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Thread: Silvertone 1484 Low Output. Possible bad OT???

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    Silvertone 1484 Low Output. Possible bad OT???

    I've got a 1484 Silvertone Twin 12 that has really low output.

    I went ahead and replaced the 'litic caps for good measure.
    Voltages all over are all pretty close, however a shade low maybe. But nothing WAY out.

    I read that these amps are notorius for the OT going bad. So I pulled the output tubes and measured the primary and got 83 and 37.

    So would this infact mean that the OT is infact bad?

    thanks in advance?

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    Old Timer soundguruman's Avatar
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    you can just put in a bassman transformer for 2X 6L6 or a twin reverb transformer for 4X 6L6
    You have to reset the output tube bias...but there are plenty generic transformers that will work just dandy.

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    Bent Member Chuck H's Avatar
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    Assuming the OT is bad.

    Voltages should actually read high in that amp because wall voltages are higher today than when the amp was made. One reason voltages could read low is excessive current. This could be a bias circuit failure, which could also cause low output. The ohm readings on the OT may or may not indicate that it's bad. This isn't to say the OT isn't bad. Just that I might do more testing before investing in an OT. Especially since the OT is considered a paramount component in a "vintage" amp for collectibility.

    EDIT: I looked at a schem for that amp that showed TWO output transformers. So further searching showed that they were indeed made this way. That isn't to say they were all made this way. But looking at the schem I would think that if there are two output transformers, and one IS bad then BOTH should be replaced with units rated at 1/2 the watts and twice the impedance. If there is only one OT then the above suggestions are fine.

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    Last edited by Chuck H; 12-04-2011 at 05:25 AM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Chuck H View Post
    ...I looked at a schem for that amp that showed TWO output transformers. So further searching showed that they were indeed made this way. That isn't to say they were all made this way... If there is only one OT then the above suggestions are fine.
    Chuck,
    All my saved info indicates that the Silvertone 1484 uses a 2X 6L6 power amp section with one output transformer.
    Anyway...please let us know if there is a model number mixup or if Silvertone did something weird.
    Regards,
    Tom

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    Bent Member Chuck H's Avatar
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    OOOoh. I Googled "Silvertone twin twelve shematic" before. The model # on the schem I looked at turns out to be 1474. My bad.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Member RWood's Avatar
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    Make sure you are ohming the OT primary correctly. You should compare the (red) centertap with each tube's plate connection (pin 3 on each power tube socket.
    With one reading almost double the other, I wonder if you went plate-to-plate, then plate-to-CT.

    Be careful if you do order a new OT because space is really tight in there and a tall one just won't fit.

    As Chuck said, though, make sure your bias circuit is running correctly (check for adequate negative voltage on both sockets' pin 5) and that you wired the voltage doubler/tripler correctly. It is a bit confusing. Also make sure the choke is not burned out, as I saw on the last one I worked on.

    Lots of folks jump to soon to the "bad OT" conclusion and that is partly the reason why they got the rep that they have. Well, that and their size (or lack of it). But thats how they get their sound!

    RWood

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    Quote Originally Posted by Chuck H View Post
    OOOoh. I Googled "Silvertone twin twelve shematic" before. The model # on the schem I looked at turns out to be 1474. My bad.
    I am currently working on a Silvertone 1396 Twin Twelve combo for a customer and it does indeed have two output transformers, one for each pair of 6L6 tubes. It is an interesting amp that sat for twenty years in a barn without speakers. I told the owner that the overhaul only gets us to the place where we get to find out why it was stuck in the barn to begin with. At the time Danelectro was using aquamarine colored Sangamo caps and every last one of them-every one!-had unacceptable levels of leakage. There's a small phenolic board in the upper chassis that mounts a single 12AX7, and it had been used as a latrine by field mice. I'm currently on the hunt for one for myself because it exudes cool.

    For the O/P, I would recommend validating the value of all resistors and capacitors in that amp. You can sub in an output transformer that's reasonably close per your suspicions but I am inclined to think the problem may be deteriorated wax paper caps or resistors.

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    Old Timer Gtr_tech's Avatar
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    Quote Originally Posted by soundguruman View Post
    you can just put in a bassman transformer for 2X 6L6 or a twin reverb transformer for 4X 6L6
    You have to reset the output tube bias...but there are plenty generic transformers that will work just dandy.
    Have you ever worked on a 1484? There's *no way* a Twin output transformer will fit given the chassis real estate available and the clearance to the top of the case. A Bassman xfmr will be tight if it will go at all. I will use undersized xfmr's similar to what Mesa OEMs on those 1484s because they physically fit.

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