Washburn SX12R amplifier Spring Reverb

[tombaker]

Hello,
I have recently dismantled one of these old practice amps; using the speaker with a Tiny Giant and harvesting the spring reverb from the bottom to build a small desktop spring reverb unit.

I was originally going to build a circuit but when i opened the amp there was a separate board just for the reverb section.
While I can only find a schematic for the SX12 (without the reverb section), I thought I would post my schematic of the traced circuit board in case anyone in the future ever wanted a reference point.

I've double and triple checked it but as most humans are, i'm prone to error. If anyone has a copy of the original could you please link me in so i can see how far off I was.

It isn't the neatest schematic but it'll do.

[Rob Strand]

Pretty good trace.  I can see a few bugs:

- Where R219 & Q204 join there should be another resistor to ground
- C207 should connect to the opamp -input (not the +input)
- The INPUT  need a cap in series with that line to stop DC paths stuffing-up the bias.

Beyond that, if you wanted to make a reverb unit out of it  you could add,
- 1MEG to ground on the INPUT line (when you add the series cap that will be on the input jack side)
- a 100k to ground on the OUTPUT line to ground.

And,

The  470pF/490pF cap in the input is a bit hefty.   Drop that to say 100pF and add a 1k or 2k2 resistor in series with the input cap you need to add.

Maybe increase R214 to 470k or 1MEG.   With that value the series input cap should be 47nF or 100nF.


Even after all that it is still not correct.
You need to mix the dry signal with the Reverb signal.
I'm trying to find a nice circuit but all the ones on line are more complicated than it needs to be.
Just just need to buffer the input then the add dry and reverb signals together.  A pot for the reverb level is nice too.

One other thing you have to decide if you want to plug in guitar signals or line level signals.  If you want both you will need a switch to add some input gain to stop the reverb signal being too low.

Have a look at
http://www.generalguitargadgets.com/how-to-build-it/technical-help/schematics/

Scroll down about 80% to the Reverb section.

[tombaker]

Thanks Rob,
Much appreciated.
I made those changes.
What does the 1M change to R214 accomplish? out of interest. Everything else makes sense to me.




I am going to use this spring reverb as an aux unit from my DAW so i hadn't intended on adding a wet/dry circuit as i would only be using the wet, but in terms of flexibility it's probably useful.

I also intended on adding this hi-pass/lo-pass filter section to the back of the reverb circuit so i could narrow the reverb frequency.



The 2 sections with back to back pots will be dual ganged and only two pots, LOW CUT and HI CUT.

[PRR]

Base of Q204 may be "bootstrapped" from the output to the tank. This would significantly increase output drive.

Have we peeked at "all" other Washburns for a similar reverb? While some designers re-re-re-invent wheels, others set a few building blocks and use them for years.

[tombaker]

Good call, I've looked around but i'm not familiar with washburn amps from the era. I've googled schems from the other amps from that particular product line and found nothing.

AND "bootstrapped" from the tank input or output?

[Rob Strand]

hat does the 1M change to R214 accomplish? out of interest.

Increases the input impedance.
For line level stuff you can get away with lower input impedance but *if* you plugged in a guitar a low impedance input would load the pickups and affect the sound.

[tombaker]

Thanks Rob, much appreciated.

Once I get the build happening I'll update any info just in the off chance other people are interested.