[Hallicrafters] Re: When do you replace tubes?

[Sandy]

There has been in the past (distant past!) many objections to using tube testers!
I was an electronic tech for many, many , many years, and I must say this is
"partially" true.  Anyone who relies 100% on a tube checker is foolish!
On the other hand, anyone who does not believe in using tube testers at all
is even more foolish!
You can immediately reject tubes that test with shorts, leakage or open
heaters/filaments.  You can suspect tubes with low emission or low
mutual conductance (gm).  Many times a tube will test good and still not
work properly!  (especially ones used at high frequencies or VHF/UHF!)

Also, many times a tube that tests "weak" my give very acceptable performance!
BEST test is when it is used in the equipment itself under operating
conditions.

Would I rely on a tube tester to help me troubleshoot a piece of gear?
Yup!  Sure would!  Just remember what the tube tester says is a relative
thing and test results must be evaluated by some experience and common
sense.

Hope this helps.
73,
Sandy W5TVW

----- Original Message ----- 
From: "Ken Kaplan" <krkaplan at cox.net>
To: <hallicrafters at mailman.qth.net>
Sent: Tuesday, January 04, 2005 10:42 PM
Subject: [Hallicrafters] Re: When do you replace tubes?


| What I do is test the tubes annualy and determine their trends. If a tube hasn't
gone legs
| up before its annual test, I'll find out if it is starting its downward spiral.
That way I hope
| to catch it while it is spinning and crashing but not yet burning. This assumes
that the
| radio is still working. Otherwise I test sooner. I'd be inclined to start getting
worried
| about a tube that tests below 50% of book Gm. Maybe that number should be 57.3% but
| I like round numbers.
|
| If I had enough tubes and time (especially time), it might be fun to measure or
hear the
| difference between a tube at 100%, 75% and 50%. Gm is the ratio of a small change
in
| plate current to the small change in gird voltage that produces it. Sounds like it
is a
| measure of its sensitivity. Does this mean that a tube with 50% of its speced Gm
which
| is used in a circuit with a designed gain of 50 will only produce a gain of 25? Or
should I
| look more at u factor (amplification factor)?
|
| 73 Ken kb7rgg - every day forgetting more & more about tube theory
|
| > On Tue, 4 Jan 2005 14:56:17 -0800 Waldo Magnuson <magnuson at mac.com>
| > writes:
| > > Hi,
| > >     My question is when do you replace tubes in Hallicrafters
| > > receivers.
| > >   I use a Hickok 6000A tube tester which is a dynamic mutual
| > > conductance
| > > and reads out Gm.  As an example in a SX-100 receiver I'm working on
| > > I
| > > read some of the following measurements:  For a 6AU6 (book value)
| > > 2300
| > > (measured) 1475 or 64%, For a 6C4 (book) 2200 (meas.)1700 or 77%,
| > > And
| > > for a 6SC7 (book) 1000 (meas.) 350 & 200 or 35% & 20%, and so
| > > forth.
| > >     My inclination is to replace any tube below 75% Gm reading of a
| > > new
| > > tube.  What are other opinions?  Thanks.
| > > 73,  Skip Magnuson  W7WGM
|
|
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