[AMRadio] Testing tubes

[John Coleman]

Hi Jim
	The idea that you have a triode connection is not exactly valid for
this type of test because there is no series load resistor.  The plate and
screen voltage are held constant regardless of the plate current.  It is
however important that you measure the plate current separately from the
screen current, if you want to measure the GM of a tetrode connection.
Changes in screen current, is not what the test is about.  The plate and
screen voltage may come from the same source.  The screen voltage is most
critical and must be maintained constant as close as possible.  Since plate
and screen come from the same source then the source voltage should be
monitored with a DVM and adjusted to be the constant of 250 Volts.  This
will require a little juggling each time you make a change in grid voltage.
The actual plate current that you start with (90-110 ma) is not so
important.  But the change (delta) Ip with grid voltage change is what we
are looking for.  The only other thing that might be a problem is that the
plate dissipation will be about 25 watts.  Is this tube OK for that during
the test?  I would not use a delta Eg of more than 5 volts to make the test
this would yield a change in plate current of more than 46.5 ma which is
about 50 percent of the quiescent current of 100 ma.  It is important that
the increase in current when the Eg is made more negative be equal to the
decrease in current when the Eg is made less negative by the same amount.
In other words it should be linear. Keep in mind that when the plate current
goes to 150 ma or more that the plate dissipation will be 50 percent more so
make you measurement fast and don't for get to hold the EP constant with
that DVM.

You could also place a 1 ohm resistor in the plate lead and capacitively
couple to a sensitive AC voltmeter (scope) and measure the AC current
produced as a change in Eg AC. A 100 ma change would be represented as
0.1VPTP on the scope.  You would need a very large capacitor to Filter the
source with.

73
John, WA5BXO
   

-----Original Message-----
From: amradio-admin at mailman.qth.net [mailto:amradio-admin at mailman.qth.net]
On Behalf Of Jim candela
Sent: Sunday, October 12, 2003 11:08 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] Testing tubes

    Hi Group,

    I got a wild hair, and made a setup to compare a set of used Genalex
KT77's to a new MP (matched pair) of Sovetek EL34's. I was following a note
in the KT77 data sheet that stated when a KT77 is considered bad (< 9.3
ma/volt transconductance when EP, Eg2 = 250 volts held constant, and IP =
100 ma). Following this criteria, I made a test setup. I am not too sure of
my technique as it is a work in progress. I am curious that the Gm can vary
considerably when changing Vg1 +/- 1 volt from the value used to obtain 100
ma plate current. Maybe this is all wrong, but I averaged the two numbers
assuming that is the final answer. Since Genalex stated to run the screen
and plate at 250 volts, I connected them together (Fluke 8012a looks at
plate current only), so this test is in triode mode (or is it not?). For a
high voltage supply I used my Sherwood S8000 stereo amp / FM with a variac
(get ~ 250 vdc at 65 volts AC input). Hey it was there (awaiting a re-cap
job), convenient, and able to do the job!

    Is the data I've taken valid? If not, can someone enlighten me? Maybe I
need to vary Vg1 less than +/- 1 volt, for better data?

Regards,
Jim Candela

Test results:
http://pages.prodigy.net/jcandela/KT77/genalex_kt77.htm

KT77 Data Sheet:
http://pages.prodigy.net/jcandela/KT77/kt77.pdf

Test Setup:
http://pages.prodigy.net/jcandela/KT77/PA110015.JPG


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