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Vintage threads from the first ten years

Gibson reverb tank

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8/16/2005 7:45 AM
Peter	Gibson reverb tank I'm restoring an old Gibson Hawk GA25RVT, and the reverb works, but the reverb tank is a short Accutronics. I think this is not original. Does anybody know anything about reverb tanks used in mid-60s Gibson amps? Long vs short? Gibbs? Input and output impedances? thanks, Peter

8/16/2005 8:14 PM
Rob Mercure	Long tanks - impedances depend on whether they were capacitively coupled to the driver tubes or transformer driven. Which is your circuit? If tranny driven a Fender style tank will do. Rob

8/16/2005 9:22 PM

Thanks for your reply. The reverb is transformer driven, but the tranformer ratio is only about 4:1, so I get the impression it needs a higher impedance than a Fender... aren't the Fender tanks very low impedances like 10 ohms or so? The tank in the amp now is 160 ohms DC, with an AC impedance of 2500 ohms or so...

Do you know anything about Hammond or Gibbs tank numbering? Lots of ex-organ tanks available but I don't know what the numbers mean. The accutronics numbers are explained on their web site.

Peter

8/17/2005 10:32 AM

Peter, 2500 ohms sounds odd to me.

A value that high is normally not transformer driven at all. Lots of old amps like Traynor would simply feed such a tank from a big coupling cap like .22 or .5 mfd from the plate of the driver tube.

Solid state reverb circuits often use 2k or 2k5 inputs and outputs on the tank.

When you say the transformer is a 4:1 ratio, is that voltage/turns ratio or impedance?

Whatever, unless you're totally stuck on keeping the amp stock I'd consider either cap feed to the existing tank or borrowing much of the typical Fender circuit, with a new driver transformer and tank.

---Wild Bill

8/17/2005 11:13 AM

Peter, try this link for a simple decoder chart for the numbers and letters on reverb tanks.

http://www.hoffmanamps.com/images/TANKCHART.gif

Wild Bill, I have the same situation on my GA-17RVT Scout amp.

http://www.informatik.uni-bremen.de/~dace/gg/ga-17rvt.gif

From what I have seen, you can no longer get the early Gibson style reverb pans. I am going to convert over to a Fender style reverb tranny and pan and then jusfogetabouit.

8/17/2005 11:26 AM

Thanks for that.

Sure enough, 2500 ohms is wrong, my mistake, it's 1475 ohms---
accutronics 1fb2a1b. It's not the original tank but it does work.

4:1 is the voltage ratio--- I measured it in the circuit with an oscilloscope. So I guess
that means the driver sees 1475 ohms times
4 squared = 23600 ohms,

which sounds like a reasonable load to drive.
The driver circuit is half a 12au7, with a 1k cathode resistor, and a 250 volt plate supply.

So it seems like the easiest solution is just to buy another accutronics with the same impedances but longer, since this is a short tank. I'm not set on keeping it all stock, it's a player, not a collector.

I notice there's lots of Hammond and Gibbs tanks on ebay, but the meaning of the codes doesn't seem to be readily available like it is for Accutronics.

thanks again,

Peter

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