The 4CX250B ceramic tube

4CX250B ceramic vacuum tube
4CX250B ceramic vacuum tube

It’s a cute little thing.  These were often used for driver tubes in FM broadcast transmitters.   With the naming conventions of ceramic tubes, we can tell quite a bit about the unit without even looking at the data sheet.

  • The first number is indicates the number of grids in the tube, 3 makes it a triode, 4 tetrote and 5 pentode.
  • The C means it is a ceramic tube
  • The X indicates it is air cooled, a V is vapor cooled, W is water cooled, M is multiphase
  • The 250 is the plate dissipation is watts
  • B is the design revision.

Thus a 4CV100,000 is a vapor cooled tube capable of dissipating 100,000 watts, something one might find in a high powered MF or HF transmitter.

Other bits of critical information about tubes would be maximum plate voltage, maximum screen voltage, maximum grid voltage, maximum screen dissipation, maximum grid dissipation and filament voltage.  Something to keep in mind when tuning a transmitter.

This particular tube is installed in the driver section of a Continental 816R2 transmitter.

Continental 816R2 driver section
Continental 816R2 driver section

I have noticed that these tubes have a much shorter life than there predecessors.  Back ten or twenty years ago, they usually lasted 12-14 months.  The latest set lasted only 8 months and both units failed catastrophically.  That points to one of two things; either something in the transmitter has changed or something in the way the tubes are manufactured has changed.  Once the new tubes were installed, I checked all of the parameters against previous maintenance logs.  I also checked things like air flow, dirt and other possible culprits.

I could find no changes in the transmitter.  The only thing I can think of is the fact that the tubes are installed horizontally, which causes the elements to warp and eventually break or short.

Continental 816R2 transmitter, WFLY, Troy, NY
Continental 816R2 transmitter, WFLY, Troy, NY

I am thinking we may try to convert the driver section of this transmitter to a solid state unit.  The transmitter itself is 24 years old, but is still works and sounds great.  I’d hate to get rid of it because of its driver section.

SOPA/PIPA protest

Yesterday, January 18, 2012, I blacked out engineeringradio.us for the day in protest of the internet censorship bill working its way through congress colloquially known as SOPA or PIPA.  There were some 17,000 or more others that did the same.

SOPA PIPA protest screen shot
SOPA PIPA protest screen shot

If the internet is indeed the new media, destined to replace the old media, then having in place draconian restrictions that allow the government to block websites and content with no due process for the website owner is censorship, plain and simple.  Imagine a country where the government can come in and shut down any newspaper, TV station or Radio station, give no reason other than some weak statement about copy write laws.  See also: China, North Korea, Cuba, Soviet Russia, etc.

It is important to check the corporate power in this country.  It is widely reported that Congress has a 9% approval rating.  It is also hard to imagine their approval rating is actually that high.  While signing petitions and writing senators and congressman may provide some relief, the shortest path to ending this is to boycott the corporate sponsors of the legislation.  Hitting companies bottom line will speak louder than any internet protest, petition, letter writing campaign, etc.  Thus, if so inclined, here is a list of producer companies that like the idea of internet censorship.

The malfunctioning STL antenna

Right after Tropical Storm Irene, it was noted that the STL signal strength at the WHUD transmitter site was low. Normally it was 300+ µV, now reading around 100 µV, which is a problem. Upon further investigation, it was revealed that the STL transmitter on the intermediate hop had higher than normal reflected power.

Time to call the tower crew.

The STL transmit antenna for WHUD’s STL (WPOU464) hop is a Scala Paraflector (PR-950), mounted at the 280 foot level on this tower:

Scala PR-950 on a guyed tower
Scala PR-950 on a guyed tower

The fact that it happened after a major storm and the transmitter was showing higher than normal reflected power indicates a problem with either the antenna or the jumper between the 7/8″ Cablewave coax and the N connector on the antenna.  A measurement with a spectrum analyzer shows very high return loss:

WHUD STL antenna return loss
WHUD STL antenna return loss

This shows distance to fault 413 feet, with a return loss of -7.4 dB.  That distance is either near or at the antenna and -7.4 dB indicates a lot of reflected power.  We had the tower climber take apart the jumper connections and terminate the jumper with a known good 50 ohm load.  The return loss did not change.  We then had him swap out jumpers and reconnect to the antenna.  That did the trick:

WHUD STL antenna with new jumper
WHUD STL antenna with new jumper

Much better, most of the power is now being radiated by the antenna, the VSWR is 1.02:1.  The impedance bump at 51 feet is a sharp bend in the coax where it is attached to an ice bridge.  Reconnecting the transmission line to the transmitter and turning it on confirms that all is normal again.  The problem with the jumper was found in one of the connectors, it was full of water.

Water contaminated Andrew flexwell connector
Water contaminated Andrew flexwell connector

I cut away the boot, water had entered the connector from the back because waterproofing and tape was not applied all the way to the coax.  This was installed in 1998 when the station moved from Peekskill to their current location in the town of Fishkill.  The fact that it happened now in the nice weather when Mt. Beacon is still accessible and not in the middle of winter means the radio gods are smiling on us.