Why Use Fans on Drake Gear
Submitted by Gerald LeMay - W1ID gerald_lemay@ti.com

     Why do we put cooling fans on our Drakes? We do it to extend the life span of our final tubes. Of course it's always better to run your equipment at cooler temperatures but other than the reason just stated, forced air cooling would not be necessary.

     A sweep tube glass envelope is rated for lower temperatures than real transmitting tubes. The maximum temperature a sweep tube glass envelope can take is in the vicinity of 220 to 240 degrees C. Transmitting tubes have "relatively" higher-rated glass temperatures somewhere over 300 degrees C. Plate temperatures could be twice that or more and that heat gets radiated through the vacuum onto the glass envelope.

     Real transmitting tubes generally use a better grade of materials for the internal elements that can withstand much more heat without permanent damage. With repeated misuse and abuse, the glass to metal seals around the pins of the tubes and plate cap leak air molecules into the vacuum of the envelope.

     Over time the tube become gassy. Another problem occurs when the tube elements themselves become softer and sag due to excessive heat during excessively long duty cycles. They may not return to their original geometry when cooled and this will affect interelectrode capacitance and may cause other internal electrical problems leading to catastrophic failure.

     If you ever take too long tuning up you probably heard a muted "ding-ding-ding" noise coming from the finals. The different types of materials in the tube have different rates of expansion when heated. Even in fresh sweep tubes, interelectrode capacitance is a problem and affects power output on 10 meters. No wonder "soft" 6JB6's don't put out nearly as much power on 10 meters compared to 80 meters over time.

     And no wonder also that when a seller is unloading an old Drake they very often state" I've tested it on 80 meters and it puts out full power". What you really need to ask is "but what does it do on 10 meters?". A sure sign you have soft finals is when you have let's say 100 watts out on 80 meters and let's say 50 watts on 10, all other things being equal, AND the power keeps dropping noticeably as you key down.

     In extreme cases, sweep tube envelopes melt or "suck in" and/or tube elements short and/or burn out. This could happen in a moment of inattention or carelessness. Sweep tubes don't tolerate prolonged and excessive overload conditions like real transmitting tubes. Permanent damage can occur due to excessive key down time in CW, TUNE, or RTTY modes or excessive duty cycle related to using certain forms of speech processing such as speech clippers on SSB.

     Another thing to consider is that sweep tubes were never rated for RF service. They were designed to be used in television circuits. Someone must have "guessed" through experimentation and/or interpolated the amount of INTERMITTENT power input a particular tube type could take without significantly shortening tube life expectancy. In other words, there were no specs provided by the tube manufacturers for transmitting purposes and the selected tube was more a matter of economics than performance. The only manufacturer I know of who ever made an attempt at specifying sweep tubes for RF transmitting service was Sylvania and they only specified 500 volts on the plate, not 650 volts used in Drake equipment.

     Some Swan rigs even had well in excess of 800 volts on the plates inside a compact enclosure and of course, no cooling fan. They also used electrolytics rated for 700 volts total where 900 was present but we won't get into that. Sylvania's data used to be available in the old ARRL handbooks and I believe it was also reproduced in QST. I wasn't at Drake but I suspect the fact that Sylvania had specs available and the other manufacturers didn't may have been a factor in selecting Sylvania brand.

     Another factor may have been product consistency in both quality and characteristics at the Sylvania factory. There were many other brands available at that time and some of it was rebranded. I remember trying to use Raytheon 6JB6's instead of RCA, GE, or Sylvania. They just didn't work in my TR-4. They worked in my color TV.

     Also taken into account was power versus cost. The designers knew at some time or other, with repeated temporary overload and without forced air cooling, the finals would need replacement but that wasn't a big deal. Sweep tubes were cheap and plentiful.

     Finally, the bottom line is that 6JB6A's will perform as well and nearly as long as 6146's in transmitting service, provided you take their temperature and duty cycle limitations into consideration.