The things one finds when you open up a defective coax line…

I thought I’d post a few photos of a repair done some time ago to show this comedy of errors, found when the VSWR on a particular FM broadcast antenna system went high:

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This shows an air leak detected at a splice in the 3″ line while up a tower several hundred feet.  Soap bubbles to the rescue!  If air can get out, water can get in.

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Here we see the leaking connector splice opened up to reveal not only a burned Teflon insulator, but a shop rag and copper chips left when the connector was field-installed!  I always wondered about the dielectric strength of a shop rag…

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Further up the tower at the antenna input proper, we see two views of a common mistake when assembling EIA coax, a split bullet.

Note how the burning is concentrated on the right side of the downward facing bullet, left loose by the lack of tension caused by the one finger being outside the inner conductor.

 

Any time this type of work is done, it always pays to hire the most professional climbing team for the job.

A lot of this type work is done at night and under time constraints, during the least revenue value time of the broadcast day.

Care and expertise must be exercised in order to prevent the illustrated types of failures.

THE LAST WEEK OF MARCH, 2013

Atop South Mountain in Phoenix, Signals worked on completing the install of a new Nautel NV40 FM transmitter for CBS Radio, at KZON-Fm 101.5.  It is an HD facility.  This upgrade replaced an aging Continental 816R4.

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Shown are some of the steps involved, to include the 400-Amp disconnect with 4/0 welding cable (to ease bending), and conduit work into the transmitter:

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A shot of the link between the site UPS units and the transmitter input for the controller and exciter power:

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Also shown is a rear penetration into a floor duct for interface cabling:

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A closeup of the transmitter main ground, with a hand nut, so checking it periodically can be accomplished without needing a wrench. (This is meant to be both fun and functional):

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The crew from CBS is Chris, Eric Schecter (DE for CBS Phoenix), and Paul.  All are exceptional broadcast engineers, and a pleasure to work with:

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I’ll be continuing this post further down the road, as we will be upgrading all the RF switching at this site.

Tube audio amps and design question…

An answer about a vintage audio driver transformer

What a friend has is a 20-Watt driver transformer, with impedances (40K CT to 2.5K CT) better suited for audio in class AB1, maybe AB2.

It would be just like a pro-audio tube power amp, just bigger.

Imagine a slightly smaller but similar transformer coupled circuit from, say a 6SN7 to a quad of 6L6’s in PPP.  Set the finals bias point for minimum distortion.  Observe maybe just a scooch of grids running positive and plate current barely bouncing under wide-open conditions into a load & scope.  See it makes about 60 or 70 Watts before it goes into really raspy distortion.

Now set the bias point and try for class ‘B’ operation.  Final grids positive a lot, pulling pretty good current, plate current rising in step with increased audio.  Heavy distortion sets in, and you determine the driver stage is running out of steam before hitting the final grids with the necessary poop.

To get more drive, you up the ante with bigger drivers (maybe those 2A3’s or 6L6’s) and a coupling transformer that will spot the maximum power transfer where you need it, in those final grids.  Weather they pull current or not, your driver cares not ’cause you designed it to work into sloppy loads via low-impedance tubes that can swing into whatever load it sees.

The triodes will handle it OK by themselves, the pentodes need negative feedback to tackle their inherent high plate impedance and get the ‘regulation or damping’ back.

That redesigned amp now makes about 120 Watts with not too bad total distortion, but it costs you more $ to get the parts for that increased power.

That choice is usually made by just what tubes you have vs. how complex a circuit you need.

Hi-Mu tubes offer a simpler setup, as most of them will work with no safety bias and are thrifty in the drive department, but the commensurate plate load impedance is always high and sometimes tough to get a transformer to match.

Low-Mu tubes are hungry for lots of grid swing voltage, and need that to perform well.  On the flip side, they have lower plate impedances, can swing lots more current in the I x E equation, and drive variable loads better.

Big tubes do the same thing little ones do, just bigger.

73DG