1975 twin reverb vibrato channel issue

[Steelwitch]

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

[Steelwitch]

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!

[Steelwitch]

Never mind y’all, swapped out the tube and it works fantastically now. Lol I’ll delete

[Steelwitch]

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...

[g1]

Do you mean the 470ohm screen resistors? If so, they generally burn due to a defective power tube.

[Steelwitch]

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

[Steelwitch]

Yes, specifically on V10

[Steelwitch]

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

[Enzo]

Put a sign on the wall:

ANY tube can fail at ANY time.

[bsco]

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......

[Randall]

I buy burned in sets when available, that seems to help with the out of the box failure rate.

[nevetslab]

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.

[Steelwitch]

.....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. ;-)

[Steelwitch]

While I have this thread open: there is a fair amount of noise when the MV is all the way up— basically from what it looks like, someone did a terrible job dressing the heaters, so I’m gonna go ahead and redo that. Do y’all 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. What’s easiest? Do these white mallory’s always come out? Lol they don’t seem to age well

[nevetslab]

Do these white mallory’s always come out? Lol they don’t seem to age well

The white Mallory's should be replaced....yes!