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Thread: Shorted reverb pan

  1. #1
    Supporting Member Randall's Avatar
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    Shorted reverb pan

    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?
    It's weird, because it WAS working fine.....

  2. #2
    Supporting Member The Dude's Avatar
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    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.
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  3. #3
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Randall View Post
    ...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.

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    Old Timer Leo_Gnardo's Avatar
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    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!

  5. #5
    Supporting Member Randall's Avatar
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    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?
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    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.

  7. #7
    g1
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    That's the correct pan. They call out 1ohm DC resistance for input right on the schematic.
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  8. #8
    Supporting Member Randall's Avatar
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    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!
    It's weird, because it WAS working fine.....

  9. #9
    Old Timer Tom Phillips's Avatar
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    Randal,
    Was the schematic you tracked down the same one that g1 posted? If not would you please post it?
    Thanks,
    Tom

  10. #10
    Supporting Member Randall's Avatar
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    Tom, that's the one.
    It's weird, because it WAS working fine.....

  11. #11
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Randall View Post
    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?

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    Quote Originally Posted by Tom Phillips View Post
    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.

  13. #13
    Senior Member Enzo's Avatar
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    SS usually drives them right off an op amp output. This is a full on push pull solid state power amp.
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  14. #14
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by 52 Bill View Post
    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 likes this.

  15. #15
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    I don't think I have either. Being so rare they must command some collector value.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by 52 Bill View Post
    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.
    I just fixed up an old Fender Taurus and it was 1970 as per the schematic. However, a look at the SS schematic for Fender 1967 it is apparent they had almost identical preamp design in many or all of the amps. The reverb is strong and sudden like that of the Dick Dale persuasion. My uncle was going to keep one of the many amps he had me work on and I told him this one sounds great!! Still need to check in with him soon to find out how he likes the sound. All I can say is that I was shocked that the amp was so quiet with nearly no hum at idle, still original filter caps. After all I heard about these amps it seemed they were horrible amps. Just goes to show that you can't believe everything you hear.
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    Quote Originally Posted by DrGonz78 View Post
    I just fixed up an old Fender Taurus and it was 1970 as per the schematic. However, a look at the SS schematic for Fender 1967 it is apparent they had almost identical preamp design in many or all of the amps. The reverb is strong and sudden like that of the Dick Dale persuasion. My uncle was going to keep one of the many amps he had me work on and I told him this one sounds great!! Still need to check in with him soon to find out how he likes the sound. All I can say is that I was shocked that the amp was so quiet with nearly no hum at idle, still original filter caps. After all I heard about these amps it seemed they were horrible amps. Just goes to show that you can't believe everything you hear.
    The astrology sign amps were the second ss series amps. The first series were named just like the tube models. They didn't sound that bad, they were just built like cheap car radios. Bad pc boards and very little thought to how things were assembled. I think it was Forrest White that quit because of these amps. It took something like one hour of disassembly to change the pilot lamp.

    These amps don't sell for much, probably due to the online reputation. Most of them have been bought up cheap and stripped for speakers, reverb tanks and hardware, as they all used the same pieces that the same era tube amps did.

  18. #18
    Old Timer olddawg's Avatar
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    Quote Originally Posted by 52 Bill View Post
    The astrology sign amps were the second ss series amps. The first series were named just like the tube models. They didn't sound that bad, they were just built like cheap car radios. Bad pc boards and very little thought to how things were assembled. I think it was Forrest White that quit because of these amps. It took something like one hour of disassembly to change the pilot lamp.

    These amps don't sell for much, probably due to the online reputation. Most of them have been bought up cheap and stripped for speakers, reverb tanks and hardware, as they all used the same pieces that the same era tube amps did.
    I remember these amps being awful. Fender was trying to make amps cleaner and cleaner while players wanted grind. They were sterile clean and were downright ugly when overdriven. Most SS amps of that era were. The SS Silvertones were utter crap as were the Kustoms IMHO. There seems to be a cult around the VOX SS amps of that era nowadays, but we "harvested" them for parts as well.

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