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Thread: '73 Sunn Model T problems and choke specs?

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    '73 Sunn Model T problems and choke specs?

    Received this head, supposedly tested and running. I opened it up and the 1.5k and 47 ohm power resistors on two of the sockets were fried.
    Turned out to be a shorted power tube. I sprung for a new quad of JJ 6550's.

    On power up I got a bout 5 minutes of fun, when it started smoking! The dang chokes were actually melting!

    I need to find specs on replacing the chokes?
    I need to replace the circuit breaker (going with a fuse). The schematic seems to suggest 4.95 amp?
    I'm going to recap the amp, but can only find a 100uf/100uf 500v cap can for the 80/80 that's in it.

    THEN I need to figure out what caused the chokes to melt in the first place?

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by glp View Post
    can only find a 100uf/100uf 500v cap can for the 80/80 that's in it.
    No worries, close enough. Melting chokes? There must be an enormous amount of current passing through, perhaps an old filter cap that has massive leakage. You're planning to replace all the electrolytics, that's good, and a necessity in an amp this old. And with the chokes melted, for safety's sake you must replace them. If their insulation has been compromised, there's over 500V that would love to arc or leak thru charred insulation, best to not take that chance.

    If the T power supply is like the other large Sunn amps, copied from Dynaco, then all the current powering the amp passes thru those chokes. The Dynaco part # is C354, and several outfits offer them: Triode has their 3-10C, 1.5 Henry, 200 mA, and boasts a 2500V hipot test so presumably there won't be any problem with leakage to the laminations & frame. There may be other sources. It would be nice to have a larger current capacity as 200 mA is barely enough to provide bias current for the output tubes plus current to run the pre tubes.

    Some data on C354+ which features a 300 mA spec. http://www.classicvalve.ca/docs/C354+_docs.pdf

    diytube had a discussion a few years back which may guide you to an alternate choice. Looks like the Hammond 194G may be a candidate, if you have enough room in, or on, the chassis.

    diytube.com ? View topic - Choke replacements

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    Last edited by Leo_Gnardo; 07-23-2016 at 03:02 PM.

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    Supporting Member loudthud's Avatar
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    The Model T runs two chokes in parallel like the 1000S/2000S series amps. You could go with one choke rated for 400 or 500mA. The main filters are 20uF at 600V. If you can't find caps rated that high, go with a series arrangement.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Supporting Member AjaxLepinski's Avatar
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    If you figure out what caused the choke to burn up, please let us know. I'm very interested.

    Good luck with the restoration!!!

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    Just take your time - wave comes. Let the other guys go, catch another one - Duke Kahanamoku

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    Supporting Member loudthud's Avatar
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    The Model T runs a full Pi filter on the B+ and an Ultra-linear output stage. So a shorted output tube could cause the chokes to melt down. Each output tube has a 47 or 50 Ohm 5 Watt resistor in series with the plate. These act as fuses, but once they blow the screen resistor (1.5K 5 Watt) usually follows.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Supporting Member AjaxLepinski's Avatar
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    Cheers, loudthud!!!

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    Just take your time - wave comes. Let the other guys go, catch another one - Duke Kahanamoku

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