Adding spring reverb to a practice amp board already printed for spring reverb?

[PeterPan]

I don't still have the circuit, but I used to have one of those Peavy Mace VT series amps, 150 w from a bank of six 6L6 tubes, but all solid state preamp side electronics. It had a pretty large reverb coil (typical 2 coil, about 20" long), and I was surprised to find those coils were driven by one side of a dual OP amp which had blown. Well seeing on the schematic that the pinout was the same as a Tl082/TL072, I replaced it with a socket so I could experiment, and I think I threw in a TL082 to start, since I had a few. It worked, and never gave me another problem. Perhaps you don't need to drive the coils that much? In fact it occurs to me that if the system were designed to offer minimal drive, the electrical drive might nicely combine with the mechanical vibration drive from the cabinet. Maybe that little OP amp would do fine?

[Timpanogos Slim]

Yes, you are missing something First, R25 sets the gain of the stage. If you have a small voltage input you must use a smaller value of R25 to get the required current. Second, you want to get the best possible signal to noise ratio and that means driving the tank as hard as possible. I did some tests a while back and found that the Accutronics suggested drive levels are very conservative. I could drive them with 10x that with negligible increase in distortion. At 20x there would be adverse mechanical effects. Third, you don't want the opamp to exceed it's current or voltage drive capability at any frequency.

My $0.02: stop over thinking it. Put in the missing components, use the tank you have and enjoy. If there is a problem, then that is the time to think more.

Point taken.

Thing is, I have an academic interest because i have a back-burner project to design a high quality stomp box that will drive an outboard reverb tank. Probably based on rod elliott's article. Haven't decided whether to use the 4556 for ease of brute force or something more refined and a discrete buffer.

For guys who like to put one on top of the bass amp, or like to kick it.

I'm not saying i'm in a noise band, but I'm not not saying I'm in a noise band.

[Timpanogos Slim]

I don't still have the circuit, but I used to have one of those Peavy Mace VT series amps, 150 w from a bank of six 6L6 tubes, but all solid state preamp side electronics. It had a pretty large reverb coil (typical 2 coil, about 20" long), and I was surprised to find those coils were driven by one side of a dual OP amp which had blown. Well seeing on the schematic that the pinout was the same as a Tl082/TL072, I replaced it with a socket so I could experiment, and I think I threw in a TL082 to start, since I had a few. It worked, and never gave me another problem. Perhaps you don't need to drive the coils that much? In fact it occurs to me that if the system were designed to offer minimal drive, the electrical drive might nicely combine with the mechanical vibration drive from the cabinet. Maybe that little OP amp would do fine?

IC preamp and tube output always seemed like an odd choice to me. Unless it's single-ended output, in which case you'd be getting some good even-order harmonics. Which would be destroyed by a push-pull output stage.

I'd totally do something champ-like with a discrete jfet preamp stage and like a 6V6 SE output.

On the other hand, I'd also be more than willing to use a tube preamp stage and a class-D output stage.

[Enzo]

Their solid state preamp sounds good, allows features built in like phaser/trem, and in the store has that row of shining tubes to sell it. And they are freaking LOUD. With a tapped OT, they can drive full power into different loads, while the solid state amp version reduces power as the impedance rises. Unless they want to add an output matching transformer to the solid state.

Looking at it the other way, many OEMs make a solid state preamp and toss a usually meaningless single 12AX7 into it so they can claim tube preamp.

[g1]

I don't mean to question Rod Elliot, but the Fender Blues Junior (and many others) drives an 800 ohm tank via TL072 so I'm not sure why he says 600 ohm limitation.
I agree with Nickb, when it comes to actual application you want to drive the tank as hard as you can. In the attached article, they state "drive the input coil as hard as possible without overdriving (exceeding core saturation)".
What seems like a lot of drive often turns out to be not as much in real world applications. See what sounds good to you.
Also in the attachment are a couple production examples, and a section on 'drive circuit design considerations' that you may find interesting.

[Timpanogos Slim]

Their solid state preamp sounds good, allows features built in like phaser/trem, and in the store has that row of shining tubes to sell it. And they are freaking LOUD. With a tapped OT, they can drive full power into different loads, while the solid state amp version reduces power as the impedance rises. Unless they want to add an output matching transformer to the solid state.

Looking at it the other way, many OEMs make a solid state preamp and toss a usually meaningless single 12AX7 into it so they can claim tube preamp.

Yeah I've seen the perfunctory 12ax7 designs.

I guess with tube power you still have the reality that when you drive them into clipping they still sound good. It's just that you can do a kilowatt of Class-D for the cost of a hundred watts or so of glass. I'm surprised you don't see more class-D in instrument amps. I have heard that the higher wattage Fender "Rumble" amps use B&O IcePower class-D, though.

I don't mean to question Rod Elliot, but the Fender Blues Junior (and many others) drives an 800 ohm tank via TL072 so I'm not sure why he says 600 ohm limitation.
I agree with Nickb, when it comes to actual application you want to drive the tank as hard as you can. In the attached article, they state "drive the input coil as hard as possible without overdriving (exceeding core saturation)".
What seems like a lot of drive often turns out to be not as much in real world applications. See what sounds good to you.
Also in the attachment are a couple production examples, and a section on 'drive circuit design considerations' that you may find interesting.

I think i was paraphrasing when i said 600 ohms is the limit. His article pertains to Accutronics / Belton tanks, and he doesn't mention the 800-ohm version at all. Which would only need marginally more voltage swing than the 600 ohm.

It's the 1k5 tanks that are best left to high voltage systems. Tubes, and potentially tube amps that have been retrofitted with high voltage mosfets. There are HV mosfet fx loop addon boards for tube amps, no reason you couldn't tweak something like that to drive and recover a reverb tank.

[Timpanogos Slim]

Anyway, thanks all for the help. It's almost done, and entirely with parts i had laying around.

At this point it's a 150pf cap and some wiring and hardware work from done. The only really BS part was ending up using a 9mm pot with wire leads soldered to the legs. I considered waiting until i could acquire the same style of, what, 12mm pot they used in this design, but i couldn't find them cheap, and this will work well enough. Just looks a little WTF, yaknow? Need to enlarge some holes in the chassis, install rubber bushings where the leads for the tank will feed through, finish making the cables for the tank, and mount the tank.

I still think the chassis was set up for a subminiature tank similar to the blue tanks inside the chassis, but I'm happy with using the 1 series tank in this project. I'll get a 4-series 150ohm (don't have the nomenclature memorized!) for the 200w bass combo amp project I have in the planning stages. Maybe the 8-ohm version, and use the lightbulb compressor circuit rod describes.

Will post results when I got 'em.

[J M Fahey]

Small cheap amps often use the smallest Belton tank, the one which does not float inside a pan but is "naked", comes with two adhesive foam pads at the ends and is sticked to chassis inside or even the PCB itself.
Does not use expensive RCA connectors but comes prewired with 2 6" pigtails ending in .1" PCB plugin connectors.
Takes all of 15 seconds mounting and wiring one in a little amp.

This is the smallest one, still floating inside a plastic tray:


exact same one is available without tray, just glued to a piece of foam.

This is a slightly larger one, with another simple plastic tray, inside a Fender Frontman:


notice both come prewired and are meant for inside chassis mounting.

[Timpanogos Slim]

Yeah, that looks like the one. Similar to the blue tank, just not enclosed.

These don't seem to be available in small quantities. Well, not at reasonable prices anyway. Belton will ship them EMS from korea at only $35 for shipping.

I went ahead and installed JST XH headers on my board, since i have headers and pigtails. The blue tanks and presumably the cheaper ones have, eh, I forget what it's called, similar to the fan connectors on a pc motherboard. Same footprint, different molding around the pins.

[Timpanogos Slim]

Got it all completed. It works, but the effect is pretty limited. It's an extremely subtle reverb.

[Enzo]

verify you have not got the ends reversed.

[Timpanogos Slim]

well it doesn't work at all the other way

[Timpanogos Slim]

So, after double checking for errors in my assembly, would it be reasonable to reduce the value of R30 to increase the reverb mix?

Edit: or, I guess, increase the value of R29 to increase the gain of the recovery stage.

[Enzo]

That would be something you could determine instantly by touching a parallel resistor across R30.

I would do a couple tests as well. Pull the return coil connector off the board, and touch the hot pin with your finger or a metal tool. It should hum. Is the hum strong or is it weak like the reverb? Or leave it hooked up and brush the springs with your finger, is that noise loud or diminished? how strong is the signal across the drive transducer? I am searching for whether the issue is on the drive or recovery end.