Shorted reverb pan

[Randall]

Anyone ever seen the input side of a reverb pan transducer short? First time for me. It's a 4AB3C1B. It's in a solid state amp, so no transformer, if that makes any difference. It's supposed to be about 8 ohms, right? It measures 1 ohm.

And now that I mention it, the output side is way off as well, measuring 178 ohms when it should be 22K. How does this happen?

[The Dude]

I doubt there's a short. Those numbers are impedance- not resistance. Look at the chart on this page. They show approximate resistance readings for different impedance transducers.

[Tom Phillips]

...It's a 4AB3C1B. It's in a solid state amp, so no transformer...

That is the model used in classic Fender tube amps. Seems like someone installed an incorrect replacement.

It's supposed to be about 8 ohms, right? It measures 1 ohm. And now that I mention it, the output side is way off as well, measuring 178 ohms when it should be 22K. How does this happen?

You are confusing DCR with impedance.

[Leo_Gnardo]

Cosi fan tutte, or "they're all like that." I'd say that's a perfectly measuring tank.

I dunno what frequency is used to arrive at the impedance figures (maybe 1 KHz?), but that's what they measure with an ohm meter. No worries!

[Randall]

OK, I looked at the chart and learned yet another thing! Then I tested it in a Fender Pro Reverb, and it works fine, how about that? So, now the question is, since the amp's send and return sections are working, how would one figure out what is the proper tank for this rather obscure amp? It's a Lab Series L5, like what BB King favored. Are there and rules for matching impedances here?

[Leo_Gnardo]

Are there and rules for matching impedances here?

Of course it would be nice to get a read off the original reverb tank in a LAB series amp.

One thing that seems to be common to just about all reverb tanks, the output coil is the same in all of 'em.

There are a couple of choices of input coils. We rule out the lowest impedance, that's only for transformer-driven tube setups. What remains, call 'em medium and high. I've never found much of a difference in how they behave sonically. What is important in most cases, is that they're NOT electrically connected, no common ground. In the current replacement tanks I've seen, MOD and RUBY brand (I think they're the same, just house-labeled for their distributors) you have input & output jacks mounted on PC boards, with solder lands where you can choose to have common grounds or not by applying or removing a dot of solder. If in doubt order the highest input impedance, you'll measure 200 ohms more or less both at input & output, make sure they're not grounded to each other & you'll probably be all right.

[g1]

That's the correct pan. They call out 1ohm DC resistance for input right on the schematic.

[Randall]

False alarm. I tracked down the schematic, and indeed that's the correct tank. Or at least those are the correct impedences. After more testing with other amp scenarios, for some reason it started working. I sprayed out the connectors and hit the solder points, and it works. As always thanks for indulging me and teaching!

[Tom Phillips]

Randal,
Was the schematic you tracked down the same one that g1 posted? If not would you please post it?
Thanks,
Tom

[Randall]

Tom, that's the one.

[Tom Phillips]

Tom, that's the one.

Thanks.
Although I know it is possible to drive a low impedance load with a SS circuit this is the first commercial guitar amp I have heard of that did so for the reverb spring drive. Interesting.

How does the reverb sound now that it is working?

[52 Bill]

Thanks.
Although I know it is possible to drive a low impedance load with a SS circuit this is the first commercial guitar amp I have heard of that did so for the reverb spring drive. Interesting.

All of the original Fender solid state series amps in 1967 did this as well as the solid state spring reverb units. Supposedly so that there would only be one tank to inventory.

[Enzo]

SS usually drives them right off an op amp output. This is a full on push pull solid state power amp.

[Tom Phillips]

All of the original Fender solid state series amps in 1967 did this as well as the solid state spring reverb units. Supposedly so that there would only be one tank to inventory.

Interesting. Nice to learn something new (to me) every day.
After all these years, I have never worked on one of the 1967 Fender SS amps.

[g1]

I don't think I have either. Being so rare they must command some collector value.