Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang
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Mike Bryce wrote:
but the tx is not running at nearly close to full power
here's what I know
1. the three 12jb6 draw 250-300ma and produce about 25-40 watts, on
most bands, 80 meters is much lower.
Not typical behavior. Should produce most output on 80 and 40M with a
little less on 20 and 15 and less on 10M. Should be at least 50 - 55%
efficiency.
2. replaced most tubes with nos, but not the finals
3. did a quick peak and tweak and all stages seem to peak correctly
4. while drawing current,(250ma) the 12jb6's plates turn red,
especially the one in the center.
Also not typical. Running at full power, you should see no color,
perhaps slight color with poor output match. One being redder than the
others indicates that one or more may not be carrying their share of the
load. One reason for matched trio tubes, although the primary reason
is to reduce distortion products. 12GJ5's should work, and are _much_
less expensive.
5. The load control must be fully UNmessed to get power into a 50 ohmload
This is also a problem. 80M it should be near fully meshed for a 50 ohm
load. Typically between 4 - 4.5 on most bands into 50 ohms.
6. when the audio drive/tx drive control is fully clockwise, there is
a squeal in the receive audio
Check _IF_ neutralization. Most likely audio feedback through one or
more stages, possible high esr in an electrolytic. Also possible lead
dress has been disturbed. There is a LOT of gain running around in there.
now before I drop for a trio of final tubes, is there any place I
should be looking??
First thing to do is to check the value and condition of R45, the meter
shunt resistor. These tend to go up in value, causing inflated plate
current readings. Verify plate current meter is reading accurately.
and... I have way too much s-meter. THe receiver receives just fine, I
can easily hear 1uV signal, but a 50 uV signal is pegged
What does the S-Meter read when the RF Gain is turned maximum CCW?
Should read near full scale, but not pegged.
looking for a good tube tester... what should I be looking for??
A tube tester isn't a particularly good investment unless you have a LOT
of tubes to check for Good/Bad condition. Most information obtained
from a tube tester can be derived from voltage measurements in-circuit
along with some Ohm's law. At least one good spare for each TYPE of
tube is a much better investment.
If you are going to get a tube tester, get one that is a mutual
conductance type that actually measures tube gain and function with
dynamic signals. The military TV-7 D/U is a very good one, and most any
of the Hickok brand are very good. Don't waste money on an Emission
type of tester. It won't tell you anything you can't determine with an
ohmmeter and a tube manual.
Keep us posted!
73, Garey - K4OAH
Atlanta
Drake B C-Line Service CDs
http://www.k4oah.com
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