Adding reverb tank - how do I determine correct impedance?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wocketpatch, Jan 19, 2015.

  1. wocketpatch

    wocketpatch Member

    Jan 15, 2015
    Hi, I'm shopping for a reverb tank to add to a vintage mixer and started looking at the MOD tanks which seem to have pretty positive reviews with lots of different sizes and input/out impedances available. From what I've read the impedance is fairly important, so I want to make sure I'm buying the right thing.

    The mixer preamp section has what looks to be basically an RCA-port effect loop which does not specifically say it is for a reverb tank or any other particular purpose but a spring reverb is what I'm hoping to use it for.

    Here are the only specs I could find but seem like they might help:

    Does anyone know how I can interpret this? What is the difference between the "for use with" and "actual" figure in these examples?

    Thanks for any help here or any other helpful hints you might have for me, reverb-tank-wise.
  2. donnyjaguar

    donnyjaguar Member

    Nov 4, 2005
    This just looks like an effects loop. Reverb tanks are very lossy and you'll need much more drive to get it to work in this application. Probably a good idea to study proven circuits and decide if its worth modifying your mixer/pre.
  3. drbob1

    drbob1 Silver Supporting Member

    Aug 9, 2004
    Reverb tanks are traditionally driven by speaker level signals, yours is going to be way low. You could grab a solid state power amp (like an EHX magnum), send the signal to that, drive the tank and recovery to the return, but no promises that it would work well. Better might be just to find a good quality digital effects unit cheap, like a Yamaha SPX90ii.
  4. wocketpatch

    wocketpatch Member

    Jan 15, 2015
    OK, thanks for the input on my output :). That's great info to know, looks like I might have to go digital to stay in my price range.
  5. J M Fahey

    J M Fahey Member

    May 22, 2009
    The digital one is the plug and play solution.

    Now if you are into DIY Electronics you can build an external spring reverb circuit.

    Of course, it involves building a cabinet or getting a metal enclosure for it, mounting it in a chassis, making a PCB, etc., or building it inside the mixer, if space is available.

    Just curious, what is exactly meant by "a vintage mixer" ?
  6. TimmyP

    TimmyP Member

    Nov 1, 2007
    Find a used TC300 series and go with that. A much better sounding unit for your purpose.
  7. OlAndrew

    OlAndrew Member

    Nov 6, 2007
    Sedona, Arizona
    My Guild Thunder 1 drives the reverb tank off of the speaker, through a cap and resistor thing. You still need a couple of good gain stages on the recovery side though. The signal out the tank is pretty small.
  8. LPVM

    LPVM Member

    Jan 21, 2009
    Dutchess County NY
    You got me thinkin'. (scary, I know)

    I've got a AO-44 reverb amp and it's matching reverb tank that were salvaged from a wreck of a Hammond M111 in a parts drawer. I should clean that up and give it a try to find out what it sounds like as a stand alone reverb unit.
  9. martinb28

    martinb28 Member

    Aug 16, 2011
    But to wocketpatch's question, how does one determine the correct impedance for a replacement reverb tank? I have a Univox U-1226 Guitar Amp. The reverb tank is long gone. How do I determine what to buy as a replacement? Here is the schematic. I tried using a Reverb Tank out of a Fender amp, but that gives a ton of hum/noise. Is there a way to use a meter to determine what to use?

    On a related note - how do I hook up the reverb tank? The amp has RCAs for the reverb that are labelled "In" and "Out"; similarly, the tank has RCA's that are labelled "In" and "Out." How do I connect it? On the one hand, DUH, put In to In and Out to Out - BUT if the labels are "Out" for outgoing signal and "In" for incoming signal, then I would connect the Out on amp to In on tank, and Out on tank to In on amp.

  10. pdf64

    pdf64 Member

    Aug 9, 2008
    Staffordshire, UK.
    The IN socket of the tank is driven from a 6AN8 triode section, ac coupled directly to its plate.
    You'll have to work out which back panel RCA socket that is.
    The tube's plate resistance is ~4700, which is effectively in parallel with the 10k plate load resistor, so the source impedance to the tank will be around 3300.
    From the tank's input transducer impedance should be ~1/5 of that, eg an E type, around 500 - 1000 ohms.
    Following the Fender convention, the tank's IN socket is isolated from the tank metalwork.

    The tank's OUT feeds a pot (trimmer type?) then a 12AX7 grid.
    I can't make out the pot value, maybe 50k?
    For a sufficiently strong signal level without excessive treble roll off, the tank's output impedance should somewhere between ~1/20 and ~1/5 of the pot value, so a standard B type output transducer of ~2500 ohms should be ok, though for a lower but more trebly reverb, an A type of ~500 ohms may be better.

    Another option may be to get rid of the pot, replace with a 220k - 1M resistor, or use a 220k - 1M pot; that should ensure a good signal level with plenty of treble from a ~2500 ohm transducer

    The Fender convention is for the tank's OUT socket to be connected to the tank metalwork.
    Note the mounting orientation / isolation notes and specs on the linked page.

    I hope it works out; reverb tanks always seem to be a bit hit and miss, in that identical units could sound rather different.
    Current production is reported to be of even lower quality, with microphonics being a major issue.
    Last edited: Oct 27, 2015

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