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

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

    I picked up a Dean Markley K-20X practice amp the other day. Very loud for 10-15 actual watts. Not bad sounding at all. Overdrive is pretty good. Probably going to be part of a starter package for my best friend's daughter who turns 16 in a couple weeks.

    Naturally i opened it up. Curiosity, plus wondering if anything needs to be replaced or upgraded. Turns out they use LED clipping in their OD circuit.

    The board is etched, drilled, and printed both sides for spring reverb. All of the component values are marked. The hole for the potentiometer is already in the chassis, just not through the control panel sticker.

    Of course, I don't know the properties of the reverb tank, but i can see that it was a small one from the mounting holes. I have an Accutronics blue spring reverb i was going to use for another project, which would absolutely fit in this but maybe not on the existing mounting holes (I haven't checked). The board is printed for JST type connectors for the reverb tank, exactly like are already on the accutronics blue tank.

    I can't find much info on obsolete DM practice amps. They've scrubbed their website of any information. Haven't been able to determine if there was a version of this amp with reverb.

    The blue tanks come with 150 ohm, 600 ohm, or 1.5kohm input . All of them have 1.5k output. Since this design would drive and recover the tank with a TL072, I think it's probably a fair guess that 150 ohm was maybe what they used, with 600 ohm also possible. There are no transistors following the output of the TL072.

    Granted, i should probably trace out the circuit and determine what was most likely from there. If it turns out they designed for 600 ohm, that tank is only $15 away.

    There is no position for a switch to disable reverb. Just the mix knob.

    Anyone ventured out this way before?

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    Lifetime Member Enzo's Avatar
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    If a TL072 drives it, I would expect the highest impedance possible.

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    Quote Originally Posted by Enzo View Post
    If a TL072 drives it, I would expect the highest impedance possible.
    OK, but with the caveat that due to power supply limitations, 600 ohms is the highest impedance possible, going by what Rod Elliott has written on the subject.

    I doubt that there is a compelling reason to still use a TL072, since i am not shaving nickles off of a production run of millions. Might require adjustments to the feedback loop, but a JRC4556 would drive the snot out of a 150 ohm tank. With no adjustments to the feedback loop, an opa2034 probably would too.

    I have a 600-ohm tank i could use for testing that is too large for the chassis.

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    Old Timer J M Fahey's Avatar
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    Donīt reinvent the wheel

    Use part type and value printd in the board, and a 600 ohm drive tank, which is the original spec.

    150 ohm tanks are driven either by a couple extra TO92 transistors boosting IC current or 2 or 3 TL07x in parallel, look at Marshall schematics.

    Never seen 1k5 tanks on my bench, guess they are made for high voltage direct tube drive, no transformer, such as Ampeg or Traynor, not your case.

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    Quote Originally Posted by J M Fahey View Post
    Donīt reinvent the wheel

    Use part type and value printd in the board, and a 600 ohm drive tank, which is the original spec.

    150 ohm tanks are driven either by a couple extra TO92 transistors boosting IC current or 2 or 3 TL07x in parallel, look at Marshall schematics.

    Never seen 1k5 tanks on my bench, guess they are made for high voltage direct tube drive, no transformer, such as Ampeg or Traynor, not your case.

    Well, you could use three tl072 in parallel, and i am not surprised that it has been done, but a 4556 can drive 70mA into 150 ohms without breaking a sweat. The reverb tank needs maybe 20mA.

    Yeah, the 1k5 tanks are for tube drive.

    Looks like no retail outlets actually sell the 2EF(2,3) blue tanks - just the 2BF (150ohm). I'll take a pass at tracing and understanding the circuit. But I guess I'll enquire with accutronics' distributors and see what they would charge to ship one of them out.

    Equalizing the input bias currents and altering the gain is no sweat. there are calculators online for those bad at math.

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    Old Timer olddawg's Avatar
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    Time vs benefit. I would find a break in point between the preamp and the power amp and use a cheap digital Reverb pedal. Done. Go Behringer... $24. Multieffects for $39.

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    Quote Originally Posted by Timpanogos Slim View Post
    a JRC4556 would drive the snot out of a 150 ohm tank.
    Ditto for the inexpensive RC4559 which is also spec'd for 600 ohms in the audio band.

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    Quote Originally Posted by olddawg View Post
    Time vs benefit. I would find a break in point between the preamp and the power amp and use a cheap digital Reverb pedal. Done. Go Behringer... $24. Multieffects for $39.
    Meh. There's something about mechanical reverb that will never be simulated.

    I have enough fx processors to have learned that they are awesome, and range from swiss army knives to leatherman tools to Gerber multi-pliers or whatever they call them. But sometimes you just need real goddamn tools. I mean, I can get up to 5 seconds of really clean delay on my Digitech Element, but there's no way to dirty it up in a glitchy organic way, and it will only repeat 36 times max.

    A reverb tank mounted near a speaker does stuff that a pedal can't. Plus it's not like this is a job. This is something I'm doing for fun. Something I'm doing to put work into a gift for someone. I could take her to a pawn shop and hand her $100 but it would be way less personal, don't you think?

    Quote Originally Posted by salvarsan View Post
    Ditto for the inexpensive RC4559 which is also spec'd for 600 ohms in the audio band.
    *shrug* I dunno about wholesale prices, but qty1000 the 4556 is actually cheaper than the 4558 from mouser. The 4559 is a little cheaper than the 4556. Penny or so.

    The 4556 is specified for 120 ohm loads, and a whole lot of expensive headphone amps drive 32-ohm and 16-ohm voice coils with them. The Grado RA-1 for example.

    The 4556 does have much lower input resistance than the 4558, so you wouldn't want to drive a guitar straight into it without some other buffer. OTOH, the njm4562 is not much more expensive than any other njm/jrc chip, way lower noise, faster, and has about the same high input resistance as the 4558. Pretty sure guitar stuff has the 4558 specifically because it is slow, choppy, noisy . . .

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    Last edited by Timpanogos Slim; 06-27-2017 at 09:55 PM.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Timpanogos Slim View Post
    Meh. There's something about mechanical reverb that will never be simulated.

    I have enough fx processors to have learned that they are awesome, and range from swiss army knives to leatherman tools to Gerber multi-pliers or whatever they call them. But sometimes you just need real goddamn tools. I mean, I can get up to 5 seconds of really clean delay on my Digitech Element, but there's no way to dirty it up in a glitchy organic way, and it will only repeat 36 times max.

    A reverb tank mounted near a speaker does stuff that a pedal can't. Plus it's not like this is a job. This is something I'm doing for fun. Something I'm doing to put work into a gift for someone. I could take her to a pawn shop and hand her $100 but it would be way less personal, don't you think?



    *shrug* I dunno about wholesale prices, but qty1000 the 4556 is actually cheaper than the 4558 from mouser. The 4559 is a little cheaper than the 4556. Penny or so.

    The 4556 is specified for 120 ohm loads, and a whole lot of expensive headphone amps drive 32-ohm and 16-ohm voice coils with them. The Grado RA-1 for example.
    Without reverb : K-20X.pdf

    With Reverb : K-30RX.pdf

    Use an 8EB2C1B tank and you can use the TL072, just as many Fender amps do.

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    Quote Originally Posted by nickb View Post
    Without reverb : K-20X.pdf

    With Reverb : K-30RX.pdf

    Use an 8EB2C1B tank and you can use the TL072, just as many Fender amps do.
    Ahh, excellent! Thank you! Saves me tracing it. I hadn't found reference to the K-30X. Duck Jean Enterprise Co? wild.

    The other tank i have on hand is a 1EB2C1B which seems to be a budget version of the 8 series. And I'm starting to realize that the reverb tank might be why there's a bar on the back of the amp right under the amp chassis, instead of along the bottom of the box. I assumed that a Really Small tank would install inside the amp chassis, rather than a medium one outside of it.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    A 1EB2C1B has just two springs - it will do fine.

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    FWIW, though I'm pretty sure I'll be using the 1EB2C1B now, it appears that they are using current drive similar to Rod Elliott's article here:

    http://sound.whsites.net/articles/reverb.htm

    So to use a 150 ohm tank, you'd use, oh lets say an OPA2132 or OPA2134, change R26 to about 3.3k, and R25 to about 150 ohms.

    Using a 4556 MIGHT be as simple as cutting the trace between those two resistors and inserting a 10uf capacitor. I'm not super well versed in balancing bias currents of bipolar input opamps in current drive configuration. I'm assuming that the ESP schematic using a 5532 has the capacitor that the duck jean schematic lacks in order to block DC.

    I'm perfectly willing to use a more expensive/exotic opamp because I'm swimming in them, from high fidelity electronics diy, which is a whole other ball game.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    No. R25 controls the input voltage in to current out ratio. Lower it for lower impedance tanks. Note current required depends on the number of turns on the tank coil, not the impedance. R26 controls the HF rolloff.

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    Quote Originally Posted by nickb View Post
    No. R25 controls the input voltage in to current out ratio. Lower it for lower impedance tanks. Note current required depends on the number of turns on the tank coil, not the impedance. R26 controls the HF rolloff.
    Aren't the number of turns and the impedance fairly closely related?

    R25 seems to be equivalent to ESP's R2, and R26 with ESP's R7.

    His suggestion that the resistor in parallel with the drive coil be roughly 10x the 1khz impedance seems pretty reasoanble to me since it also sets the output impedance of the opamp.

    It's interesting that he suggests higher values for the R2/R25 position given that the power supply situation is similar, but he's using a buffered opamp in his example schematics. He suggests 33 ohms for a 8 ohm tank and a 5532 with transistors added to the feedback loop.

    Am i missing something about how that is selected, or did they just pick the "as much as possible without going up in smoke" value for the tl072?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Timpanogos Slim View Post
    Aren't the number of turns and the impedance fairly closely related?
    Yes, up to a point.

    R25 seems to be equivalent to ESP's R2, and R26 with ESP's R7.
    Correct

    His suggestion that the resistor in parallel with the drive coil be roughly 10x the 1khz impedance seems pretty reasoanble to me since it also sets the output impedance of the opamp.
    It's a good place to start.


    It's interesting that he suggests higher values for the R2/R25 position given that the power supply situation is similar, but he's using a buffered opamp in his example schematics. He suggests 33 ohms for a 8 ohm tank and a 5532 with transistors added to the feedback loop.

    Am i missing something about how that is selected, or did they just pick the "as much as possible without going up in smoke" value for the tl072?
    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.

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    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?

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    Quote Originally Posted by nickb View Post
    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.

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    Quote Originally Posted by PeterPan View Post
    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    Don't forget the joker g1's Avatar
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    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.
    Attached Files Attached Files

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    Quote Originally Posted by Enzo View Post
    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.


    Quote Originally Posted by g1 View Post
    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.

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    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.

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    Last edited by Timpanogos Slim; 06-28-2017 at 07:09 AM.

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    Old Timer J M Fahey's Avatar
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    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.

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    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.

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

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    Lifetime Member Enzo's Avatar
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    verify you have not got the ends reversed.

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    well it doesn't work at all the other way

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    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.

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    Last edited by Timpanogos Slim; 07-05-2017 at 07:04 PM.

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    Lifetime Member Enzo's Avatar
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    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.

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