express differentiator ?

[Andy Le Blanc]

I've been playing with the values on paper of what looks like a RC differentiator
before the clipper stage in an express.

Using: Fc = 1/2piRC

So..... a .002 cap and a 150k resistor gives a xover around 520 Hz?

I have have seen this pop up in forum discussion, any recommended threads?

I'm curious what the range of component values are in extant wrecks,
and where the xover has been achieved with favorable results in clones?

[JamesHealey]

values other than 150k/2n2 have been seen in wrecks but the main two Francesca and Orphan Annie (Glen's Wreck) that the community here have been cloning both have the 150k/2n2.

I quite like 68k/4n7 which is interesting, same frequency or there abouts but it's a cleaner more dynamic version.

Some Liverpools have 56k/2n2, I saw a documented wreck with 150k/4n

I guess this is the fine tuning point of the wreck, in a 5W SE clone I've just made im using 330k/100n which is the Komet concorde value which sounds great also..

Go experiment and trust your ears not your conception on how close to the original it should be, you can be 100% correct and find an amp you're not keen on, experiment/mod and enjoy chasing that tone.

[jjman]

I've used from .02 to .002 all with the 150k. But I use a half sealed half open 4x10 so I need more low end potential. So the .02 is in there now.

[gearhead]

Anybody played around with adding a pot to the 150k? Adding resistance above 150k drops the corner freq. From your experience, JH, imagine that would make it less clean and dynamic.

Kinda hesitiant to dynamically monkey with the cap (switchable); it's got the plate voltage on one end.

[Andy Le Blanc]

Here's the math, for the rest of us bone heads, and a way to make a variable control.