l- ----- (ground)
(grid) (zener) (1n4007, etc)


---(parallel with grid reference resistor, 220K, 470K, etc) ---

[tubeswell]

Is this what you mean Alexo?

[labb]

The Paul Ruby Buzz fix is here:

http://www.18watt.com/storage/18-watter ... fo_311.pdf

[solderstain]

That's exactly what I remember being described to me. THANKS to ALL. Yet again, this place rocks.