Danelectro DM 25 Reverb problems

[Resonator Guy]

I recently got a Danelectro DM 25 amp in really nice shape. It sounds good and the tremolo works great.
There is just the slightest bit of hum and I am waiting for new electrolytics to arrive in the mail.
The only problems with it have to do with the reverb. Now I know Dano reverbs aren't anything Dick Dale would like, but it barely seems to work. I have never worked on a reverb before, other than accidentally getting one in my Teisco Checkmate 50 going, so I will proceed carefully. I am also waiting for a couple of NOS reverb driver tubes to see if that will help.
The biggest problem is, with the head sitting on the speaker cab, the reverb just wants to howl. Sort of like feedback but one steady note that disappears when you turn the reverb knob right down or hit the footswitch.
This howl is there whether a guitar is plugged in or not, or whether the driver tube is in or not. Put the head away from the cab and no howl, not much reverb either. The reverb does crash nicely when you bump the head so something is working.
Does anyone have any suggestions on what to look at to get this howling to stop, and also what to check if a new tube doesn't beef up the reverb.

[VintageSeb]

Spring reverb or electronic reverb?

[Resonator Guy]

Strictly spring reverb in a 60s Danelectro.

[gmoon]

I'm sure swapping out tubes first is the way to start...

Does the DM25 have the weird piezo element tanks, like some of the Silvertones? I've heard of at least one attempt to replace those elements with cheapo piezo disks.

Obviously, it could be other stuff in the signal path, regardless.

The reverb on those isn't subtle. Personally, I like the option of swampy, "garagey" reverb at times... I have a DM10--no reverb, but love it. Always wanted a DM25, too.

[Resonator Guy]

No it has spring reverb, you can hear it quite clearly if you bump it.
I am posting a couple of pics and a schematic. I really would like to get it working right because the amp is a beauty otherwise. There is a slight stain on the grill cloth from many years ago but other than that it cleaned up real nice, and the grill cloth is coming pretty clean.
The howl is the main problem and a new tube won't fix that because it does it whether the tube is in or not.

[gmoon]

Nice!

The piezo tanks are true spring reverbs, but with piezo rather than magnetic transducers. Here's an example in a Dano 9100 Reverb.

I didn't catch the "bumping" part (why am I so lazy?) That's good, at least.

[VintageSeb]

I would have a look into the reverb unit, maybe theres a faulty connection, or something with the spring.

[52 Bill]

The problem is mechanical. The speaker is vibrating the case, which in turn is vibrating the head, which in turn is vibrating the reverb tank/spring, which is feeding back to the speaker.

You can try adjusting the mounting springs or you could try and dampen the metal reverb tank with some foam weather striping, etc.

Worst case will be that the tank will need to be taken apart and rebuilt.

[Resonator Guy]

52Bill, I can understand that happening if the reverb was actually on, but it does it even when the driver tube is pulled. Does that sound right?
If so, once I get it working, I will just not have the head on the cab.

[52 Bill]

The drive tube has nothing to do with the return circuit.

The return side of the reverb tank connects directly to the input of the power amp and so if the reverb control is turned up it will allow the spring to make sound through the amp. Try shaking the amp head, do you hear the spring rattle through the speaker?

Think of it this way, in the olden days recording studios would build a reverb chamber by placing a small speaker at one end of a room, or oil tank, or clay pipe, with a microphone at the other end. The signal would then be sent to the speaker and the sound would bounce around the room and be picked up by the mike. If the speaker went bad and there was no sound going into the room, would that mean that the mike would also be dead? Of course not, the mike would still pick up any ambient sound that was in the room.

[Resonator Guy]

Thanks 52 Bill. That explanation works for me. Now I will wait on the new driver tube to see if it puts a little more reverb on and use it with the head off the cab. I don't really want to take the tank etc apart as who knows where that might lead me.