Hi Wade -
First, and most important, is the fact that many (most) of the meter
shunt resistors in 30 year old T-4XCs have gone up in value, causing the
plate meter to read high. The resistor is R59, (3.3 ohm, 1/2W,) and
is located on the T-R relay board. It should be replaced with a 1W
ceramic comp or wirewound 'flameproof' depending upon what you can
find. You will probably also need to replace R58, which is a 'select
at test' resistor up on the meter switch (behind the LOAD control). It
should be somewhere between 51 and 330 ohms, and be selected so the
meter reads 70 mA when the DC voltage across R59 is 230 mV (for a 3.3
ohm resistor). This is the important reading, because this is what you
set the final stage operating bias with, and determines the linearity
and efficiency of the stage. The plate meters cost Drake a little under
$1 each, so are not precision meters!! The 70 mA reading is far more
important to tube life than the absolute resonant plate current at full
output.
The 6JB6 is specified at 17.5W plate dissipation. Maximum plate voltage
is 770V (Drake uses 650V), and peak plate current is 550 mA, so plate
dissipation is the only specification that is being stretched. This
is at 100% operation, 24 hours a day, inside a closed TV cabinet with 20
other tubes in it. These conditions are worse than CCS (Continuous
Commercial Service) in transmitter use. Using this specification, a
pair of tubes would be capable of producing approximately four times
this power, or about 70W INPUT in Class AB1 at 50% efficiency.
Fortunately, amateur transmitter service is MUCH less demanding, and
tubes are a LOT tougher than transistors!!
Sylvania is the only tube manufacturer that actually tested sweep tubes
in Linear amplifier service and published the data. There was an
article in Ham Radio Magazine for April 1968 by Bill Orr that stated
full-blast ratings (250 W PEP input) were not unrealistic, and good
tube life may be achieved ... PROVIDED the operator does not 'cook'
the tubes during tuneup. Cooking was defined as more than 30
seconds at full power in a 2 minute period. Obviously off resonance
periods will cook them a LOT faster. Drake used Sylvania's data
(published in 1964) in the design of the T-4X.
The original tune-up instructions for the T-4X transmitters stated to
adjust the TUNE and LOAD controls for maximum output. Some time later,
the FCC modified the 'purity of emissions' regulations in Section 97.73,
and Drake issued new Tuning Alterations for the T-4X series of
transmitters. These changes specified limiting the final plate current
to a maximum of 340 mA with the LOAD control on 160, 80, 40 and 20M. No
change was recommended for 15 and 10M operation.
The BEST thing you can do to get maximum tube life from 6JB6 finals is
to add a small fan to the top or back of the PA cage, (BLOWING OUT,) to
extract the hot air and move it away from the area. It doesn't have to
be a big or noisy fan, a small, silent fan will drop the PA area
temperature to approximately that of the rest of the transmitter.
Obviously, tuning up quickly is also important, as is making sure you
have the right antenna connected before starting tuneup! Off-resonance
plate currents can exceed 450 mA! Neutralization is also important, and
is worth the extra few minutes to get it right when replacing finals.
Back in the 60's I ran three A and B lines in Autostart RTTY service on
20M at plate currents in the 350 - 380 mA range. The transmitters all
had small fans, and ran 24/7. The stations were used in RTTY repeater
service, ran at full power, and would often transmit in excess of 30
minutes key down. Under these conditions, final tubes lasted about two
years, being replaced when the output at 14.075 MHz dropped from 150W to
100W. Obviously in CW and SSB service they should last MUCH longer. I
have B and C Line transmitters in regular operation that have had the
same final tubes for 10 or more years, and still have full output.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line TR-4/C Service Supplement CDs
www.k4oah.com
wmacleod wrote:
Hi Drakelist
Would someone out there know the maximum plate current a pair of
Sylvania 6JB6 / 6JB6A sweep tubes can handle without damaging the tube
and what the T4X-C PA stage typically draws when tuned for maximum
power output? I have read some conflicting information and would like
to hear from someone with practical experience. I have a T4X-C which
can deliver 150 W output on all bands but requires almost 350 ma to do
so.
Regards
Wade
VA3HM
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