Vintage Danelectro reverb unit - help needed

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by John Phillips, Sep 28, 2006.

  1. John Phillips

    John Phillips Member

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    I have here a 1966 Danelectro 9100 Reverb Box a customer brought me. All original apart from one tube :). I've never seen one before - they were never imported into the UK AFAIK, but I assume one of you in the US may know something about it...

    Fault symptom - the reverb doesn't work, although the rest of it does. I told the customer it should be an easy enough fix - how difficult can a reverb unit be? I assumed it would be very similar to a Fender one inside, and that it was probably a dead recovery tube or a broken transducer wire.

    But of course the dead part is the one thing that's critically different from a Fender - the reverb transducer. It looks like nothing I've ever seen in a reverb tank... a little package wrapped in cloth, possibly some sort of piezo/ceramic element, as it doesn't appear to be magnetic. The return one is simply producing no signal. It meters open-circuit - but then it would, if it's a piezo - and the send one does too, so I have no idea yet if that one's working or not either.

    Anyone know for sure what the transducer is or its spec? Is it replaceable with any common substitute? Is this a well-known fault?

    Thanks in advance.
     
  2. brad347

    brad347 Member

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    I would guess tthat it's actually an old phono cartridge. These were sometimes used as reverb transducers (like the Acos GP71 cartridges in my Farfisa organ). They have a crystal in them that liquefies over time. An easy fix has not yet been found. Good luck!
     
  3. John Phillips

    John Phillips Member

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    Yep, that's it - but there's no info on the transducers.

    I think you're probably right. I found out the send one is dead too - I paralleled the return transducer with a cheap acoustic guitar contact pickup and poked it into the end of the spring :), and now I get reverb crash when I shake the unit, but still no reverb with a signal input. The rest of the send circuit is definitely working fine.

    I'll have to find some substitute for the transducers. The return one is easy, an Accutronics type would work perfectly - but the send is harder as it's an ultra-high-impedance type. I may just fit a drive transformer in place of the 6C4's plate resistor and use a standard tank.

    The unit is well worth fixing, the sound is really great even with the reverb not working - the straight-through signal gets a really nice grungy-surf overdrive tone :), surprising with only one tube stage.

    Thanks! Any other ideas still welcome...
     
  4. brad347

    brad347 Member

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    John... what did you figure out with this?

    I actually picked up one of this as part of a "package deal" with another broken single-ended amp recently.

    The recovery on mine works fine. When I tap it it goes "KKKRRRRASSSSSHHH" and the reverb is very lively.

    But the send transducer (these appear to be peizo elements of some sort) may not be working. I get only a very faint reverb signal with the reverb all the way up.

    Did you ever find a fix?
     
  5. John Phillips

    John Phillips Member

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    No - I've still got it, but the owner can't afford to have it done at the moment. It's a fairly big job to convert it to a Fender-type low impedance circuit, which I think is the most sensible solution.
     
  6. brad347

    brad347 Member

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    i found a site on the internet where someone took material from a cheap piezo buzzer/tweeter and repaired it the original way.
     
  7. John Phillips

    John Phillips Member

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    Aha... I didn't think of that!

    Now, I have a large number of piezo disc transducers that someone gave me ages ago, that I've never found a use for... I did wonder about using them as acoustic instrument pickups or something but since there are so many good ones of those about already I never bothered.

    Hmmm... :)
     
  8. brad347

    brad347 Member

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    i might have to try to source some piezo and try that fix.

    John, if you do happen to try this at some point, could you post anything you learn as far as what to watch out for, advice, etc?
     
  9. John Phillips

    John Phillips Member

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    [Lara Croft]

    Ah-hahhh...

    [/Lara Croft]

    :D

    Fixed it - at least I think I have! I'm not totally sure how it should sound, but at least it's working. And it was incredibly easy...

    The transducers I have look very like that one, but larger - just over 1" in diameter. I simply removed the old dead ones, by separating them from the little metal ring and the spring end that are glued to them, drilled a couple of small holes in the rims of the transducers, threaded the springs and the rings through, and hooked them up, then put a spot of glue on to lock them into place. Ten minutes work for both ends.

    Now... it doesn't sound that great, although it's quite cool in a very lo-fi sort of way. The reverb is thin, crashy and slightly distorted, and there's substantially less reverb level than dry signal, so that if you max the mix control to give wet only, it's pretty quiet. But there's a nice spot at about 1 o'clock where you get enough reverb to be useful without reducing the level of the dry signal that much. From reading some of the comments in the link you posted above, I suspect that actually it's working fine, or at least nearly as well as it should be. Even just looking at the crudeness of the circuit and drive system, there's no way it could ever be as deep and lush-sounding as a Fender.

    The only other problem I ran into was that the cathode cap for the 6C4 driver tube was almost completely shorted, so it was distorting badly. Actually, it made the reverb sound even more gritty and oddly cool like that, and a little louder, but unfortunately it also bled back into the dry signal via the tube grid, so I had to fix it... this was the source of the 'surfy-grungy' distortion I was hearing earlier without the reverb - and why it happened with only one tube stage - but it definitely isn't right and detracts from it when the reverb is working.
     
  10. brad347

    brad347 Member

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    Thanks John!

    Do you have any idea where I might be able to source the part you used?
     
  11. mr coffee

    mr coffee Gold Supporting Member

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    using the piezos from those cheap little tweaters... never have done it but what they are getting on ebay for those junk heaps I'm sure i'll start to see them again
     
  12. John Phillips

    John Phillips Member

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    PM sent... :)

    Here it is in place:

    [​IMG]

    Be careful to connect the baseplate side to the ground connection otherwise it will short itself via the suspension ring :).
     
  13. brad347

    brad347 Member

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    That picture will be helpful to me when I try and fix mine. Thanks!

    The unit happens to be about 19" wide which means I might put some ears on it, and rack it for a lo-fi analog reverb texture in my home studio. :)
     
  14. tiwouaf

    tiwouaf Member

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    Hi , I'm new member , I'm French ( excuse me for my bad english !!!) , I recently buy this reverb unit on ebay and she dosen't work , I have all of voltage beetween difference to 6c4 I have only 100 volts , not 120 volts but I don't think is the problem. The reverb tank is replaced by a classical tank , I have no nsound when I place the mix to 10 , I have no kkkrraasssh when I broke the spring , I think is not a good imp├ędance of transducteur . I try with a tank reverb of classic 30 peavey and it's the same problem , can you send me a picture of complet assembly of you original spring and piezo system , were I can buy a piezo and spring ? I can remplace by a accutronics tank ? what model ? I play essencialy blues and rock with a 335 like ( ibanez AS100 ) and kawa├» solibody in a SOUND amp ( italian 70' tube amp ) , thansk for any information
    kind regards
    Denis
     
  15. John Phillips

    John Phillips Member

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    Hi Denis

    Don't worry, your English is better than my French :).


    It won't work at all with a normal reverb tank. The impedance of both transducers is totally different.

    I don't have any other pics of the unit - it's gone back to its owner a long time ago - but it's just the same as that at both ends, very simple.

    Do you have the original spring? If not, you can probably make a standard one work, although the original is a little larger size and looser than an Accutronics type.

    I still have several of the transducers in that picture, and I'd be happy to send you two if you can't find anything like it in France. Send me a private message if you need them - postage cost plus a couple of Euros is all I'd want.
     
  16. tiwouaf

    tiwouaf Member

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    HI , and thanks you for your answer , I don't have the original spring , the unit is remplaced for a unit like accutronics , one spring is broken and they small compare to your picture , But I think I can found a good size spring , but for the transducteur I haven't any idea for found it , I accept your proposition . I think the rest of the unit work correctly . the tube are ok , I just have a low voltage in a 12AX7 second grill , but it's easy to tune with resistor , do you have any idea of the size of the original spring ?

    kind regards
    Denis
     
  17. Valvetrain

    Valvetrain Member

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    hmmmm... sorry about being late to this thread... but... it seems to me that an Ampeg style Accutronics tank may solve the repair and possibly the tone problems... (probably not the volume one though)... since it's a capacitor coupled circuit, it's fundamentally the same as the early Ampegs... for a tank, try a tank with the 2nd and 3rd characters as FB (for example 9FB2C1B (long tank, medium delay) or 8FB3C1B (short tank, long delay) Mojo has a 4FB3D1B that may also work) the 6C4 is basically half of a 12AU7 which is similar to a 6SN7 used in the original Reverbarockets... so the impedances should be close... this is just a thought, so your milage may vary...
     

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