My reverb tank diagnostics

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bradb

Well-known member
Joined
May 5, 2005
Messages
523
Location
Brooklyn, NYC
I just bought a broken external reverb tank. Its pretty old, something like 50's/60's...its a PANoramic brand.

its got 3 tubes in it... 6X4, 6C4 (i think)and a 12AX7. Anyway, my background is all solid state, so i got online and got some info on how tubes work (cool!)...

Basically the tank will give me the spring crash if i hit it, but it won't "process" the incoming signal thru the spring. It will also pass dry signal. If i attach my AC voltmeter (no oscilloscope) to the input side of the spring and strum my guitar, I "see" signal. So basically, I'm thinking the transducer on the input side of the spring is bad, while the output side is good.

Now on to my question.. the transducers look like something rolled up in some kinda sticky paper, not what i'd think of as a transducer. Does anyone know what these are or how they can be replaced??

Thanks for your patience with my lengthy post...

bb
 
The typical spring reverb tank has a coil on each end of the spring - the "send" and "receive" pick-up. Both coils can be evaluated simply by measuring with an ohm meter. continuity - good, open - bad. Usually they either work or they don't. You can liken the reverb tank the same way a guitar pickup works - there is a pickup coil, and a small ferrite bead within the coils field that is attached to the end of the spring.
 
What Bill says. It is an electromagnet (coil) and a hunk of iron. If current flows in the coil, the iron moves. Vice-versa at the other end.

You have voltage on the input coil. You have output thump-boooingggg. You have everything you should have, except input-output reverb. So I would see if a voltage on the coil actually forces current through the coil. An ohm meter answers that. The ohms might be 10 or 1,000... as Bill says, don't care what the actual number is. If it is "infinite" (anything over 100K or 1Meg) then it is "Open", which means zero current and no iron-wiggle. A wire inside broke.

That's not real good news. You can't get an exact match for a factory tank. And to get a generic, you would like to know the input impedance, which you can't measure if it is open. Check the Accutronics site, there may be a color-code for coil impedance.

First: get under a bright light with a magnifier. There are leads from the connector to the coil. There is some chance you just have a bad joint or a yanked wire, something you can fix. If you can ohm the coil directly and prove the break is inside the coil, I suppose you could un-wind the coil and attempt repair. Guitar pickup winding repair skills would be valuable.
 
hey guys, thanks for the info... I'm going to investigate this more and if I get anywhere significant, I'll post back.

all the best,
brad
 
I might add, check the coil leads very carefully. I have seen old reverb tanks where the insulation gets brittle with age and the wire actually breaks inside the insulation due to flexing and bouncing around during transport. If this is the case, it can be salvaged by carefull soldering some new wire onto the leads at the break. If you can find it, use some fine, highly flexible wire such as used on turntable tone-arm leads or something similar. The light mass will resist breakage better than using a heavy wire.
 
The leads coming off the coils are very low mass, flat wires. I'm going to triple-check the connections and then perhaps open up that coil.
 
Be really careful when working on the coil. The spring end is usually hooked on a short piece of flexible steel wire soldered into the back of a brass cup. This wire can be broken easily and is near impossible to find a replacement. Disconnect the spring carefully and try not to handle the coil any more than is necessary.
 
"Panoramic" sounds kinda familiar... it might be the one I couldn't fix a few years ago.

Not all tanks use the standard types of pickups. Some, including the ones in the Silvertone amps, use some kind of piezo deal for both driver and pickup.
 
I took some pictures of the tank and transducer last night...

check it out:

http://instrument.50megs.com/images/dsc00138.jpg

http://instrument.50megs.com/images/dsc00135.jpg


thanks all


{EDIT} damnit, only the first link is working... need a better free image hoster..
 
bradb-end.jpg


Yes, that IS weird. Not at all an Accutronics. The 2 turns are probably a clamp, not an electric coil. I would then guess that the flat strip is a piezo bender/twister. These fail from over-voltage, mechanical abuse, and cheap manufacturing.

As you see, your second link did work for me.
 
No idea where you would find a peizo wafer like this. There was a company called Vernitron in the mid west states that made raw peizo products, perhaps try them. Another option might be to take a peizo transducer from a peizo buzzer and try and adapt this to your application.
 
yep the first coils (off the mount) are mounting clamps.

The transducer is the greyish flat thing pinched in there. the black piece seems to be electric tape, the other transducer does not have this.

I've put a picture of this here:

DSC00136.jpg


maybe this will work... if not, the url is http://instrument.50megs.com/images/DSC00136.jpg

these things are really weird and thought you guys would get a kick out of them... its almost as if someone put a roach in some wax paper and attached some electrodes to it...

I'm probably going to try a cheapy piezoelectric element...ah, i just see someone has replied to the post as i'm posting this....

thanks guys,
 
Amp made/makes? plastic piezo materal

Maybe if you can't find a ceramic one you could glue a small plastic piece on the ceramic.
 
It certainly is a subtle, understated front panel. Had to be for accordians.

:green:
 
> Amp made/makes?

Pancordion Inc PANaramic

bb's image-host is being picky about linking so I've posted a smaller version on my host.

PANaramic.jpg


Unless A Miracle Happens, I think the reverb-tank is as dead as my dog. Which opens the question: how do you stuff a standard Fender/Accutronics reverb tank in there?

> 6X4, 6C4 (i think)and a 12AX7

6X4 is a power rectifier. You could replace with a couple 1N1007 sillycon diodes, but there is no good reason to do so.

12AX7 is two high voltage gain low current amplifying triodes.

6C4 is one low voltage gain high current amplifying triode (pretty much half a 12AU7 twin-triode).

This is near-enough a Fender reverb chain. It probably comes into half the 12AX7 for voltage gain, then to the 6C4 to get more drive current, then to the piezo to waggle the spring. The other half 12AX7 recovers the weak signal out of the spring. Since the whole PANaramic box is instrument level output, they probably mix the amplified reverb with the dry signal passively (does a wire run right from the one knob to the output jacks?).

The piezo impedance is probably 5,000pFd to 50,000pFd, a few thousand ohms at the top of the guitar range and higher at low frequencies. Accutronic tank input coil models run from 10 ohms to several thousand ohms at the bottom of the audio range, rising at high frequencies. Fender nearly always used a 10K:10 transformer out of a small-medium triode like a 6C4 into a 10 ohm reverb input coil. Those transformers are stock items in guitar electronics websites. (On-Sale at Triodeel: $9.95, such a deal.)

So trace enough of the 6C4 plate and cathode circuit to see what they did. Voltages if possible (if you never worked around 300V, be careful!). Almost certainly you will find a plate resistor that can be replaced with a transformer primary. You may have to increase the size of the cathode resistor to keep the 6C4 current from doubling. That should drive a standard Accutronic tank, and your piezo recovery amp should do OK.

The remaining question: mechanical contraptions have slanty frequency response. The Accutronic uses coil inductance to cancel some of their mechanical slant. The piezo surely did something similar, but possibly the opposite way because it is voltage-drive. So you might end up with very shrill or dull reverb tone. We might have to look for odd-size bass-coupling or treble roll-off caps and change the values to get the reverb tone semi-flat.

Guide to Accutronic part numbers

OK prices on tanks and transformers
 

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