Garey Barrell
Tue, 16 Sep 2008 08:20:51 -0700
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 DrakelistWould 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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