Silverface Bassman 100 tube arcing

[glebert]

Working on a Bassman 100 that looks like it has sat in a shed for 20 years. Currently powering up with 2 out of 4 tubes and an 8 ohm load. Amp powers up and puts out good signal, but after a minute or so I see an arc in a power tube and fuse blows. This has happened twice with two different pairs of tubes in different sockets. First time it was in socket 1, next time in socket 3, so not even same side of push/pull.

Schematic here: https://www.thetubestore.com/lib/the...-Schematic.pdf

I did later find and repair a broken solder joint, but it wasn't on the power tubes and unless there was oscillation I don't know if that could be the problem or not. Broken joint shown in picture below.



Replaced fuse, put first pair of tubes back in (checked for hard shorts on them first), powered up on LBL first, no bright light, power up normally, static coming to the speaker but have not put signal through it yet and powered it down after a quick check of voltages. Plates and screens all at about 478 and bias voltage at -52V.

Thinking I should probably retension the socket pins before I get too far. Any other ideas? Don't want to keep shorting it out as a troubleshooting method.

Thanks,
Greg

[glebert]

Hmm, doing some research and thinking the tubes I am using might not be up to the task. Will have to risk some new ones.

[g1]

When you test for shorts on the tester do you tap on them? (I kind of snap at them with index finger nail)
Then I also do the tap test at working voltage prior to applying signal and again while delivering power to load.
Any response is bad.

[glebert]

When you test for shorts on the tester do you tap on them? (I kind of snap at them with index finger nail)
Then I also do the tap test at working voltage prior to applying signal and again while delivering power to load.
Any response is bad.

I do not do that, great suggestion!

[glebert]

Put in a couple new tubes and tensioned and cleaned the socket pins. Did the tap test with less than full voltage and again with full voltage. No problems. Let it run at idle for 20 minutes with no problem, put a signal through and it sounds good. Not going to say it's fixed yet, but it is at least much better.

[The Dude]

Out of curiosity, what kind of tubes were the ones that were arcing and were they new?

[glebert]

Out of curiosity, what kind of tubes were the ones that were arcing and were they new?

Used tubes, one pair was Bugera stock 5881s out of my 1990 120W head, second set was some Groove Tube 6L6-CHPs. Both pairs were in the Bugera when I got it and it worked but had blown a fuse for the previous owner (which is why I got it cheap).

[SoulFetish]

When you test for shorts on the tester do you tap on them? (I kind of snap at them with index finger nail)
Then I also do the tap test at working voltage prior to applying signal and again while delivering power to load.
Any response is bad.

To illustrate g1's point, it may be worth noting that some testers will not show shorts that will demonstrate under actual operating conditions in an amplifier. My tube tester is completely useless in this regard, which renders it pretty much useless- period. An amp came in with a blown HT fuse and I tested each EL34 for shorts, emission, etc. All test came back fine.
When bringing up the HT and mains on a variac, one of the tubes went hard short only after the mains voltage reached maybe 40-50V (or so).
If you can monitor a moving vane meter with a quick trigger finger response, you can often save fuses and identify the faulty tube. But be extremely cautious, as it will almost never be a gradual increase in current. So when you see it move, it will most probably pin the ammeter. Of course, this is a great application to an incandescent light bulb limiter as well.

[nevetslab]

I'm always having to address power tube contact tension and cleaning of both tube socket contacts as well as tube pins. I haven't yet gone searching for industrial grade small diameter circular cleaning brushes, but just looked to see what I could find on line. I found these so far:

https://www.mscdirect.com/product/details/03716172
https://www.torringtonbrushes.com/br...e-brushes.html


The circular brush in the first instance is 1/8" dia Brass twisted wire bristle brush, spiral wound on a 0.085" dia shank. Similar to cleaning brushes I have for my Pace Desoldering Iron tip shanks. Has anyone looked into this sort of cleaning brush for tube socket contacts?

[glebert]

I'm always having to address power tube contact tension and cleaning of both tube socket contacts as well as tube pins. I haven't yet gone searching for industrial grade small diameter circular cleaning brushes, but just looked to see what I could find on line. I found these so far:

https://www.mscdirect.com/product/details/03716172
https://www.torringtonbrushes.com/br...e-brushes.html


The circular brush in the first instance is 1/8" dia Brass twisted wire bristle brush, spiral wound on a 0.085" dia shank. Similar to cleaning brushes I have for my Pace Desoldering Iron tip shanks. Has anyone looked into this sort of cleaning brush for tube socket contacts?

I haven't, since I am a noob with old tube amps (but I am learning!). Could you use something like these rotary tool bits or would a low power rotary tool tear up the socket contacts? I'm envisioning one quick, light pass around the cup, not grinding away at them.

https://www.harborfreight.com/diamon...-pc-69653.html
https://www.harborfreight.com/20-pc-...set-62379.html

[doombass]

I haven't, since I am a noob with old tube amps (but I am learning!). Could you use something like these rotary tool bits or would a low power rotary tool tear up the socket contacts? I'm envisioning one quick, light pass around the cup, not grinding away at them.

https://www.harborfreight.com/diamon...-pc-69653.html
https://www.harborfreight.com/20-pc-...set-62379.html

I'd imagine those would be too harsh on the plating.