I have two pairs of 500Z (one pair from my L-7, one from a big Henry) that suffered from this (one tube in each pair). The solder can develop enough resistance inside the pin to keep the filament from lighting, and still look OK. I resoldered the pins and they are both fine. There is a ton of advice on the Amps reflector on how to safely do this; basically, I used a high-temp silver solder to resolder the pins,with a wet paper towel tucked around the base of the wire where it enters the tube to keep the glass from overheating. It does take a little doing, as the old solder should be cleaned out as much as possible and the flux needs time to clean up the oxidized metal.
Whatever you do, don't throw those tubes out. :) 73 -Jim On Fri, 17 Dec 2010 20:51:20 -0600, Joe Loverti wrote: >Hello to All- > >Let me preface this by stating that I am very "green" at >troubleshooting amplifiers, so please bear with me :) > >I've owned and operated a Drake L-7 amplifier for several years >without incident. The other day I noticed my output pwr was diminished >by about 400 watts. I had a look at the back of the RF deck and >noticed the outboard 3-500Z filament was extinguished. After the >sinking feeling set it in, I un-plugged everything and took the cover >off the amp. I pulled the tube that was un-lit and inspected it >visually. I could see no signs of anything cooked, cracked, burnt, et >al. I decided to swap the tube positions and put the good tube in the >outboard socket and put the bad tube in the other socket. I put >everything back together and lit the amp off again. I noticed that >both filaments were now glowing brightly and I was getting full output >again in my dummy load. > >All was OK for a day or so. Then yesterday, I noticed the filament was >out again on the tube that was switched. Only now this tube was in the >other socket. I've read on the web that it's common to the solder on >the filament pins on the 3-500Z to run out due to heat over time. This >causes a bad connection between the filament pins and the tube socket. >I've inspected the pins on the tube and they do not reveal any signs >of solder leaking out of the bottoms whatsoever. They all look >perfectly intact. I also inspected the socket and the "pin grippers" >in the sockets. They look to be in good shape and seem fairly snug. I >cleaned the contacts with a Q-tip and some De-oxit while I was in there. > >Some research on the Web suggested that a check of continuity is >recommended to see if the tube is the culprit and / or if it can be >fixed. I tested the filament pins (1 and 5) on the suspected culprit >3-500Z and I AM reading continuity. But... the reading jumps around >and isn't stable. I also tested the wires above the pins that lead up >into the envelope of the tube. The readings here are also showing >continuity but, aren't stable. The meter jumps around a lot. I'm >planning on checking the "good" tube tomorrow to see how the readings >compare on it. I did not have time enough to get to it today. > >Does anyone have any other suggestions as to what might be the cause >or if there are other tests I could conduct to help diagnose the >issue? I suppose I could go and buy a new set of bottles for the >amplifier. The tubes that are currently in the amp are most likely the >original Eimac's. They have had a long service life. I think I'd >rather get do some more testing before "throwing tubes" at a problem. >Especially, if there is something else that I may be overlooking due >to inexperience. > >As usual, thanks for your time and assistance going forward! > >Happy Christmas and New Year, > >Joe Loverti Jr. >WS8X > > > -- Ham Radio NU0C Lincoln, Nebraska, U.S.S.A. TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time! "Give a man a URL, and he will learn for an hour; teach him to Google, and he will learn for a lifetime." HyGain 3750 User's Group - http://groups.yahoo.com/group/HyGain_3750/ http://radiojim.exofire.net http://incolor.inetnebr.com/jshorney http://www.nebraskaghosts.org _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
