[AMRadio] Testing Transmitting Tubes

Larry Will lwill at voicenet.com
Mon Sep 20 16:02:33 EDT 2004


I just completed a static DC tube tester for 4CX150 and 4CX250 tubes that 
uses HV and is set up with adjustable bias, screen, and plate along with 
appropriate metering and cooling.  I can set the screen and plate voltage 
to match the load curves in the TT manual, and adjust the bias per the 
curves and read Ip, Isg, etc.

Seems to work well.  You need fil, bias, screen, HV, air, and a sealed grid 
cavity and lots of panel meters.  With only about 500 volts on the plate it 
found all the shorted tubes, those with marginal or low emission, etc as 
well as the good ones.  Normal 200 ma on a 4CX250 with 250V screen and HV 
at least 300V is about -22 volts of bias per the load curves in the manual.

One note on 4CX tubes, the screen can and does go negative and the screen 
supply must be loaded down all the time (I used a 15 k resistor), to 
prevent the screen voltage from rising when Isg goes negative.

The only thing better is an AC Gm Test like the Hickocks (and some others 
do) but thats difficult to do at RF.

Larry W3LW

.At 10:19 AM 9/20/04 -0400, you wrote:
>Has anyone built a transmitting tube tester? Or seen any plans on the web? 
>The only thing I have seen on this subject is the issue of GE Ham News 
>that covers the subject (Vol 6, #3, May/June, 1951). I have some power 
>supplies I can use to run the static test. But I need to build something 
>to hold the tube sockets, filament transformers and plate current meter. I 
>hate to reinvent the wheel and would be happy to follow a pre-existing 
>design if I could find one...
>Any ideas?
>73, Don Merz, N3RHT
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