I'm getting ready to install another 3CX800A7. Been tossing these in for 20+ years and still have to think there's a better way. The tube is mounted upside down and it's within a collar, making visibility almost zero. On top of that, the tube has many small bendable pins that go into a relatively small socket. It's always been a 20 minute (if I'm lucky) hit or miss guessing game. The owner in question described the process in the past as including "swearing, sweating, grumbling and wondering why he ever agreed to attempt this again". Makes me feel like a patient man by comparison. Has anyone found a better way?
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Changing the 3CX800A7 in a CCA Transmitter
- Thread starter BobOnTheJob
- Start date
Yup. Same as RF Burns said. I always remove the collar and do it that way. I also got a teflon collar from Van after losing two bakelite chimneys to high voltage arcs and breakdowns.
rfburns said:
I will second that as well - I've done it that way for more than a decade.
Dale H. Cook, Market Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html
Thanks for the collar advice...will try it that way in 22 hours and see how that goes. I did lose one of those collars to an arc around 1993 but it never failed again...that was on a 10KW...this one's a 4.
RFB/Donald/Dale..I owe you one. The owner's jaw almost hit the floor after a complete re-tube in 15 minutes. This is a 4KW running less than 3KW...some folks are getting up to 4 years from a 3CX800A7. This one was toast at 14 months (owner says that is typical for the 21 years he's had the transmitter). 20W exciter running about 15 watts. Closed loop room at 80 degrees year round. Filament at 13.5V for 7 days & then dropped as low as possible without losing emission. 10.5-11V seems typical. Thoughts on extending the tube life?
What's the rise across the tube? Or across the whole radio if you can't get to the tube with a probe or IR.
Don't have that answer Littlejohn, but I'll get it. The PA tube always looks shiny and new when removed...never any trace of heat. The 3CX800A7 looks almost perfect, but does show some very minimal signs of heat.
Also have a look at grid current, which may take some mental arithmetic. In a previous life, I had an owner who thought it was suave to run the drive up on an old CCA 25 KW. Over currented the grid on a regular basis. Kills the tube.
Interesting that you mention that...the PA Grid current on most of these rigs runs around .22-.27 amps...this one runs .57. But the exciter is only outputting 12-15 watts and the power control is set to about mid point. I've never seen one of these that you could get to 0.3 no matter what you did. If the grid current is reduced to .25 amps, the TPO misses the mark by a lot, even with fresh tubes. This in a situation where the 4KW is running less than 3KW. The original test data shows the PA grid current to be in what I've historically regarded as the normal range. I've always assumed that it was metering error. But the fact remains that since the station came on in '91 (original owner) and neither tube has ever lasted longer than 14-18 months. And I recall getting 3-4 years from the IPA on a 10KW. The owner is taking a meat thermometer with him when he turns the filament voltage down Saturday.littlejohn said:
Interesting. This is the only one I've seen that was significantly above the sweet spot...wonder if someone modified something? I did note that the IPA Cathode current is higher than the test data as well. That could point to a reason for both tubes having shorter life spans...Good analysis...gives me something to dig into.Kent T said:
Measure cathode and anode current with a known good meter... and be careful doing it. Then subtract and you got grid current. Now, if the grid is running 570mils and it's 300 volts below the cathode (for instance) you're dissipating 171 Watts in the grid itself. What's the tube absolute grid dissipation rating, relative to what you're dissipating? And, you should always be able to get the radio very close to the initial tuneup report values (a trick Jack Sellmeyer taught me years ago). If it won't, something isn't what it was when the thing was built, return it to that setup and get it right. Then proceed to wherever you need to be with it.
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