[AMRadio] 75A2


John Carrington, M.D. jmc05 at charterinternet.com
Fri Feb 20 13:36:16 EST 2009


Excellent, Jim...
Thanks so much!

John
WB5CW 

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Wilhite
Sent: Friday, February 20, 2009 9:28 AM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] 75A2

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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