Music Man 2275-130 redplating one side

[Prairie Dawg]

I've got a recently arrived Music Man 2275-130 amp head (12AX7 PI variety) that runs for a short while, the volume goes away and two tubes on one side redplate.

It looks pretty clean inside and hasn't been bodgered.

As an update I found the same voltage on all four grids-about -34.75v and figured maybe it was one of the coupling caps that had gone south. Replaced all the .047/630v coupling caps (C-34, 35, 36 and 37) and a new preamp tube and it just got worse.

The first time I fired it up when I got it here it had been sitting for a few months, and I was able to play through it for a few minutes before it went belly up.

Back to fundamentals, I guess. Something's rotten in there.

Thanks

[ThermionicScott]

Could be the bias-feed resistors or dirty tube sockets.

[Twist]

It sounds like the tubes may simply be bad. One of the red plating tubes will also cause its mate to redplate too.

[Prairie Dawg]

Dang it Twist, how come I didn't know you when I was a razorback in Fayetteville? I lived in Johnson for a year in 96-97.

I've got a number of things to check. Luckily enough the causes of redplating are relatively uncomplex. One thing I will do before too long is ring out the output transformer, because if one side was shorted it could do this. My handy dandy insulation tester should work well for that.

When this amp first started exhibiting problems a local music shop sold the guy a set of tubes that did not cure much of anything except an excess of currency. I do have a new set of tubes that I can try after I get done checking out the rest of the system.

[Twist]

You could check the tranny out but I would think a short in the transformer would blow the fuse.
Anyway, I do know of Johnson and I avoid it like the plague. It's one of the worst speed traps I've ever seen. I was born in Springdale in 1960 but didn't move back till 2002.

[Prairie Dawg]

I did get to see the chief of police herding a mob of cattle with his police car, they'd gotten out of a pasture south of town and were wandering around in the road.

[Prairie Dawg]

Update on this amp. It was a bad tube, a new JJ EL34, that tested good on my Hickok tube tester-go figger. I'm going to renew the small electrolytics and validate all the stuff in the bias system before I call it good.

[jrfrond]

Update on this amp. It was a bad tube, a new JJ EL34, that tested good on my Hickok tube tester-go figger. I'm going to renew the small electrolytics and validate all the stuff in the bias system before I call it good.

I'm always leary with JJ power tubes. I've seen too many go south. They SOUND good, but they have issues. Not like OTHER new tubes don't have issues as well, but JJ's seem to be pretty consistent in that department.

[Twist]

I agree 100% with jrfrond. I try to steer customers away from JJ's. If they insist on using them then I tell them to buy them off the net and that way I wont have to guarantee them on repairs. You may get a good set or they may short right out of the box.

[km6xz]

Speaking of tube failures, I have a brand new Yerasov Detonator amp here just delivered from the factory with 4 JJ EL34s and one failed in a short performance test on the bench.
Does anyone keep track of new tube quality, how many of which brands fail quickly among currently available brands?
I am trying to line up sources for reliable tubes to stock or distribute here and have run into a few road blocks. I did secure a deal to get Svetlana(the real one) directly from the factory but haven't gotten anywhere with JJ. I might have to import tubes from the US which adds a lot of cost just to get tubes that are already here but unavailable to the country. I got some 6P3SE tubes to test that are cheap and seem sturdy but have not done torture tests yet.

Also, who is a good source, reasonable prices, good consistency supplier? I always had good luck with Ruby, it did not hurt that Tom was located just down the street a ways. But that was from 2004 and before, and tube sources are getting more questionable and drifting towards esoteric audio hucksterism. The number of names these same tubes are called, all with different performance claims and prices is not very ethical.
What I really would like it to find cheap untested tubes in bulk and do my own testing to match curves.

[Prairie Dawg]

This is the third, and last amp here that has had a JJ octal based power tube failure. I've ordered a set of something different.

[km6xz]

Out of how many total installed? Were these 3 failing during bench tests or after the customer had the unit? ow long on average, if it failed on the customer, did the tube survive? Do you recommend a particular brand for reliability or does the customer's tone request take priority over longevity?
From purely technical and business reasons I would prefer to stock only the most reliable tubes but players don't always care if they "heard it somewhere" that they should use XXX brand.

[Prairie Dawg]

These are all amplifiers that came in for service with relatively new JJ power tubes installed. Two were Mesa Boogie Mark IIs, and this third was the Music Man.

I have had a couple of Sovtek stepped base 6L6GCs that have also been in a shorted condition, but those were 6-8 years old and had seen long service.

I have been using Ruby tubes as replacements for 6L6GCs and the results have been good-no comebacks or early failures. They are Chinese made, and they seem pretty robust. I installed a set in a Mesa Mark II, and eight months later it came back for service for another issue, which allowed me to recheck the current draw. Between all four tubes it was still 3 ma between the highest and the lowest.

[Zouto]

I'm always leary with JJ power tubes. I've seen too many go south. They SOUND good, but they have issues. Not like OTHER new tubes don't have issues as well, but JJ's seem to be pretty consistent in that department.

I got lots of troubles with different JJ power and pre amp tubes, many of them blew after short time, in some 6L6s the getter came loose, many of the 12AX7s became microphonic after 6 months.
So I never use them again, it's just wasting time and money.