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fold back reverbs

9/9/1999 10:23 PM
SteveR fold back reverbs
I was just reading about power amp fold back reverbs in Kevin O'Connor's book and was wondering if anyone has built, used, or heard one of these.  
He didn't come right out and say it, but at one point pretty strongly implied that this kind of reverb implementation doesn't sound very good.  
I imagine that it can't sound as good as the "classic" reverb design otherwise the fold-back type would BE the classic. However, I want to put reverb in a project but don't really have space or filament power for two more tubes and transformer. I don't really need Dick Dale surf reverb, and I wouldn't mind saving 50 bucks either.  
Any thoughts?  
9/10/1999 5:24 PM
Steve Dallman

I recently bought a Champ 12 which uses this type of reverb. It didn't sound very good. The reverb 'tank' is in the amp head and was missing one of the 2 springs. I rebuilt a full size, 3 spring Acutronics tank, chopping it down to 15". It is a decent servicable sound now. I tried several tanks. My favorite, long decay, full size 3 spring sounded the best, very close to classic fender. Look up the schematic for a Fender Champ 12 and you will see the basic circuit.
9/10/1999 6:12 PM

I had a Champ 12 for a while. The rvb was the weakest part by far IMHO. Kinda neat that they could cram the tank into the chassis like that w/o all kinds of hum. That was impressive. Nice idea, putting a real tank in there!  
9/10/1999 7:34 PM
Mike D.

I have added this feature to a couple of old Harmony guitar amps. If you only need a moderate amount of reverb, the concept works ok. If you try to make the reverb too strong, you will get a high-pitched feedback. You will need to experiment with the voltage divider on the output stage to provide the correct "dwell" to the tank, you will also need to futz with the gain of the recovery stage. I believe the Champ 12 incorporates a trim-pot to adjust the recovery gain.  
As an alternative, I have used a Jfet/Mosfet cascode driving a standard reverb transformer along with a jfet/mosfet cascode recovery stage with excellent results.  
I have used both the fender reverb transformer and an output transformer scavenged from an old tube radio. They both worked fine, and were capable of Dick Dale reverb.  
There is information on Steve Ahola's page regarding the use of Fets in tube circuits. I am the one who wrote this doc.  
Use a long tank if you can, but the world will not stop turning if you use a short tank.  
Mike Donovan
9/10/1999 9:38 PM

I worked on a Bassman that a guy had added reverb tp. He stuck in a large value wirewound power resistor in series with the ext. spkr. jack and fed the tank (normal Accutronics / Gibbs / Hammond type) with that. I think it may have been a 50ohm resistor... The output of the tank was connected to the "Bass" channel. Fed back at voumes above 6, but sounded pretty good up to there.  
Seems like it'd be REAL easy to whip up a circuit that used a TL072 or something to drive and recover the reverb signal, then mix that in with the amp's signal via a couple of resistors. HR Deluxe, anyone? (Or Blues Jr....)  
9/10/1999 10:25 PM
Thanks for the feedback guys.  
I think I may go ahead and explore this option a bit more. It sounds like a lot of the poor sound that comes from this circuit has a lot to do with a cheap reverb unit (since the whole scenario is a cost cutting measure).  
I intend to get a nice long 3 spring Accutronics tank since it will sit in the bottom of the cabinet. The project is essentially a brownface super with reverb. Since I'm using the full 2 1/2 tube compliment for the vibrato and an extra gain stage, tubes and space are a little scarce.  
O'Connor does mention the potential for feedback with this circuit which I'll have to watch out for.  
Thanks again for the replies.  

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