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Thread: SVT-Classic blows R14

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    Question SVT-Classic blows R14

    I was chasing the extra hum in the SVT-CL few days ago - well it wasn't that bad, but it did not depend on tubes, biasing and volume, it was just more than I know these amps should have. I disassmbled it to clean some places - found a suspicious power supply 100uf cap which I replaced - along with main input jack, 220ohm screen r's with updated handling etc. It was running the same (the same extra hum), so I thought I disassemble it once again and look for the dirty grounds or something else maybe.

    When I put it back together, shortly after I hit standby, both RED and GREEN bias light were lit, horrible, super loud squaling noise in the speakers appeared (then I shut it off) and it smoked the R14.. well quite unusual place to blew one!? I have disassmbled it 3 times already, looking for a reason, cleaned and looked for solder problems, PCB trace problems, measured all the components and I see nothing suspicios. I even took a preamp from another SVTCL to be sure it's not there (changed the driver tube and the 12AU7s too). Now running out of the 220ohm R14s and no idea how to trace the real problem...

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    Last edited by boroman; 01-14-2020 at 12:46 PM.

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    Don't forget the joker g1's Avatar
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    It sounds like the OT phase has been reversed and the feedback you hear is burning that resistor.
    Is there a connector somewhere that could be put on reversed, like to the OT primary?

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    Supporting Member nevetslab's Avatar
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    One thing I noticed immediately is the solder joint fractures you have on the two Bias pots in the first photo. This is typical on SVT-CL's, since the back side of that PCB is supported by the bias pots, the phone jacks & the XLR DI output connector. De-solder and re-solder those bias pot terminals PLEASE! Also, when the main PCB is put back into the chassis, the standoff on the Output Xfmr side of that PCB (lower-right corner relative to your photo) is the ground reference for that PCB. Make sure the standoff is super tight to the chassis, and then the screw that secures the PCB to it is also tight. That too is a hum source if it's loose.

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    It sounds like the OT phase has been reversed and the feedback you hear is burning that resistor. Is there a connector somewhere that could be put on reversed, like to the OT primary?
    There are two connectors near the Speaker Board. One is gray ribbon that goes to the preamp, and second is 2pin with 2 red wires (#pic1). Both can be put reversed. I rely on two shots of the SVT guts I have somehwere and connect it that way (#pic2).

    today When testing power tube board (amp fully on, no power tubes, no driver tube), when I put the tester in PIN2 or PIN3 (don't remember) I smoked the 47k R34... What's going on here?!

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