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Re: [Amps] 3CX1500A7/8877

To: Jeff DePolo <jd0@broadsci.com>, amps@contesting.com
Subject: Re: [Amps] 3CX1500A7/8877
From: John Lyles <jtml@losalamos.com>
Reply-to: jtml@vla.com
Date: Mon, 13 Feb 2017 23:06:41 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Jeff,
Yes, I was there after the first transmitter was built, which was the largest, the 30 kW. I started from the lower power end and designed the cavity amplifiers and transmitters from 3.5 kW up. The 1.5 kW was a separate special project that the late Bill Harness conjured up, and I just built a model for him on the computer to help. And I was there for the redesign of the FM30, into FM30A, where we got rid of the 1 bit stoplight controller and put a hard logic fault protection board in it, plus the new "Wider" band solid state drivers.

The half wave cavity circuit itself was not new, having been done by RCA in TV, Gates in FM, and other applications. But making it re-entrant, folding it with a grounded stub at the opposite end from the tube, was the late Jim Aurand's idea. He also came from RCA and maybe Harris. This shortened it and made it easy to tune with a movable stub having a bellows and no sliding contacts. Also, feeding the B+ at the fundamental RF voltage null was a good idea. All in all, the BE design was a good departure from all the other FM rigs that used wrapped film plate blocker caps and finger stock tuning contacts on a sliding short circuit plane.

Thanks for the kudos, glad you were able to experience mostly pleasure with those rigs. A few remain on the air.
John
K5PRO

On 2/13/17 9:15 AM, Jeff DePolo wrote:
Around 1980-84, I was an RF engineer on a team that built a series of
new FM broadcast transmitters in Quincy, Illinois. All were tetrode
designs using the newest tubes with handles from Eimac at the time,
4CX3500A, 4CX7500A, 4CX12,000A/8989, and 4CX20,000A/8990.

John,

Were you on the design team for the BE transmitters of that era that started
the "folded half-wave" cavity design, eliminating plate blockers?  If so,
kudos to you.  I've had the pleasure (mostly pleasure anyway, hi) of
installing and maintaining many of them over the years.

                                --- Jeff WN3A



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