Teisco King Stereo Pre-Amp Reverb not working

[gui_tarzan]

I finally got this amp ready to roll with the exception of the reverb and trem not working. The trem comes later...

I turned on the reverb and there is little "wet" signal even with the reverb volume all the way up. What is there is very, very weak and sounds horrible. So I un-soldered the leads and put the ohmmeter on it. I get at best a half ohm through one tank and nothing when I try connections across all three tanks. I checked the terminals every way I could think of and I'm getting open readings on almost all of them. As you can see in the picture there are three tanks connected in parallel. Nothing ventured, nothing gained, right? I pulled the tanks and heard a rattle. Uh oh. When I opened the first one, this is what I saw.



Gosh, I wonder why it wasn't working??? lol

I don't suppose anyone makes a tank that would fit in this chassis if I can't repair these? It's only about nine inches long.

[g1]

The typical accutronics style 8 or 9 series tanks are about 9" long. The trick is which impedance do you need?

It looks like one end of the tank runs to a .02/400V cap to V3. What type tube is V3, would that be the drive end of the tank or the pickup? The other end of the tank goes to the Reverb pot, then a 250K and .005 cap into the hot lug of the vol. control. I would kind of expect that to be the pickup end, but may be mistaken.

If it's a cap coupled tank driven by V3, a 8 or 9 EB or FB type might do the trick.
I guess there is no actual schematic available?

[The Dude]

Possibly a stupid question, but I can't see exactly what is broken on my screen- even zooming into the part where the springs attach. Obviously the springs are detached, but is it not something that's repairable?

[gui_tarzan]

The typical accutronics style 8 or 9 series tanks are about 9" long. The trick is which impedance do you need?

It looks like one end of the tank runs to a .02/400V cap to V3. What type tube is V3, would that be the drive end of the tank or the pickup? The other end of the tank goes to the Reverb pot, then a 250K and .005 cap into the hot lug of the vol. control. I would kind of expect that to be the pickup end, but may be mistaken.

If it's a cap coupled tank driven by V3, a 8 or 9 EB or FB type might do the trick.
I guess there is no actual schematic available?

The third connection goes to the reverb switch jack. The tube is pin 6 of v3, one of the five 12AX7s in this. The only schematic is the one I drew from this layout. I'm still putting the final touches on it before I share it to schematic sites. I have searched for a few months for the right one and it seems to be unobtainium so I have used the opportunity to create one from this amp.

[gui_tarzan]

Possibly a stupid question, but I can't see exactly what is broken on my screen- even zooming into the part where the springs attach. Obviously the springs are detached, but is it not something that's repairable?

It's not a stupid question, the mounting points on the transducer ends don't seem to want to hold the spring assembly anymore. I would joke about it being a new wireless reverb but that means it would have to work.

I may try to figure something out with it but I'm not sure yet what might hold.

[g1]

Ok, that confirms that V3 is the driver tube. So those tank numbers from post #2 should apply.
The layout shows the reverb footswitch coming off the .02 cap so a minor discrepancy in the schematic (shows separate connection).

[Enzo]

Agree with g1, from the schematic a EB or FB tank would be appropriate.

[52 Bill]

It's been awhile since I've seen one of those tanks, but I think that the transducers were ceramic chips similar to the Danelectro tanks. The normal magnetic tank may work, but you will probably need to increase the drive signal to the tank to get a good reverb sound.

Post a better close up of the transducers and that may give us a better idea of what type of technology we're dealing with.

[gui_tarzan]

Ok, that confirms that V3 is the driver tube. So those tank numbers from post #2 should apply.
The layout shows the reverb footswitch coming off the .02 cap so a minor discrepancy in the schematic (shows separate connection).

There are three positive connections from the tanks that I highlighted in the snapshot: one goes to the reverb volume, one to the .02 cap off V3-6 and one to the switch jack. I thought it was odd to have three connections but it does. Keep in mind even though there are two wires coming off the left side of the tanks, the three tank positive terminals are all connected together in parallel.

[gui_tarzan]

Here's another shot of all three opened up. Only one was still attached, explaining why I was getting a tiny reverb signal. Upon closer inspection there are pieces broken and I don't think it's worthwhile repairing this one.

[gui_tarzan]

It's been awhile since I've seen one of those tanks, but I think that the transducers were ceramic chips similar to the Danelectro tanks. The normal magnetic tank may work, but you will probably need to increase the drive signal to the tank to get a good reverb sound.

Post a better close up of the transducers and that may give us a better idea of what type of technology we're dealing with.

There is half a 12AX7 not being used if we go that direction.

[Helmholtz]

It's been awhile since I've seen one of those tanks, but I think that the transducers were ceramic chips similar to the Danelectro tanks.

And piezo transducers (not having coils and rotor magnets) will measure open with an Ohmmeter.

[J M Fahey]

Those MUST be Piezo transducers, no way a 12AX7 plate loaded with 100k can drive them.

Maybe a transducer *real* closeup (one good, one damaged) can show how they are attached.

FWIW each transducer might be a Piezo disk (thing piezo tweeter driver or watch buzzer) with the funny triangular spring yoke glued to it, no electrical connection is needed.

I have made cheap Piezo reverb transducers out of spare tweeter replacement ceramic disks, go figure, and they worked very well.



The original piezo driver/pickup was:



No magnetic pickup tank (think Belton - Accutronics) I know can be properly driven from a 12AX7 plate, period.

From memory, Ampeg used a beefier tube with around 10k or lower (6k8?) plate resistor, a very different thing, and capable of 10-15X the drive current.

And that into the highest input impedance tanks.

[g1]

No magnetic pickup tank (think Belton - Accutronics) I know can be properly driven from a 12AX7 plate, period.

From memory, Ampeg used a beefier tube with around 10k or lower (6k8?) plate resistor, a very different thing, and capable of 10-15X the drive current.

And that into the highest input impedance tanks.

The ampeg V amps used 6CG7 as you say.
Traynor YGM3 used two halves of a 12AX7 paralleled, and he said there is an unused half available so that may be an option.

[g1]

There are three positive connections from the tanks that I highlighted in the snapshot: one goes to the reverb volume, one to the .02 cap off V3-6 and one to the switch jack. I thought it was odd to have three connections but it does. Keep in mind even though there are two wires coming off the left side of the tanks, the three tank positive terminals are all connected together in parallel.

The pic is washed out a bit so I can't see if the wire to the FS jack is going to the same lug as the other white wires or not.
It's no biggie anyway, I just mentioned it because the layout shows that wire directly connected to the others.