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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?

  • #2
    If a TL072 drives it, I would expect the highest impedance possible.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      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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      • #4
        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.
        Juan Manuel Fahey

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        • #5
          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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          • #6
            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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            • #7
              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.
              When they close the gun shops, I'll know that the Texas gov't is taking the pandemic seriously.

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              • #8
                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?

                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 . . .
                Last edited by Timpanogos Slim; 06-27-2017, 08:55 PM.

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                • #9
                  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.
                  Experience is something you get, just after you really needed it.

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                  • #10
                    Originally posted by nickb View Post
                    Without reverb : [ATTACH]43944[/ATTACH]

                    With Reverb : [ATTACH]43945[/ATTACH]

                    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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                    • #11
                      A 1EB2C1B has just two springs - it will do fine.
                      Experience is something you get, just after you really needed it.

                      Comment


                      • #12
                        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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                        • #13
                          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.
                          Experience is something you get, just after you really needed it.

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                          • #14
                            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?

                            Comment


                            • #15
                              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.
                              Experience is something you get, just after you really needed it.

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