JFET Vibrato/Tremolo...?!

[alex.s]

Hi everyone, here's a potentially very silly question I've been asking myself for a while...

We all know about the lovely JFET adaptations of famous preamps/amps etc and how nice a job they do at emulating the tonal character of different amps. There's a couple that do well at having a crakc at the AC30 tone, which is great as I love my Vox AC30 very much, but one of the things I really love about it is its Vibrato/Tremolo channel.

Would applying the original ROG JFET-for-tube process to the AC30's vibrato channel result in vibrato/tremolo? Or would the modulation process get lost in translation when using JFETS?

Thanks in advance for any help, suggestions and tips...!

Alex

[Frank_NH]

Interesting idea.  Do you have a schematic in mind?  I suppose you could try it out on a breadboard.  I'm still stuck on that JCM800 amp emulator.  Similar idea - replace tubes with jfets but retain the circuit topology and tone stack.   I've been researching all of the mods on the real amp and have some ideas on how to tame the bright treble response.

[alex.s]

This  is the schematic for the preamp section of my AC30 (the UK made Korg-Marshall 90s/early 00s one, which should be very close if not identical to the original Top Boosts, including valve rectifier).

If you compare the TB channel with the ROG English Channell you'll see it's pretty much 1:1 with the TB channel... If no one know in theory if the modulation of the Vibrato channel would work with JFETs I might take the plunge and do the same kind of work done for the English Channel, or perhaps simplify it omitting the first gain stage...

[Frank_NH]

Well, you should go ahead and apply the ROG approach to see what you get.  Here is a similar project where a the Fender 6G15 reverb tank is emulated using JFETs.

JFET Reverb Project

Frankly, I think there are a lot a nice tremolo/vibrato circuits out there that would get you close to the sound you wanted.  JFET emulation is only an approximation and needs to be tweaked to make sure the JFETs bias correctly.  Moreover, as noted in the article linked above, you should first build a JFET measuring rig so that you can sort your JFETs according to Vp and Idss so as to get optimal performance from your circuit (I plan to build one soon so I can do this).

Good luck, and let us know if you come up with something interesting.  

[alex.s]

I've started last night by redrawing the circuit for clarity's sake, still need a couple tweaks to it.

After that I'll start thinking about breadboarding/vero-ing it, possibly omitting the preamp part of it proper, and keeping the vibrato effect only. If I could get anywhere near the subtle Vibrato wobbly sounds (not so fussed about the tremolo as indeed there are plenty of options out there) I'd be very happy...

Think it will be a while before I'll get the time to get it all up and running (and debug the f*** ups that will come along the way...!) but I'll report my findings should anything interesting come out of it.

[Surgeon]

If memory serves, the EA trem is a jfet based tremolo no?

[Frank_NH]

There are a lot of "inspired by" effects out there, and to honest, if it sounds like the original, it shouldn't matter if the circuit topology is the same or not.

Here's a case in point.  A guitar vibrato based on the Vox AC-30:

Vibrato Circuit

This circuit looks a little complicated, but it could probably be put on a vero.  I would probably opt for a simpler design.