Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Tom -
It sounds like you have an oscillation in the final caused by a defect
in one of the tubes. It's a little puzzling why it only shows on 20M,
but probably has to do with the neutralization circuitry and the
defective tube. The fact that the plate current won't dip implies that
the frequency of the oscillation is outside the range of the pi-net when
set for 20M. You might be able to find it with an external general
coverage receiver, but it's really not that important.
I would expect one of the defective trio of tubes to show some color in
the plate, since that one is probably drawing the excess plate current.
If not, what I would do is take one of the tubes in the "good" trio, and
swap it with one of the tubes in the "bad" trio. If the oscillation is
still evident, replace the tube removed from the "bad" trio and try
another one until you find which one is the cause.
If the two tubes in the "bad" trio are otherwise in good shape, you can
probably get a good new one and it will be a close enough match. You
can check for "match" by putting in one tube at a time and noting the
plate current at idle. It should be about 33 mA for one tube. More
accurate would be to measure the voltage across the 15 ohm cathode
resistor of the socket that the one tube is plugged into. Swap in each
tube into the same socket and note the plate current of each _without
adjusting the bias voltage_ and select the three that are closest in
current. Hopefully they will be within a couple - three mA of each
other. This is not a "real" matching, but will be close enough for all
practical purposes.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Thomas Wright wrote:
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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John:
Well, I tried swapping finals. In fact I swapped with my TR-4 finals,
which are weak. The finals that were in the TR-4C that was acting up
do the same thing in the TR-4, and vice versa. When in the TR-4 the
tubes produce very good outputs on all bands except 20, just like they
do in the TR-4C. Obviously the problem is the Sylvania 6JB6's, or at
least one of them. 20 meters simply won't dip on either radio when it
has those tubes and output stays very high even when reducing xmit
gain to full CCW. Can someone explain that to me? Do I sense that
the only fix is new finals? I hate to give up such a strong trio.
Tom
----- Original Message ----- From: "Thomas Wright" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Saturday, December 01, 2007 2:44 PM
Subject: Re: [drakelist] AC3 Bias
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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I assume you mean finals. I have a set but they're weak.
----- Original Message ----- From: "john" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Saturday, December 01, 2007 1:43 PM
Subject: Re: [drakelist] AC3 Bias
john <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Sounds like the PA might be oscillating on 20M. Do you have another
set of tubes to try, to see if behaves differently?
John
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