[R-390] R-390 Digest, Vol 100, Issue 4

[bernie nicholson]

Hi To all  , On the subject of tube testers , Most military radio gear and 
test equipment from the Tube era , will have a list of electrode voltages in 
the Handbook relating to the potential on  various pins ,I have always 
located faults by measuring the voltages and checking against the list,  one 
can see what's happening by for instance measuring the voltage drop across 
a screen resistor  and plate resistors ,and cathode resistors , if there 
isn't a voltage drop NO current is flowing ,Mental arithmetic will give you 
a picture in your mind of what's happening or not! In the 1960 s when tubes 
were the go People who resorted to tube testers were known as TUBE JOCKEYS, 
and were looked down upon by REAL Service Men , When you look at the signal 
Corps specs , for a Receiver like the AR88 , They state that ANY or ALL  of 
the tubes can be down to 30% emission and the radio still has to perform to 
spec , Most tubes in the 390A are running VERY conservatively,When using a 
tube tester I would imagine that lots of tubes are discarded prematurely , I 
have ALWAYS Regarded the circuit the tube is working in as the best tester 
for the tube , Tube Testers Have their place ie. in a Lab.  to check batches 
of tubes  are in Spec ect .  Our family had a TV set that was purchased in 
1956, it was still working well with most of the original tubes in 1975 
Tubes like vertical and  horizontal output, damper diodes and the  Cascade 
triode in the Front end were replaced a few times as they work harder,  but 
about twenty tubes were original and had survived daily use for two decades 
, BUT if one had tested them on a tube tester regularly They would mostly 
have tested  '' BAD ''  Regards to All  Bernie VK2ABN