3 amps, same fault in one week: arced-over power tube sockets

[g1]

I'll give the conspiracy theorist angle here just for fun (you know the guys but I don't and sometimes good folks don't like to own up to gaffes).
a) The internal 10A fuse (tin foil protector) was blown so someone must have replaced the 6A something big. This makes me suspicious of the operators. (possibly the 6A was a slowblow and they blew at the same time but...)
b) 3 amps with arced tube sockets, only one tube has evidence of arcing. A bit of swapping going on? Yeah I know they are different type of output tubes but like I said, just for fun.

[jamesmafyew]

I'll give the conspiracy theorist angle here just for fun (you know the guys but I don't and sometimes good folks don't like to own up to gaffes).
a) The internal 10A fuse (tin foil protector) was blown so someone must have replaced the 6A something big. This makes me suspicious of the operators. (possibly the 6A was a slowblow and they blew at the same time but...)]

Haha I'd probably think the same except that I pulled the blown mains fuse out myself (they had just left it there) and it was indeed a 6A slow-blow, and the 10A fast-blow was blown too.

[jrfrond]

Must be something in the air!

My son plays my an early prototype of an amp design I built and sold for awhile. Unlike the production models, it sports bakelite power tube sockets. It's a cathode-bias 6550 design.

It blew the main fuse last week. I opened it up to find the 100 ohm heater balance resistors cooked. I replaced them, the fuse, and the 6550's (one looked like it shorted, because the silk-screen red paint cooked). Turned it on and got red-plating. No bias. As it turns out, the tube short took out the 22uF/100 cathode bypass cap (shorted to ground). So I replaced it. Upon power up, I got a very weak output, and then I saw an arc jump across pins 2 & 3 (plate and heater) and the resistors went up yet again. In a bizarre twist, the condition also took out the OT. So much for the the R3000 protection diodes installed on the plate! New ceramic sockets, a new OT, and new tubes and resistors, and all is good again.

Coincidence? Of course! But it just goes to show that no matter how solid everything is, disaster is always looming around the corner.

I NEVER trust anything electronic!

[J M Fahey]

For me, an *arc* through sockets is not a *parts* failure but a *functional* failure.
It's not caused by "bad quality air" having a lower dielectric constant but by having higher voltage from leg to leg, so I must seasrch for a reason why way higher than expected voltage is there.
3 same failures in a row, same place (people), same week?
Unloaded (inductive) output transformer?.
I would bet the house on that.

[jrfrond]

For me, an *arc* through sockets is not a *parts* failure but a *functional* failure.
It's not caused by "bad quality air" having a lower dielectric constant but by having higher voltage from leg to leg, so I must seasrch for a reason why way higher than expected voltage is there.
3 same failures in a row, same place (people), same week?
Unloaded (inductive) output transformer?.
I would bet the house on that.

Actually, I was just kidding about the air.

The transformer was loaded, but the secondary was open (unbeknownst to me at the time) due to the catastrophic tube failure. As I said, the output was very weak when fired up.

Sh*t happens!

[J M Fahey]

Hi JR.
I was not referring to your

Must be something in the air!

(popular semantic expression) but mentioning

air

, presumed guilty dielectric present there.
The fact we both used the same word in widely diverging concepts is just another demonstration that the Universe is not Newtonian or Einstenian but ENZIAN, as described in His Parable of the Red Toyota Corollas.

[melvin]

No matter what the cause for three amps blowing up, it's always the tech who gets stoned. Widely diverging inferences can be made about this statement too.

[kenda47]

Hi guys. I know this is an old thread but I thought this might be of interest. I would go one step further than Pedro and Steve on this and say wrong speaker cable. I think the power company bit is a red herring as we say here in UK. About 6mths back a road crew brought a fried Dual Showman and cab to me for repair. I asked them where the speaker cable was and they said it got lost before the gig. I said yeah so where's the one you used for the gig? They said oh it burned up just before the amp did. Turns out they didn't have a spare speaker cable and had used a jack to jack jumper cable! Now I don't want to accuse anyone but studios are full of jumper leads but probably not so with speaker cables. My money is on these amps were all connected to the speaker cabinet with a screaned lead rather than a speaker cable. Dissaster!!

[protojim]

I have seen loose faulty grnds to a bldg/home that cause this!

I would say you have a "floating" grnd, or it's disconnected entirely.

You would see 130v on an outet if there were flourescent lighting on that circuit- (from the ballasts)

I have witnessed this destroy amps, stereos, appliances, and nething with a standby xformer in it!

as soon as the wind blows and interrupts grnd- there's a sag and then a surge- a big one too!

It took me almost a week to help a friend diagnose this prob before I made the official statement- It has to be outside your bldg, or at the drop.

Had him call the electric company to check outside ,at the pole, and sure enough there was a broken loose grnd from a tree limb- they made the repair, and he hasn't had a single issue since.