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Author Topic: How to know when to stop working on the radio..  (Read 1069 times)
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K8DI
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« on: February 02, 2024, 08:53:44 AM »

So..  I've been back at my RCA, trying to get a modification finished in time for the kickoff of tonight's AM Rally.  When working with a tube transmitter with a pi output network, there’s an easy way to pre-tune the network if you have any sort of VNA, and know the plate load.  With the power off. you clip in an equivalent resistor across the plate and ground, then feed the output with your VNA. Tune for 50 ohms, and when you power it up, it will be close.

The important thing to remember is to remove the resistor when you’re done.

1500v/4k=375mA
375mA*1500v=562w.  1/4w metal film lightbulb...

(this was last night...time to call it quits)


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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KD6VXI
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Making AM GREAT Again!


« Reply #1 on: February 02, 2024, 09:42:10 AM »

Lol.  I've done that.

With 130 uF at 6.2 kv and a 1.7k resistor!

6200/1700 = 3.65A

3.65A x 6.2kv = 22.6kw!

Thats why I started using 1/8 watt fuses for those!  Lol

Input too.  Always nice to tune the input with the vna, then throw 3 to 5 hundred watts of drive into the tube.  Nice smell that will make you change your drawers

--Shane
WP2ASS / ex KD6VXI
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w9jsw
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« Reply #2 on: February 02, 2024, 10:49:59 AM »

BTDT

Hooked up the test rig but forgot to connect the HV. It was tucked against the chassis.

Glitch resistor went off like an M-80. Blew a breaker as it should have. Should not have exploded the glitch.

After careful study, I had too many strands on the K1JJ HV fuse (RG-8 strands from the braid) and a too small glitch R. Was 25 ohms at 20W. According to Eimac, should be at least 50 ohms and 50-100W.

Time to take a break and change the whitey-tightys.

John


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w8khk
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This ham got his ticket the old fashioned way.


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« Reply #3 on: February 02, 2024, 12:34:47 PM »

you clip in an equivalent resistor across the plate and ground, then feed the output with your VNA. Tune for 50 ohms, and when you power it up, it will be close.

The important thing to remember is to remove the resistor when you’re done.


At least, be thankful you remembered to remove the VNA from the RF output!  Easier to view a zorched resistor with humor!

When I am working on any of the high-voltage equipment, if the task requires several steps of disconnection or disassembly, as I proceed, I create a simple checklist, starting at the bottom of the page, working upward, listing each part that was disturbed.  Then when I am ready to restore it to test status, I start at the top, and make sure I have left nothing in an unsafe condition. 

It may be a sign of getting old, as I did not do this years ago.  But if a checklist works for licensed pilots, both young and older, it can't be all bad.  If I am too tired to go to the trouble of keeping the safety list, it is time to do something else!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
W3SLK
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Just another member member.


« Reply #4 on: February 03, 2024, 09:38:22 AM »

W8KHK said:
Quote
It may be a sign of getting old, as I did not do this years ago.  But if a checklist works for licensed pilots, both young and older, it can't be all bad.  If I am too tired to go to the trouble of keeping the safety list, it is time to do something else!
This isn't being old, it is experience! Or as Harvey Penick referred to it as 'well-seasoned!' Remember what George Santayana said: "Those who fail to remember the past are condemned to repeat it!"
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
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