unusual failure with a happy ending...

[212Mavguy]

So I was hanging out in my living room with Rhonda Jean the Trailer Trash Queen.   She came to life in 1965 from Illinois.  She ain't no spring chicken, she's a dirty looking blonde with a little tatter around the edges.  She was hanging out after one sweet singing session.  I was taking a break for a few when all of a sudden this howling hum came out of her like pop, out of the blue.  I got to the power button quickly enough to beat the fuse blowing.  

So that meant that it was trouble shooting time.  One time in a 6l6 amp, a tube had a poor socket connection at one of the pins, and when that connection stopped passing current one of the tubes started to brightly redplate and the same king of howling hum was happening out of the blue.  So I thought that something was similarly happening, after all, I had bought the amp used and had never cleaned the sockets myself.  After several minutes to let stuff cool down,  I wiggled those power tubes in a circular motion in their sockets and fired it up again with bated breath waiting to hear the dreaded noise, the amp this time was it's normal quiet as a mouse noise floor and when I quietly touched the strings it made sound that was pretty normal but less loud than before.

I pulled one of the 60's vintage Tungsram el84's out of this old girl and it was OK.  Grabbed hold of the other and the bottle moved with a squeaking sound but the pins were still in the socket...What happened is that the glass cracked and broke from thermal stress between the bottom where the pins come out and the sidewall that was a lot cooler.  There is a glass weld going all the way around the bottom of every el84 joining the sides and top to the bottom pins and glass plate they go through.  This stress put into the glass from that process couldn't handle the temp differences caused by the cooler.  The welded area was where the break happened.

I have been a fan and user of IERC military tube shields because they are actually very effective tube coolers and the use of them will greatly, greatly extend the life of vacuum tubes due to the glass bottles being kept cool enough to avoid the in-gassing porosity problems associated with high glass temperatures.  That is one of two ways power tubes get old, the other is gassing from inside the bottle as the parts heat and cool down. 

The reason for the initial noise was the introduction of air into the vacuum during operation, it caused a huge increase in current draw.  A few minutes later, the tube was completely filled with air and would not pass current at all, so normally quiet the second time around was the result when I turned 'er back on. The getter, a silver colored deposit of barium inside the bottle top was now displaying the white color of  tube death.

I went to the closet tube stash and found a tightly matched pair of military US el84's that were built by Sylvania.  Got them from Silk Electronics in SLC, UT. several years ago.   His ebay auctions go under the handle of silkytubes.  I used some De-ox-id electronics parts cleaner that I got off ebay and used it to spray a proxibrush kind of dental pick for going between teeth for gum health.  They make exlellent socket cleaners.  I'd spray the brush and plunge it into the sockets, going around each soket time and time again until the tool came out of the socket holes clean.   It was easy to see the difference of the metal color as it got cleaner and cleaner... also sprayed the cleaner directly across each tube's pins and used the terry wash cloth that was catching the overspray to do some further wiping on the pins...then straightened the slightly bent ones from the cleaning process where necessary and resocketed all the tubes into now clean sockets.  I waited a good half hour for the cleaner to evaporate.  Right out of the gate she filled the room like the proverbial fat lady, a good bit louder and throwing out harmonics like a Hammond B3.  Rhonda Jean is one of the best old school sleeper amps in my stable,  I had yanked the cheap little brown plastic turdlet tone caps that were original and subbed in some Russian military hardware units.  These are the type of tone caps found in Siegmunds and Alessandro's boutique backline babes.  Vintage cheese is now underground boutique, please.

I'm not worried about a repeat incident, I am now using a US made military tube along with a US made mil spec tube cooler.  I had gotten hundreds of hours out of those old Tungsrams, as it turned out they had experienced a lot of use before I got them.  The good one left over from the pair will sound really good in someone's Marshall class 5...too bad I don't and won't have one.  Igor the Frank-en-Champ has the SE situation covered and then some....

 

[Genericmusic]

Good read. Thank you. 

[FredZepp]

You have to love stories with a happy ending.... 

[Gitfiddler]

I always feel a little smarter...and a whole lot dumber after reading one of Dr. 212Mavguy's amp autopsy reports.