Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Rasmus -
No, this is not good! It will lead to early failure of the "red" tube
as well as the others as they try to take the load when the "red" one
weakens and fails.
Does the "red" tube always appear in the same socket? You might check
the Final cathode and screen resistors, as they do change value with
age. They should be within 10% of their marked value and preferably
within 5% of each other.
A method for "matching" final tubes is to plug the tubes in one at a
time, (into the same socket each time,) and measure the voltage at Pin
3, (cathode,) in the "idle" condition (transmitter keyed with GAIN
control at minimum). The idle Plate current should be reading ~30 mA
with only one tube. Select the three tubes that measure closest to the
same voltage, ideally within 0.1V or less of each other. Be sure the HV
is discharged safely before changing tubes, and that the "unused" plate
connectors are well clear of everything else in the Final compartment!
If you are unable to get a good enough set, RF Parts in California still
offers matched "triplets" for $75.
73, Garey - K4OAH
Atlanta
Drake C-Line Service Manual
<http://www.k4oah.com>
Rasmus Weijola wrote:
Hello- I burned my triple 6JB6 : s in a missing
bias situation and have tried to replace them
with good secondhand tubes. I have tried with
four tubes changing combinations but always
end up with one tube getting remarkably red in a few seconds.
How bad is this for the tube and can it endure it ?
Rasmus OH6KH
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