Champ kit looks fun. I'm currently just toying with a Mosrite Fuzzrite clone. Very leisurely pace...

Wes
SoCal ex-pat with a snow shovel

DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices.

Apart from the aluminium binding the guitar is done! Still has to be set up and tweaked, but overall a good result. Great tone too!

I'm more of a clean amp guy, but I have to say this thing rocks on a Tweed Deluxe!

Some time ago, JObeast asked me if it would be possible to build a FET version of the Magnatone vibrato. I got interested and couldn't resist the challenge

The interesting thing about this vibrato is the use of varistors to modulate the phase shift. Magnatone describes this like this:
http://www.magnatoneamps.com/FMvibrato.html
Varistors (variable resistors) are a vital part of the Magnatone vibrato. These are high-voltage, obsolete, old components that are difficult to obtain. The principle is explained at the bottom of this page:
http://www.geofex.com/article_folders/phasers/phase.html

This pdf describes how varistors can be replaced with ordinary zener diodes. You can even see references to Dinko Tomljanovic (from Bambi Molesters):
http://dalmura.com.au/projects/Vibrato%20effect%20measurements.pdf

Dinko is using a home built amp with the Magnatone vibrato.
https://www.youtube.com/watch?v=6gcVPo-qp9M

With very few changes of the original schematic it is possible to emulate the tube/varistor circuit using jFETs/zeners. Tubes are replaced with jFETs and varistors with 3.3v zeners.
http://www.magnatoneamps.com/schematics/magnatone_280B.png
The phase shift oscillator need high gain to oscillate, so a 'mu-amp' circuit was added to increase the gain. (Same as used for the FET Reverb recovery gain stage)

++ The sound is very good, unfortunately I have no real Magnatone to compare with.
++ The principle of the original circuit is based on a few compromises and non-linearities.
I think this JFET version has retained most of the original tone.
++ Only 2.6 mA current consumption (9V battery)

-- 60 components for a mono version. Probably ~100 for a stereo version, quite a lot.
-- 9V is close to the minimum possible supply voltage. Probably need some tweaking for optimum operation at 9V.

Last edited: Apr 24, 2015 15:57:47

Perfect addition to my SB. Hope you will sell those.

The Hicadoolas

Glad I put that bee in your bonnet, Bjorn! You pull this one off and you will single-handedly dominate the DIY vintage-emulation circuit marketplace and win a special place in the hearts of all us vintage-tone-addict guitarists who aren't afraid of a soldering iron.

So, somebody please lend Bjorn a Magnatone by which he can compare his new circuit gto perfect it and bring it to market!

Squink Out!

Thanks guys

This forum is fantastic, I got lots of interesting suggestions and comments from the nice people here!! I just wish I had more time for building stuff.
Right now I work hard with new FET Reverb stuff that I will showcase at Surfer Joes in June (if everything goes as planned).

After that it would be fun to continue with the vibrato experiments.
What do you think, is it important that the PCB is designed to fit in a small stompbox?
100 smd components is a lot, but doable.

THE uber-cool solution like what eddiekatcher suggests doesn't care too much about the small PCB, as long as we have the spring tank, we need a big cab anyway! the DIY lads might even be rather skeptical towards smd stuff in terms of service- and moddability?
for the bigger market, those micro-pedals with tiniest PCBs are certainly trendy.
on the other hand, also modular effects pedals are quite hot, and that's what eddies idea might aim at... to add a fuzz, a driver or a delay, there's enough space in a reverb tank. patchable, of course

Finally we got the faux 'HarmoMetal' around the body. As soon as the fret file arrives we'll be able to give it a final setup and then we're really done.

If I was fiddling around with mating up SB Reverb and Vibrato cards, I'd build a small flat cabinet just tall enough to have the a sized tank lying flat in it, suspended by springs as per a normal tank but flat with the PC cards directly above it, then I would arrange the controls so that they were addressable from the top side. Put some tall rubber feet on the cabinet to clear the amp's handle and have this bad boy sitting on top of my amp where I could get to it. Put in a tuner circuit and wah-la! I hate crap on the floor. I am always stumbling over cables and tuners, my tank..... If it is on the floor, I am going to trash it.

By the way Gillette, that is one cool "workbench project."

Clumsy bumpkin, Ed

Traditional........speak softly and play through a big blonde amp. Did I mention that I still like big blonde amps?