[Central-Electronics] 600L issue

[Jim Candela]

  I have been using my 600L now for a few months teamed up to my Retro-75 AM 
transceiver. Since the 600L is pretty old, I have been waiting for it to fail on 
me. Maybe I jinxed it.....the 600L did die. :-)

  What started happening is the screen regulator tube, the 812A, would fail to 
light up on occasion. So I had a little rod through the perf metal, and a prod 
to the tube would light it up. After several days this quick fix stopped 
working. Ok, time to juggle the 100# chassis, and take the enclosure off. I 
pulled the tube, looked at the soldered ends on the two big pins, and they 
looked fine. So I roughed up the pins with 400 grit emery paper, and took a 
scribe to the tube socket pins to expose some fresh metal. So, put the tube back 
in, and everything worked fine. I operated the rig this way several days, no 
issue. Then I put it back in the cabinet. Still worked great.

  Then Sunday I turned it on, and the 812 failed to light again! OK, juggle the 
chassis again, and replace the 812 with my spare that was NOS in the box. I had 
it in storage, and actually remembered having it...my only spare. So I put that 
tube in, and it lights up...but wait, the 'M' shaped filament was only half lit! 
Damn, my spare is bad...:-(

   So I then re-solder the pins on the original 812A using solder that has 2% 
silver. It works!! Since this was my only good 812A, and not trustworthy at 
that, I looked for an alternative. I have over a dozen 808's back from the day 
when Fair Radio Sales sold them for 75 cents apiece (in the 1970's). These are 
medium Mu tubes with a 7.5V 4A filament, and a grid cap on the side. I wired the 
grid to the base, and plugged it in. It lights, and has 6.5 volts on it.

   Does it work? I am a little concerned running a 7.5V tube on 6.5V, but the 
application here is not taxing the filament for a lot of emission either. So the 
808 seems to work fine for my application. Now I got several good 808's and one 
good 812A in case I need it.

  Back in the case.... :-)

Jim
WD5JKO