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Engineering Extra
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The Reference Room
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Milestones
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| Gates Built a 1 kW Classic |
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by Charles S. Fitch,
10.11.2006
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Charles S. Fitch, W2IPI, is a registered professional consultant engineer, member of the AFCCE, senior member of the SBE, lifetime CPBE with AMD, licensed electrical contractor, former station owner and former director of engineering of WTIC(TV) in Hartford, Conn., and WHSH(TV) in Marlborough, Mass.
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Photos
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In appearance high-tech for its time, the sleek Gates BC-1T was styled to compete against the RCA `New Look' cabinetry.
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Illustration from the BC-1G instruction book shows the perforated front screen removed. The assembly on the right swung into the void between it and the dummy, allowing easy front and back access.
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The cost-effective and readily available 6BG6 tube was used in the 1T. In this case two are paralleled for use as an RF driver. The dashed area is accommodated on a printed circuit board.
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Even at this late date, nearly every cluster of stations has an AM station. There, we often can find at least one classic 1 kW transmitter.
Many of these stalwarts are from the generation 1950 through 1980, when tubes ruled the roost.
When you study Rocketry 101 in college, you are taught that if you want the rocket to take off for certain once, you need to design and construct it to take off a thousand times. Similarly, if manufacturers wanted a 1 kW rig to last until at least to the end of the warranty, they had to design and build the box to last at least a decade.
Most succeeded far beyond their expectations. Many of these transmitters remain in main and standby service even now. The surviving silent sentinels are a tribute to those who made them and maintained them over the years.
As we've mentioned in a previous profile of the Bauer 707, tube manufacturers offered mainly two power tube choices for 1 kW: the 4-400 tetrode and the 833 triode. The Bauer was a 4-400 unit, so let's look at an 833 box this time, the Gates BC-1.
Fast service
Gates made two all-tube models in this period, the BC-1G and the BC-1T. The company's idea was that a new transmitter was a major capital investment for even a medium-sized station, which might be able to afford only one. Keeping that rig on the air was critical, so ease and speed of service was a major concern. To facilitate this, the major subassemblies of modulator and RF final swung out from the sidewalls to increase access to components.
A dummy load, this time made out of discrete power resistors, was built onto the wall of the transmitter and in the exhaust airflow for cooling.
For the portion of the price the design represents, the quality and reliability of the final product are extraordinary.
A designer spends little or no time reviewing aspects of the product that are right, focusing instead on what is having difficulty or needs improvement. That's the only explanation for the nearly universal low-voltage transformer failure in these Gates units.
Apparently the transformer is running well up its current curve. It was durable and reliable when all the filter components were correct; but the transformer failed as soon as the input choke shorted, upping the current (overall the circuit was lower-resistance, hence higher-current).
One should not complain about a problem that occurs years after purchase; but almost like clockwork, after eight years you'd have new doorstops on hand, because you had to replace the transformer and choke.
The two transmitter models were similar in circuitry concepts, but there were notable distinctions.
The BC-1G had 807 tubes in many of the low-level stages, where the 1T used the 6BG6 pentode. The manufacturer viewed this as a more readily available, TV repair shop-type item; it also was notably less expensive than an 807. The 6BG6 was used as a horizontal driver in many big-screen black-and-white TV sets.
The 1G mainly was wired discrete, point to point, whereas the 1T had several subassembly units with printed circuit boards.
Many of us in the industry had our modest beginnings in the great outback of America, where we were able to make mistakes without causing too much trouble. Almost universally, we remember a 1 kW AM transmitter as part of the story. Like the car in which you learned to drive, or the first plane you learned to fly in or your first serious love affair, you never lose that tender feeling for your first rig.
Please send along your Gates or any 1 kW stories and share your memories. Write to radioworld@imaspub.com.
The author learned to drive in a 1960 Morris Minor sedan and to fly in a Cessna 152 and says he'll never forget them. Neither vehicle has survived but we're certain that was not Buc's fault.
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