January 2017
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Longest tube life?

We may be going for a record here; this Broadcast Electronics FM20T was placed in service on June 6, 2001:

Broadcast Electronics FM20T, WYJB, Albany, New York

Broadcast Electronics FM20T, WYJB, Albany, New York

The original 4CX15000A tube is still in use.  I wrote about this a few years ago in this post: Longevity.

I thought by now, we would have changed out that tube.  A few quick calculations shows that the tube has been in use for 118,289 hours or 4,929 days or 13 years 6 months and 3 days.  Anyway you look at it, that is a long time for one tube in nearly continuous use.  I noticed the hour meter is lagging a bit:

Broadcast Electronics FM20T hour meter, WYJB, Albany, New York

Broadcast Electronics FM20T hour meter, WYJB, Albany, New York

Reads 113051.24, which is 5,238 hours different than what I calculated from the maintenance log.  I noticed a slight discrepancy in hours two years ago and attributed it to various off air periods.  However, between then and now, this transmitter has not been off at all.  Thus, the hour meter is wearing out before the tube.  I would say that this is because of excellent filament voltage management,  but I think we simply have a really good tube.

Has anyone else had a tube that lasted this long or longer?

The Raytheon RL10 Limiting Amplifier

Update: Apparently this is quite interesting to a number of people.  I have rescanned the manual, properly compressed it and which you may find it here.

Found this manual at one of the older transmitter sites:

Raytheon RL10 limiting amplifier manual cover

Raytheon RL10 limiting amplifier manual cover

Entire manual is available for your reading pleasure here: Raytheon RL10 limiting amplifier

As this is an older design than either the Gates Sta level or the Collins 26U, it may not be as useful to tube audio enthusiasts.

Raytheon RL-10 Schematic diagram

Raytheon RL-10 Schematic diagram

The main issue with the Gates and Collins unit is the GE 6386 remote cutoff triode used, which were great tubes, but very difficult to come by these days.  This design calls for a 1612 or 6L7, which is a pentagrid amplifier.  Feedback is provided by the screen of the following stage, a 6SJ7GT.  Anyway, perhaps it will give somebody some idea on how to make a good tube compressor limiter.


One of the stations that we do contract work for installed a Broadcast Electronics FM20T transmitter on June 6, 2001. It is still running on the original tube, a 4CX15,000A. By my calculations, that is 11 years, 7 months and 9 days, or 101,712 hours.

Broadcast Electronics FM20T transmitter

Broadcast Electronics FM20T transmitter

BE FM20T filament meter

BE FM20T filament meter

The hour meter shows 101,168 hours, which accounts for some maintenance, and other anomalies.  Overall, the transmitter has a 99.5% up time. I do not think the transmitter suffered any failures, rather, things like the generator and the STL failed instead.

EIMAC 4CX15000A tetrode

EIMAC 4CX15000A tetrode

Almost twelve years on one tube is pretty impressive.  I know that other Broadcast Electronics T model FM transmitters have similar tube life expectancies.  I wonder what the Broadcast Electronics secret is.

Look at my wonderful tubes!

I found this promotional picture in an old NAB conference technical papers book dating to 1969:

RCA TT-30FL promotional picture

RCA TT-30FL promotional picture

So here we see an obviously qualified and appropriately dressed technician gesturing to all the components she is about to install in the transmitter behind her. I wish I worked there.  No, wait, I wish I had that transmitter and perhaps this fetching young woman would come and work at my station.  Well, hell, I don’t need a TV transmitter, just the woman.


I wonder how many of these rigs RCA sold before the broadcast division went out of business.

By way of reference, the RCA TT-30FL is a VHF television transmitter, 30 KW peak visual power, 7.5 KW peak audio power, air cooled.

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.

The BE FM20T transmitter

This is the main transmitter for WYJB in Albany, NY. The backup is the Harris FM20H3 on the right. I haven’t turned that unit on lately, but it normally makes quite a fuss the first time the Plate On button is pushed. The FM 20T on the other hand, is mellow and even tempered.

WYJB 95.5 Mhz, class B, transmitter Albany, NY

WYJB 95.5 Mhz, class B, transmitter Albany, NY

One other thing of note; The FM20T is still on its original tube.  I looked up the maintenance records for this transmitter, it was installed in December of 2000.  Eleven years later, the 4CX15000A is still cranking out 15 KW TPO, which is impressive.  I found that high power ceramic vacuum tubes actually seem to last longer when run closer to their limits than those that are running at half power.

Judicious management of filament voltage is required to achieve this type of longevity.  There is a set procedure for installing a large ceramic vacuum tube:

  1. After the tube is in the transmitter, run it a full filament voltage for at least an hour or so before turning on the plate voltage.  This allows the getter to absorb any stray gases in the tube.
  2. Once the plate voltage is applied, proper tuning should be completed a quickly as possible.  Tuning procedures vary from transmitter to transmitter, however, the general idea is to obtain the maximum power output for the least amount of plate current while keeping the PA bandwidth within acceptable limits.  Some transmitters can get narrow banded at high efficiencies, which manifests itself as higher AM noise.
  3. After the tube has been in use for 90-100 hours, the filament voltage should be reduced gradually until a drop in the transmitter output power is noticed, then increased by 0.1 volts.

This maximizes the filament life for that particular transmitter and power output.  Once the filament can no longer boil off enough electrons, the tube power output drops and it is time to replace it.

This site also has two other radio stations, WZMR, 104.9  and WAJZ 96.3 , both class A using solid state transmitters of less than 1,000 watts:

WAJZ and WZMR Energy Onix solid state transmitters

WAJZ and WZMR Energy Onix solid state transmitters

Not the prettiest sight in the world, but it does stay on the air.  There is no money to go back and neaten up this work, unfortunately.

The tower supports all three antennas.  There was some discussion of a common antenna for all three stations, however, WZMR is a directional station, thus it would require it’s own antenna.  Doing a common antenna for the other two stations was cost prohibitive, so the tower supports three two bay antennas.

WYJB, WZMR, WAJZ FM antennas, New Scotland, NY

WYJB, WZMR, WAJZ FM antennas, New Scotland, NY

The stations are all located in the New Scotland, NY tower farm.  WYJB is licensed to Albany, WZMR is licensed to Altamont and WAJZ is licensed to Voorheesville.

RCA receiving tube manual, 1964 edition

RCA receiving tube manualI found this in the great clean out of 2010, Bridgeport, Connecticuit.  Once upon a time, I had a slightly newer version of this, I think from 1972 or so.  This version is from 1964 and gives a complete run down of most small tubes that were manufactured back then.

There is something about a well designed, well maintained piece of tube gear.  I remember an old Collins tube console that was in a production room at a small AM station.  The console went dead (paper clip shorted the B+) and I fixed it.

I recall listening to the test recording of my own voice from a reel to reel machine when I fixed the console.  It sounded better than I’d ever heard it, not that I have a great radio voice, by any means.

A tube is a voltage amplifier versus a transistor, which is a current amplifier.  A tube does not have the same fidelity as a transistor, as the voltage reaches it’s peak, it gets a little fuzzy, adding some distortion and harmonics.  Tube gear adds warmth, what a musician might call Timbre. The combination of fundamental frequency and varying amplitudes of harmonic frequencies that allow a listener to tell the difference between a piano and a guitar playing the same note.

This is what the current crop of tube mic preamps and other tube products tries to reproduce.  Several companies have come out with an amplifier design that has mostly transistors and one tube, usually a 12AX7.    Unfortunately, at least in my opinion, the fall a little short.  If you want to have the “tube sound,” it needs to be all hollow state.

What is intriguing to me are the schematic diagrams in the back of the book.  There is one for an audio amplifier:

RCA receiving tube manual audio amp schematic diagram C 1964

RCA receiving tube manual audio amp schematic diagram, C 1964

This is a single channel unit, for stereo, it would need to be duplicated.  Also, I would loose the tube rectifier in favor of a solid state full wave bridge, that would simplify T2 somewhat.  The OA2 could also be changed to a diode.  Looks like unbalanced audio in, which could be modified with an input transformer.

Another interesting diagram is this one, which can be used as a mic preamp:

Microphone Preamp schematic diagram RCA receiving tube manual C 1964

Microphone Preamp schematic diagram RCA receiving tube manual C 196

That looks like a pretty solid design, a few tweaks here or there to add some gain reduction and some type of output level adjustment and I would be a really cool piece of gear.  Again, the tube rectifier could be replaced with something solid state.  The output transformer would likely have to be changed to something like 600 ohms.

Couple that to something like this, the Collins 26U compressor/limiter, and one would have a great sounding microphone processor:

Collins 26U compressor limiter

Collins 26U compressor limiter

Looks pretty cool.  R10 is used to balance the two plate currents.  I would be interested in the transformer values, input/output impedances, voltages, etc.  The 6386 tube is very hard to find these days, a good substitute would be a 5670 which are still made by several manufactures.

Update:  This is a picture of a Collins 26-U sitting on my bosses floor.

Collins 26-U compressor limiter

Collins 26-U compressor limiter

A great online source for tube information is: Electron Tube Data Sheets.

If I have some time this winter, it may be a fun project to fool around with.

Western Electric 212E vacuum tube

The company I work for is in the midst of cleaning out a studio location.  Most radio engineers are some form of pack rat.  I know I have been guilty of this myself, not wanting to throw something away because tomorrow, it might be needed.  That was carried out to the extreme at this location.  One of the things that I found in my clean out was a Western Electric 212E vacuum tube.

Western Electric 212E vacuum tube

Western Electric 212E vacuum tube

It is an impressive thing, measures about 12 1/2 inches tall, including the pins.  I am thinking this is pretty old, it probably came from a pre-WWII Western Electric AM transmitter.  This would make the most sense, as the station signed on in 1926 with 250 watts.  Back in the day,  Western Electric was the patent holder for AM technology.   In fact, there was some talk of suing General Electric for patent infringement after the airing of the world series by WJZ and WGY in 1922.  Parent company AT&T was working on radio modulation techniques to implement with their telephone system.

These tubes were used for audio amplification, according to the spec sheet, the plate could dissipate 275 watts.  Filament voltages is 14 volts at 6.2 amps, the plate voltage was 3,000 volts, maximum.  It is a tetrode.  The RF counterpart to this tube is the WE 308A.

From what I am to understand, these have not been made since 1960 or so.   I also understand there there is quite a cult following for this tube amongst Asian audiophiles.  There are several examples of extremely low distortion class A and AB amplifiers using this tube type.  Some prices on Ebay are in the $1,500 to $2,000 per tube range.  Unfortunately, I don’t think this one works anymore as there is a loose screw and little bits of what looks like control grid wire in the bottom of it.  It does light up with 12 volts on the filament, however.

Rebuilt tubes

As broadcasters, we don’t really hear that much about ceramic power vacuum tubes these days, as more and more broadcast transmitters migrate to solid state devices.  Once upon a time, however, power tubes where the engine that drove the entire operation.  Tubes had to be budgeted for, stocked, rotated and replaced on a regular schedule.  Some of those dern things were expensive too.

Take the 4CX35,000A which was used in the Harris MW50 transmitters.  The transmitter used two of these tubes, one in the RF section and one it the modulator.  As I recall, new tubes cost somewhere north of $8,000.00 each from EIMAC.  Plus, in the A models there were two 4CX1500A driver tubes.  All of which could add up to an expensive maintenance cost every two years or so.

The next best option was to buy rebuilt tubes.  Rebuilt tubes were about half the cost of brand new ones.  Some people complain that rebuilds don’t last as long, or only last half as long as the new tubes.  I never found that to be the case.  I often found other factors effected tube life far greater, such as filament voltage management, cooling and by extension, cleanliness.

I can say I never had a warranty issue with ECONCO tubes.  I cannot say that about EIMAC, as during the late 90’s and early 00’s (or whatever you call that decade) I had several brand new 4CX3500 tubes that were bad right out of the box.  These days, ECONCO and EIMAC are both owned by CPI.

I spoke with John Canevari from ECONCO who had a lot of information.  For example, as the tube ages, the filament gets more flexible, not less.  Most ceramic power tubes use a carbonized tungsten filament containing some small amount of thorium.  As the tube ages, the filament can no longer boil off enough electrons and the emission begins to drop off.  That is the normal end of life for a power tube.  Occasionally, some catastrophic failure will occur.

There are many steps in the rebuilding process:

  1. Dud is received from the field, the serial number is recorded and the tube is tested in.
  2. The tube is prepped by sand blasting the sealing rings
  3. It is opened
  4. Filament is replaced.  In 60-70% of the cases, the grid is replaced.  In those tubes that have a screen assembly, 20-60% of those will be replaced.
  5. Interior of the tube is cleaned
  6. Tube is resealed and tested for leaks with a gas spectrometer
  7. Tube is placed on the vacuum machine.  Tubes are evacuated hot, smaller tubes take 12 to 24 hours, very large tubes can take up to one week.
  8. The tube is nipped off of the vacuum while still hot.  When the tube is fully cooled the vacuum scale is normally around 10-12
  9. Exterior of tube is cleaned and replated.  Silver for tubes that are socketed and Nickel for tubes that have leads.
  10. Tube is retested to manufacture’s original specification or greater.

After that, the tube is sent back to its owner or returned to stock.  John mentioned that they are very proud of there vacuum tube processing machines, so I asked if he could send along a picture.  They certainly look impressive to me, too:

vacuum tube processing machine

Vacuum tube processing machine, photo courtesy of ECONCO

Not exactly sure which tube type these are, but they sure to look like 4CX15,000:

vacuum pump on rebuilt ceramic power tubes

Vacuum pump working on rebuilt ceramic power tubes, photo courtesy ECONCO

Econco has been in business since 1968 and rebuilds about 600-1,000 tubes per month.  In the past, broadcasters used most of the larger tube types.  However, with the majority of broadcast transmitters shifting to solid state, other markets have opened up such as industrial heating, military, research and medical equipment.

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.

There 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 accident, I did it by maintaining the filament voltage, 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 intensity 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 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 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 tube, follow manufacture’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 manufactures 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 manufactures 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 it’s 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.


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

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