[AMRadio] 75A2


Jim Wilhite w5jo at brightok.net
Fri Feb 20 12:27:58 EST 2009


In experience I have had with tubes checking good but not working in 
a radio had a lot to do with the design of the circuit they are in. 
If the circuit doesn't take into account things like plate to grid 
capacitance and such properly, they can be a problem for high gain 
tubes.  There is a lot of discussion about this if you want to read 
it.  I had an FM tuner that used a 6BE6 mixer that would not work 
with tubes that were in the normal range on my tester.  If I put one 
with low transconductance in the circuit, it worked fine.  Some 
tubes would even cause an oscillation if they had high enough gain. 
I did a little circuit redesign and solved the problem of high gain 
tubes.  I don't think you will experience anything like that with a 
75A2 so if you  find a bad tube with a tester, chances are it will 
be bad in the radio.

One of the things about using a tube tester is you can find shorted 
tubes before you put them in your radio.  If they are shorted screen 
to G1 you can cause damage to some radios.  A tester will find this 
type of problem before you put the tube in the socket.

I use a Western Electric KS-15650 L2 tester.  It is a Hickok and is 
a transconductance tester.  Don't get one that measures emission, 
look for one that tests transconductance.  There are several that 
will, made by Hickok, B&K and the venerable I-177 made for the 
military by Hickok and others.  One of the things about tube testers 
is if you get a late model it may not have the test setting for the 
old tubes.  If you get an early model it may not have the sockets 
and test setting for some of the newer tubes.  So I suggest you find 
an emission tester of later manufacture that has the obsolete tube 
data available.

Are tube testers the end all of repairs?  No but used correctly and 
knowing their limitations, they are a useful tool.

Jim/W5JO

----- Original Message ----- 

>I can see a tube tester in my future... Any recommendations?
> Thanks Bob,  John WB5CW
>
> -----Original Message-----
>
>
>
> John, Todd is quite right.  The only thing for sure with a new 
> tube is that
> it is new.  I've had several radios that worked quite well with 
> tubes that
> tested poor to bad. Some receivers will perform well with a fair 
> tube and
> others won't with the same tube, If it works, leave it alone. 
> Good luck,




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