Month: June 2010

Filament Voltage Management

4CX35,000C ceramic vacuum tube
4CX35,000C ceramic vacuum tube

There are still many hollow state (AKA tube type) transmitters floating around out there in the broadcast world.  High power, especially high power FM transmitters are often tube types and there are many good attributes to a tube transmitter.  They are rugged, efficient and many of the well-designed tube units can last 20-25 years if well maintained.

The downside of a tube transmitter is tube replacement.  Ceramic tubes, like a 4CX20,000 or 4CX35,000C cost $6-9K depending on manufacture.  A well-maintained tube and last 3-4 years, I have had some lasting 8 years or more.  My personal record was for a 4CX35,000C that was a final PA tube in a Harris MW50A transmitter.  The tube was made by EEV (English Electrical Valve, now known as E2V)  and lasted approximately 84,000 hours, which is 9.58 years.  When it finally came out of service it looked like it had been through a fire, the entire metal plate body was dark blue.  I took it out because the power was beginning to drop a little and it was making me nervous.

This was not an accident, I did it by maintaining the filament voltage and keeping the tube and transmitter clean.  The tube filament supplies the raw material for signal amplification.  Basically, the filament boils off electrons, which are then accelerated at various rates and intensities toward the plate by various control grids.  The plate then collects the amplified signal and couples it to the rest of the transmitter.  When a tube goes “soft,” it has used up its filament.

I had a long conversation about this one day with Fred Riley, from Continental Electronics, likely the best transmitter engineer I have ever known.  At the time, the consensus was to lower the tube filament voltage by no more than 10%.  On the 4CX35,000C, the specified filament voltage is 10 volts, therefore, making it 9 volts was the standard procedure.  What Fred recommended was to find the performance “knee,” in other words, where the power began to drop off as the filament voltage is lowered.  Once that was determined, set the voltage 1/10 of a volt higher.  I ended up running that EEV tube at 8.6 volts, which was as low as the MW50’s filament rheostat would go.

The other important thing about tubes is the break-in period.  When installing a new tube, it is important to run only the filament voltage for an hour or two before turning on the plate voltage.  This will allow the getter to degas the tube.  New tubes should be run at full filament voltage for about 100 hours or so before the voltage is reduced.

Tube changing procedure:

  1. Remove power from transmitter, discharge all power supply caps to ground, hang the ground stick on the HV power supply.
  2. Remove the tube, and follow manufacturer’s procedures.  Most ceramic tubes come straight up out of their sockets (no twisting).
  3. Inspect socket for dirt and broken finger stock.  Clean as needed.  Finger stock, particularly in the grid section, is important for transferring RF.  Broken fingers can lead to spurs and other bad things
  4. Insert new tube, follow manufacturer’s recommendations.  Ceramic tubes usually go straight down, no twisting.
  5. Make all connections, remove grounding stick, half tap plate voltage supply if possible, close up transmitter
  6. Turn on filaments and set voltage for manufacturers’ recommended setting.  Wait at least 90 minutes, preferably longer.
  7. Turn on plate voltage and tune transmitter.  Tune grid for maximum current and or minimum reflected power in the IPA.  PA tuning should see a marked dip in the PA current.  Tune for dip, then load for maximum power.
  8. Turn off transmitter, retap plate supply for full voltage
  9. Turn on transmitter and plate supply, retune for best forward power/efficiency ratio.
  10. After the 100-hour mark, reduce filament voltage to 1/10 volt above performance knee.

Of course, every transmitter is slightly different.  There may not be a dip in the plate current if the transmitter is running near its name plate rating, in which case one would tune for maximum forward power.

This system works well, currently one of the radio stations we contract for has a BE FM20T with a 4CX15,000A that has 9 years on it, still going strong.

FCC authority to conduct warrentless searches of Private Property

FCC seal
FCC seal

I was this interesting tidbit on the Radio World website the other day.  The question is, how much authority does the FCC have to conduct a search of a private residence?   The Electronic Frontier Foundation wanted to know therefore they sent a FOIA request to the FCC seeking documents supporting this claimed authority.

The documents received seem to be redacted and some are mostly blank, such as the training module on how to obtain permission to enter private property is supposed to take 6 hours to complete, but consists of 3 paragraphs and 2 questions.  Hopefully, that is redacted and does not reflect on the quality of agents the FCC is employing these days.  The upshot seems to be the agent either needs a warrant or permission.

It may be surprising to some citizens, however, the FCC does have the authority to investigate radio signals, whether they are intentionally generated, as in a pirate broadcaster, or unintentionally generated, as in a piece of gear gone bad.

According to federal regulations, an FCC agent may request entry to inspect a private building anytime he/she believes there may be a device emitting radio frequency energy.  This includes anything with an FCC part 15 sticker, which can be computers, TV remote controls, garage door openers, WiFi network routers, etc.  This basically covers every house in the US as well as most businesses.  Those rules were written when most homes and businesses did not have any RF generating devices and there was little to indicate that they ever would.

The consequence of failure to allow an FCC field agent into a residence or business appears to be the issuance of a citation in the form of a threatening letter.  Continued intransigence would be met with a NAL (Notice of Apparent Liability) followed by a forfeiture notice also known as a fine.  The typical FCC fine these days seems to be $10,000.00.  If it is a repeated and willful violation, the equipment can be ceased and the perpetrator arrested.

In instances where the safety of life is in question, then every step necessary to disable the offending device needs to be taken.  Things like transmitters spurring into aircraft frequencies or TV antenna amplifiers running wide open, also interfering with aircraft frequencies come to mind.

One of the examples given details a field agent trying to track down a noisy cable TV amplifier.  From the FCC field agent’s perspective, the homeowner appears to be a royal pain in the ass.  In this day and age when everything has a computer and most of them generate RF, tracking down interference can be a painstaking process, especially where housing is dense.  Uncooperative homeowners, especially dumb ones, who have no idea what they have plugged in only make things worse.

Still, there is the issue of fourth amendment rights, which, if the above law was misinterpreted, misused, or applied with the wrong standard would likely be trampled.  In these days of extra-constitutional activity, giving Los Federales entry into one’s house might invite unwanted scrutiny by other agencies.  As far as changing the rules, with the current group of scoundrels and rouges in the legislative branch, one might end up with something ten times worse than before.