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.

Sigh.

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.