Author Topic: Fet spring reverb, is a transformer necessary?  (Read 319 times)

HeavyFog

Fet spring reverb, is a transformer necessary?
« on: March 08, 2018, 11:36:56 AM »
Hello everyone, iv'e been working on a fet adaptation of the fender 6g15 spring reverb. Yes i'm very aware that fets are not tubes and i'm not trying to emulate the original, rather i'm using the circuit of the original as the basis for the circuit i'm working on. In a nutshell the it's just 2 fet stages driving a tank directly without transformers, with a recovery stage after, and a signal fet stage for the dry signal. The stages are more or less close to the original in topology, but i'm not sure weather or not il need transformers like in most reverb designs. In some schematics like the stage center reverb i noticed that no transformer is needed but i don't know if the same applies to fets like op amps given the higher input impedance (310 ohms) of the tank compared to the fender unit's lower impedance.

Never built a spring reverb before so i have no clue if this would work or not or what kind of impedances i should be looking for in the tank in order to drive it without (or with) a transformer). Il try and get a schematic up soon. 

mth5044

Re: Fet spring reverb, is a transformer necessary?
« Reply #1 on: March 08, 2018, 01:20:23 PM »
Check out the Surfy Bear using MOSFETs

 http://cackleberrypines.net/transmogrifox/Misc/surfybear-jfet-reverb-r3.pdf

Plenty of opamp versions too, check out Forrest Cook’s Reverb on Solorb.

wavley

Re: Fet spring reverb, is a transformer necessary?
« Reply #2 on: March 08, 2018, 01:40:48 PM »
There are plenty of capacitor coupled tube reverbs out there.  The 70's version of the Traynor YSR-1 deletes the transformer, so you can look over the revisions of those schematics for some clues.  Most modern tube amps these days have solid state driven reverb circuits, all of the Hot Rod Fenders, Mesa Boogie, Peavey Classic series...

See if you can find the schem for the tube Danelectro reverb, fixed one of those recently, it's a strange beast.  Capacitor coupled driver tube into a crystal clipped to a screen door spring in a wood frame, then another crystal for a pickup, no transformers at all, not even in the tank.  The crystals were bad so after trying with limited success to replace them with piezos we just redid the driver and make up stages and put a regular tank in it.

Short story, you aren't barking up the wrong tree.
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PRR

Re: Fet spring reverb, is a transformer necessary?
« Reply #3 on: March 09, 2018, 09:45:05 PM »
300 Ohms and 100-900mW is a big job for common JFETs.

Would you be excommunicated for using a chip or BJT or MOSFET to drive the spring?

2K springs are available; does that break the bank?