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.

Nautel V-40 FM transmitter

Yesterday, I threw out a transmitter.  I know there is probably some radio station out there that may have been able to use a 5 KW FM transmitter but believe me, not that one.  There are limits to how much you can help out a fellow broadcaster.  Donating an FM transmitter that never really worked right in the first place is counterproductive.

Anyway, to demonstrate that I am not a total heel, here is my favorite brand of transmitter, Nautel:

Nautel V-40 transmitter (4 V-10 transmitters combined)
Nautel V-40 transmitter (4 V-10 transmitters combined)

I like Nautel because they are rugged, reliable, and good-looking.  Okay, good-looking is low on the list of transmitter attributes, however, you have to admit, it is good looking.  It is also good sounding.  The night we switched over from the long-in-the-tooth BE FM30A to the Nautel V-40 I noticed a marked improvement in the station’s sound.  It was like somebody switched off the background noise generator.

As the caption states, this is 4 V-10 transmitters combined with an ERI magic T combiner.  It is set up so that if any one transmitter fails or reduces power, the magic T combiner automatically adjusts for minimum rejected power, then the SC-1 controller turns up the other three transmitters to maintain the station’s Transmitter Power Output (TPO).


In this case the TPO  is 28 KW, which is getting into the semi serious range for an FM station.   Nautel has updated their transmitter line, which now consists of the NV series transmitter.  The differences mainly have to do with the IPA module/PA module interchangeability (not interchangeable in the V series, fully interchangeable in the NV series) and the “Advanced User Interface.”  I don’t know, fancy touch screens are optional on FM transmitters as far as I am concerned.  It’s the underlying RF generating sections that I am most concerned about.

Nautel V-40 transmitter
Nautel V-40 transmitter

Another view.  Just for the useless trivia that is in it, the “V” in these transmitter names stands for “Virtuoso.”

Gates FM5G transmitter

Takes its rightful place in the world today, the scrap heap:

Gates FM5G carcass
Gates FM5G carcass

As EDWARD I of ENGLAND once said, “A man does good business to rid himself of a turd.”

Of course, he was speaking about Scotsman John Balliol and not some old cranky FM transmitter, but I understand that feeling.  The Gates and later Harris transmitters always seemed to be somewhat less than top-notch. The 5G was no exception to this rule.  The final step for tuning the transmitter was to turn off the lights in the room and look down through the screen on top to make sure there were no little arcs in the PA tuning section.  It also had a way of self-oscillating, which could make for some exciting tuning.

Gates FM-5G transmitter prior to disassembly
Gates FM-5G transmitter prior to disassembly

Goodbye, I will not miss you.

In one of my past jobs, I worked in a RCA town.  I worked there long after the broadcast arm of that company went out of business, however, all of the broadcast transmitters, AM, FM, TV were made by RCA.   I had an RCA FM-20ES1 which was 22 years old, built like a tank and just kept going along.  I think that transmitter was finally destroyed in a fire, caused by it’s replacement transmitter.

Old Collins, Contenental, RCA and even Broadcast Electronics transmitters had some heft to them.  Of course, not every RCA transmitter was well thought out, the amplifuze series of AM transmitters were a maintenance nightmare.

If it ain’t broke, break it

One thing that I find a little annoying is the continuing need to reboot everything at some interval.  Computers in the studio, audio vault servers and workstations, e-mail servers, files servers, network routers, and so on.  Got a problem, first thing to do is cycle the power off and on…

One of the most irritating pieces of equipment is the audio processors on one of our FM stations.  A few years ago, we purchased the whiz-bang Omnia 6 processor.  Every 6 or 8 months the thing losses its mind and sounds terrible.  The station gets all bassy and the high-end sounds distorted.  I have tried everything I can think of to prevent this, including installing a UPS, extra grounding, extra shielding, software updates, etc.  In the end, it just has to be rebooted, which of course, means several seconds of dead air.  Naturally, this processor is at the FM transmitter site, which it is difficult to get to.

Truth be told when it is working, it does sound pretty good on the air, but is it $10,000 dollars better than the older Optimod 8100A?  No, it is not.

The old Orban Optimods sound pretty good as long as they are re-capped and aligned every so often.  If fact, our number one billing station has an AC format and uses an Optimod 8100A and nothing else.  Our other station in the same market uses an Optimod 8100A and a pair of Texar Audio Prisms. In the ten years, I have been working for this group of radio stations, I have never had to reboot the Optimod or the Audio Prisms, they just seem to work continuously without problems. Imagine that.

I have seen this called a “retro audio chain” by some.  Nothing retro about it, but a little care and feeding and I’d stack this equipment up against an Omnia 6 any day of the week and twice on Sunday.

This is a grainy video of an 8100A  in action:

That was taken in our rack room using off-air audio on the rack room speakers and a cheap video camera. You get the idea.

So here is to Frank Foti and his marketing gurus that have sold all of the program directors in America on the need to “update” their air chain processors, because, you know, the Optimod, that is old skool.