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Thread: 1975 twin reverb vibrato channel issue

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    1975 twin reverb vibrato channel issue

    Hey yíall, I have a 1975 fender twin on the bench. The vibrato channel has low output, and Iím losing signal before the 12at7 phase inverter. Clean channel is coming through crystal clear, so itís somewhere before the channels merge. It sounds easy but itís really taking a long time for me! Can someone help me out with where I should be probing? I have a feeling itís probably a bad connection. It was working on the bench all of yesterday and this came up right before I left. Thanks!

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    There was no signal across the 10pf cap before pin 7 of V4B. clipped a new one in and we have signal going into pin 7 but not coming out pin 6!

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    Never mind yíall, swapped out the tube and it works fantastically now. Lol Iíll delete

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    Actually, after I put it in the cabinet for the final check, it started smoking and the fuse blew. So, there are obvious power supply issues with this amp. Output tubes were holding a bias, everything looked good. I see now that there is a crack in one of the resistors between the grids of the output tubes, but I'm not sure if that is a result of overheating just now. Just blew my variac too...

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    Don't forget the joker g1's Avatar
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    Do you mean the 470ohm screen resistors? If so, they generally burn due to a defective power tube.

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    So, that resistor went up in smoke. Just wondering if there is a short in the OT that could have caused this

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    Yes, specifically on V10

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    It was a bad tube.. straight from the box of a brand new JJ matched quad wow!

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    Lifetime Member Enzo's Avatar
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    Put a sign on the wall:

    ANY tube can fail at ANY time.

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    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member bsco's Avatar
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    Quote Originally Posted by Steelwitch View Post
    It was a bad tube.. straight from the box of a brand new JJ matched quad wow!
    I have had new tubes fail right out of the box...happens more than you think.....they don't make them like they used to......

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    Supporting Member Randall's Avatar
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    I buy burned in sets when available, that seems to help with the out of the box failure rate.

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

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Steelwitch View Post
    Just blew my variac too...
    .....blew my variac too....? Hopefully it was a fuse for the variac, and not the coil or wiper assembly of the variac. I have both General Radio and Superior Electric variac's (Variac is a trade name for General Radio, not that it matters), and have over the years pulled them apart to service the wiper assembly, replace the wiper brush, spring in that assembly, etc. I've never had a coil fail.

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    Quote Originally Posted by nevetslab View Post
    .....blew my variac too....? Hopefully it was a fuse for the variac, and not the coil or wiper assembly of the variac. I have both General Radio and Superior Electric variac's (Variac is a trade name for General Radio, not that it matters), and have over the years pulled them apart to service the wiper assembly, replace the wiper brush, spring in that assembly, etc. I've never had a coil fail.
    Yep! Just the fuse. I mean who can argue with the design here: fuse on the variac blew, 470ohm grid resistor blew up, everything else is unharmed. ;-)

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    While I have this thread open: there is a fair amount of noise when the MV is all the way upó basically from what it looks like, someone did a terrible job dressing the heaters, so Iím gonna go ahead and redo that. Do yíall have more success with lead dressing against the chassis, out of the way from crossing over other potential sources of noise, or elevated, but neat and tidy! Also wondering if any oscilloscope wizards have tips for identifying noise coming from old caps either with ac or dc coupling. Whatís easiest? Do these white malloryís always come out? Lol they donít seem to age well

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Steelwitch View Post
    Do these white malloryís always come out? Lol they donít seem to age well
    The white Mallory's should be replaced....yes!

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