Danelectro reverb box

[Tech. Tom]

Hello:
I am working on this reverb unit. I have signal to the reverb spring and I can input
a signal on the output of the spring and get signal at the output jack, but no signal goes through
the spring.

It has some kind of coupling component at each end that I don't recognize. They don't look
like transformers and I don't get any resistance reading.
See attached picture.

Does anyone know what I could use for coupling, or any suggestions at all.

Thank you.
Tom

[g1]

Is that a 9100 unit?
I think they use peizo transducers or maybe phono cartridges. They can sometimes be fixed with peizo buttons like this:

[52 Bill]

The right side transducer in the photo should be hooked under the metal clip, just like the one in the left side of the photo. The springs are not stretched out as they should be.

G1 is right, the small flat taped blocks at the ends of the spring, are ceramic pickups. After all these years, they will deteriorate and crumble. I have fixed a few with the piezo plates like his photo, but I needed to increase the drive to the spring to get any real reverb.

I would guess that the circuit could be used to drive a set of Accutronics type springs, but that would definitely change the sound of the unit.

[J M Fahey]

Agree and add: I see both transducers properly attached (glued) to "keychain rings", but I *guess* the left one is the driver, so its free end can "safely" (ugh!!!) be attached to a terminal strip hook, while the right one is the pickup, very sensitive to mechanical shock, so it's floated, in a VERY crude way, just by letting it hang from too long wires, which is an accident waiting to happen.

I used Nylon/Grilon fishing line for suspension and thin coiled wire for connection, getting better floating and safer connection.

*Maybe* an earlier user extended the original copper wires (ugh!!!) beyond "factory spec" trying to achieve that, of course flexing copper cuts easy.

Drive is definitely very poor.

I have made similar ones (using spare tweeter piezo disks as drivers as in G1's example) and got quite acceptable results, but had to drive them from bridged Op Amp outputs to get 60Vpp , but specially 5 mA current capability, which that poor 6C4 simply can NOT provide (think 0.5mA at best).

Those piezos are capacitors and a modern disk is between .082 and .1uF , a beast of a load to drive for a triode plate , since it has around 1600/2000 ohms at 1kHz .

Sound is very bright and extended, compared to nasal/sproingy conventional reverbs.

I made a star shaped, multi spring type , all springs different lengths, each having its own driver and all joined by a center pickup one which provided Studio quality reverb.

It was as large as a Plate reverb, of course.

Some Danelectros have the best of both worlds: a magnetic driver and a piezo pickup and use Ampeg type circuits.

[Tech. Tom]

YES g1 it is a 9100, I should have noted that. Do you have a source for that piezo button?

[52 Bill]

I'll disagree with Juan here in that both original transducers are the same and both should be connected to the case with the small metal clip. The green wires are the connecting wires to the transducer and may have been pulled out of the crystal from being pulled on by the springs.

Maybe if you inspect the transducer and wires it may be fixable. Is there any sound from the springs at all?

If one of the two is still working, you can switch it to the drive side and use a piezo disc for the recovery side. The piezo discs work really well as pickups, not so great as the drivers.

The discs can be found almost anywhere. They are sold as signal beepers or as pickup transducers. They are often housed in a small blue plastic shell that can be removed to expose the contact surfaces.

[J M Fahey]

and both should be connected to the case with the small metal clip.

I didn't say that, quite the contrary
I did say that hanging from copper wires is poor practice and even how I split functions: nylon fishing line for support and flexibility and thin copper wires, coiled so they are not pulled by vibration , for connectivity.

Piezo disk drivers perform very poorly in the original circuit, no doubt about that.

After all, it's a single triode plate with 100k resistive load , average current around 1 mA, availability to outside loads about half that, trying to drive a capacitive load of 2k ohms.

With proper current capability I have had those twist and vibrate in my hand , not far from what a cylinder magnet twists in a conventional reverb.

FWIW VOX used a step up transformer to drive their phono pickup driven reverbs.
Wouldn't be surprised if finding 60 to 90V RMS there.

Now, the puny triode with 100k load?
And extra 27k in series with the crystal?
Doubt more than a couple volts reached it.

[Tech. Tom]

You get a little sound from the spring if you tap on the cabinet, but nothing through it. I did open the outer sleeve and peeked inside.
It looks like the transducer device has crumbled. It sounds like aside from redesigning this unit there's not much hope I can
find any original part to fix it.

[J M Fahey]

To keep it simple and keep socket (and chassis hole ) you *might* replace 6C4 by a 7 pin power pentode, 6AQ5 comes to mind, add a "reverb driver transformer" and put a Fender type reverb there.
The rest of the circuit will do; of course you will have to rewire the socket and add a couple parts for the 6AQ5 .

Or you might replace 6C4 by parallel 12AT7 , just like Fender does, but you'll need to replace 7 pin socket by a noval and add chassis hole (or mount it on spacers so it floats 1/2"above).

[52 Bill]

You get a little sound from the spring if you tap on the cabinet, but nothing through it. I did open the outer sleeve and peeked inside.
It looks like the transducer device has crumbled. It sounds like aside from redesigning this unit there's not much hope I can find any original part to fix it.

I haven't found anything that will substitute exactly for those crystal elements. If the one still works, I'd use it on the drive side and replace the receive end with a piezo disc. That would be the easiest fix.

Short of that, I'd look at using the original drive circuit to drive an Ampeg style tank. The inner channel might fit inside 9100 box.