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.

What the inside of a ceramic vacuum tube looks like

In case you have wondered it yourself:

4CX3500A
4CX3500A

This is an EIMAC 4CX3500A which came out of a Harris HT5 transmitter. As you can see it the ceramic cracked in half. When I arrived at the transmitter site, the unit was on, full plate voltage, no plate current, no overload lights. I figured it might be something with the tube, so I tried to pull it out, but only the top half came. One of those “Ah ha” moments.

Fortunately, there was a working spare at the transmitter site and we got back on the air relatively quickly.  That, in and of itself is amazing considering the building that this transmitter lived in.  One of those abandond former studio sites with the transmitter jammed into a back room somewhere.  To get to it, one has to dodge pigons, beware of rats and wade through piles of garbage.

It is a little bit hard to tell in this photograph, but there are to “cages” which are the Screen and Grid.  The post in the center is the filament/cathode and the top detached part is the plate/anode.  In an FM transmitter, the exciter is coupled to the grid, the screen accelerates electrons toward the plate and therefore controls the power, the plate collects the electrons and is coupled to the output stages and the antenna.  Good stuff.