Hi all - putting this in "Instruments and Amps" since I'm guessing this is where all the reverb junkies lurk 😎
I picked up a mid-60's Shure mic mixer over the weekend for next to nothing. After cleaning the pots and re-attaching the audio transformer that had come loose and was rolling around the inside of the chassis, it works like a charm (could use new e-caps, but I'll get to that). Happy to post photos soon, but here is a link to the data sheet (kudos to Shure for still providing these!). Mine has 4 hi-Z inputs and only one low-impedance XLR jack, unfortunately.
After doing some research, Shure had a similar model with onboard reverb, which got me to thinking... how cool would it be to build an outboard spring reverb unit into a rackmount chassis for this thing! What's nice about this little unit is that is has a 28vdc accessory tap, which could be close for driving a tank circuit.
This is my first true from-scratch DIY build, but I have a well-outfitted bench (minus a scope). Just starting my research... here are my initial questions:
1) Accutronics has a whole mess of sample diagrams for drive and recovery circuits, and they all seem pretty straightforward. Some call for 15vdc, but a few don't specify voltage. Will any of these work with the power output of the Shure? The specs call for 5mA maximum load, which seems low to run a reverb circuit... but an EE I am not.
2) Just to complicate the build, I am hoping to be able to use a Modutec VU meter I picked up for $1 yesterday. This meter driver circuit calls for balanced audio inputs, and also runs on 12vdc... am I really going to be stuck stepping down that handy-dandy 28vdc output for this build? I didn't want to have to use an Xfmer if possible to save weight and $$$.
3) Anything else I should work to understand before getting started?
Thanks in advance, gurus. Photos to come.
-Mike
I picked up a mid-60's Shure mic mixer over the weekend for next to nothing. After cleaning the pots and re-attaching the audio transformer that had come loose and was rolling around the inside of the chassis, it works like a charm (could use new e-caps, but I'll get to that). Happy to post photos soon, but here is a link to the data sheet (kudos to Shure for still providing these!). Mine has 4 hi-Z inputs and only one low-impedance XLR jack, unfortunately.
After doing some research, Shure had a similar model with onboard reverb, which got me to thinking... how cool would it be to build an outboard spring reverb unit into a rackmount chassis for this thing! What's nice about this little unit is that is has a 28vdc accessory tap, which could be close for driving a tank circuit.
This is my first true from-scratch DIY build, but I have a well-outfitted bench (minus a scope). Just starting my research... here are my initial questions:
1) Accutronics has a whole mess of sample diagrams for drive and recovery circuits, and they all seem pretty straightforward. Some call for 15vdc, but a few don't specify voltage. Will any of these work with the power output of the Shure? The specs call for 5mA maximum load, which seems low to run a reverb circuit... but an EE I am not.
2) Just to complicate the build, I am hoping to be able to use a Modutec VU meter I picked up for $1 yesterday. This meter driver circuit calls for balanced audio inputs, and also runs on 12vdc... am I really going to be stuck stepping down that handy-dandy 28vdc output for this build? I didn't want to have to use an Xfmer if possible to save weight and $$$.
3) Anything else I should work to understand before getting started?
Thanks in advance, gurus. Photos to come.
-Mike
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For what it is worth, hammond used ~13 ma to drive the spring reverb in my H100 organ. 275 volt through a 500 ohm choke, then to a hot 7247 tube plate with a 1000 ohm cathode resistor. Then parallel out from the plate through a 1500 ohm resistor and DC blocking cap (30 uf) to the spring.
The signal back seems to be ~25 mv.
It would be a lot easier these days with op amps and lower voltage. I have trouble getting the signal back through the tube socket to the grid of the EF86 tube. Changing the tube socket is a ****.
The signal back seems to be ~25 mv.
It would be a lot easier these days with op amps and lower voltage. I have trouble getting the signal back through the tube socket to the grid of the EF86 tube. Changing the tube socket is a ****.
Thanks, that's helpful. I would like to build the drive circuit with op amps... as much as I love tube stuff, I don't really want to mess with 1 gajillion volts and a bunch of tubes and sockets just to power a reverb unit.
If it helps, Fender used the TL072 chip for drive and recovery in their Hot Rod series amps. They have a +/-16v supply. See how Fender did theirs and maybe you can get enough of an idea to help studying those. I see your meter uses a TL071, maybe they are close enough in spec to be used for both? Or are interchangeable? BTW, that is for a 4EB3C1B Accutronics tank.
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Or just about any Peavey tube amp uses the same circuit. Probably very similar to the Fender, and both similar to the stock Accutronics suggested circuit.
OK, folks. It sounds like between ~15-25mA is needed to drive a low-impedance tank (like this one).
I've getting divorced from the idea of using the Shure power supply to power the tank. I'd also like to be able to use it without the mixer if I wanted to. Here's what I'm thinking about:
Driver Circuit (from here):
Recovery Circuit (also from here):
VU Meter Circuit (hoping this will run on the 15vdc for the driver circuit???):
I would wire some kind of bypass switch so that I could use this with an effects send/return on my board...
...and power the whole mess with a 15v 1A wall wart.
What am I missing??? If this passes everybody's sniff test I'm going to try to amalgamate these schemos with a bypass and see what it looks like before I start ordering parts.
You guys rock.
Mike
I've getting divorced from the idea of using the Shure power supply to power the tank. I'd also like to be able to use it without the mixer if I wanted to. Here's what I'm thinking about:
Driver Circuit (from here):
An externally hosted image should be here but it was not working when we last tested it.
Recovery Circuit (also from here):
An externally hosted image should be here but it was not working when we last tested it.
VU Meter Circuit (hoping this will run on the 15vdc for the driver circuit???):
I would wire some kind of bypass switch so that I could use this with an effects send/return on my board...
...and power the whole mess with a 15v 1A wall wart.
What am I missing??? If this passes everybody's sniff test I'm going to try to amalgamate these schemos with a bypass and see what it looks like before I start ordering parts.
You guys rock.
Mike
To get +-15 you will need a 32 v wall transformer. They make them, the copier/printer/fax machines all use them. They are about $4 at salvation Army resale. You'll need 4 e-capacitors, a couple of regulators or zeners, some resistors to make the current low enough not to blow the regulators or zeners. I used +-8v for my project out of an 18v race car transformer, since my hottest signal was 7 VAC. You want about 7 VAC driving the spring if you follow hammond's practice. (I measured it).
Your mystery R2 and R7 set the gain, you must decide on that. Rf/Rin is the gain.
Spring reverb drivers usually have a low cut filter to keep the bass out of the spring. Another op amp stage usually.
I don't like TL072 for home projects, they lock up if the input ever gets near the power rail. My first op amp project did that every few milliseconds, but I was using ST33078 so it didn't matter. It recovered every few milliseconds. 5532 I don't know, they seem safe from a distance. Don't forget the power bypass capacitors, ~10 uf near the op amps and 0.1 uf ceramics, too. Slow old 5532 probably doesn't need a 33 pf ceramic cap around the feedback resistor like every op amp with a higher slew rate does.
Your mystery R2 and R7 set the gain, you must decide on that. Rf/Rin is the gain.
Spring reverb drivers usually have a low cut filter to keep the bass out of the spring. Another op amp stage usually.
I don't like TL072 for home projects, they lock up if the input ever gets near the power rail. My first op amp project did that every few milliseconds, but I was using ST33078 so it didn't matter. It recovered every few milliseconds. 5532 I don't know, they seem safe from a distance. Don't forget the power bypass capacitors, ~10 uf near the op amps and 0.1 uf ceramics, too. Slow old 5532 probably doesn't need a 33 pf ceramic cap around the feedback resistor like every op amp with a higher slew rate does.
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About the TL072, Fender used it in their Blues DeVille,Hot Rod Deluxe and Deville, the red knob Super series -all three, and the Blues Jr. These are some of their all time best selling amps. They must have been doing something right. Many players say they can't hear the difference between these and tube driven reverbs. And these were the ones I found in 5 minutes of searching. I'm sure there are others and other manufacturers. Just need proper design to avoid problems.
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