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Re: [OT] Broadcast Engineering (was:Re: Google Earth)

On Monday 14 January 2008, Lamar Owen wrote:
>On Monday 14 January 2008, Gene Heskett wrote:
>> I still do the transmitter cuz Dave never saw a vacuum tube till he went
>> to work for us about 11 years ago now.
>Former employment involved a Doherty linear running a pair of 4CX15000's as
>carrier and peak, driven with a 30W solid state exciter, making
>10KW+125%-100% AM into a two tower DA-N.  Also involved a 3rd
> harmonic-driven RCA BTA-5T with a single 5762 in the final, modulated by a
> pair of
>3CX3000F1's (this transmitter was later replaced with a Harris DX-25U; in
>fact, I was on my way to this station to work on the DX-25U upgrade when the
>South Tower was hit on 9/11) into a 4 tower inline DA-N (and, of course, the
>highlights of the year were the twice annual phasor rocking expedition....).
>And then there was the BTA-5F which was supposed to use three 892R's, but,
> due to the extreme cost and shortage of that tube (rebuilds, assuming a
> rebuildable dud, were over $3600 per, in 1990, and new ones were simply not
> available for any price (the last they actually quoted for purchase were
> over $10,000 each; I've not checked recently to see if this 50-pound-plus
> beast was even still available at all)) the previous engineer modified it
> to be 1KW only using a pair of 833's mounted to a spruce 2x4 sitting on top
> of the 892R modulators' air stacks....
>Working with glassfets (firebottles, whatever you want to call them) has
>always been fun.  Not terribly reliable (compared to a Nautel ND-10 I
> babysat for years; from 1989 until 2000 the only parts that had failed were
> one power FET and a crystal; after that a couple of modules went due to FET
> failure, but still....) (But, then again, that Continental 316F Doherty had
> the same pair of 4CX15000's from 1984 until 1998; the 164 amps through the
> filaments finally caused the filament fingerstock in the socket to
> carbonize, causing an arc that breached vacuum integrity).....

Chuckle, that fingerstock for the filaments hasn't been a huge prob with the 
old GE TF-3A since they are just 2 piece rings bolted around the flanges of 
the tube.  I did lose a socket under a 3cx2500 about 10 years ago, but most 
of the fireworks here have been rodent or snake related.  Built back in the 
50's long before the NEC codebook, no entrance breaker.  One snake managed to 
take out 2 of the 4 pieces of 750mcm from the substation pole to the 
building, had to replace the weatherhead too as it was nicely toasted too.  2 
tons of plate tranny was toast of course. The pole fuses have been blown so 
many times the porcelain tubes are conductive so we often still have at least 
one phase worth of lights even when all three flags on the 14.4 line are at 
halfmast.  Makes for 'interesting' repairs.  That water cooled final?  Had to 
change it a couple of years ago, it had only been out of the socket once 
since '92, to replace the rotted rubber water gaskets under it.  That one 
since failed shorted, and the old one is back in the socket but with the 
heaters set at 11.25 volts now, its a 12 volt tube.  Takes about 190 amps to 
light it...

>But I digress....

So do I, Lamar.  But all these frogs here should understand that computers 
ain't the only things that make electrons do useful work.

Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
An American is a man with two arms and four wheels.
		-- A Chinese child

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