Continental Shortwave Transmitters

I started my radio career working in HF radio, albeit somewhat different than broadcasting.   I enjoy the long distance aspect of HF communications and there is something about the high power shortwave (HF) rigs that interest me. This is a video of a Continental 418E HF transmitter. The carrier power is 100 KW capable of 100% modulation, when means peak output power is 400 KW. This particular model has a solid state modulator, which is in the cage where the guy is walking around. From the video, it would appear they had several blown fuses in the modulator section. The fuses protect the individual IGBTs in the modulator.

This is an older transmitter that is getting upgraded to a 418F. The heavy cable is the connection between the solid state modulator and the RF final section. Depending on modulations levels, it carries around 33 KV.

From the Continental Electronics website that details the SSM unit:

The modulator consists of 48 series connected modules which are switched on or off to provide the high voltage DC and the superimposed high level audio voltage. The switching is accomplished with Insulated Gate Bipolar Transistors (IGBT). A low pass filter follows the series connected modules which removes the switching signals and allows the DC and audio signals to pass to the RF amplifier. Because each of the modules is either in full conduction with very low loss, or turned off, again with very low loss, the overall modulator efficiency is in excess of 97%.

A full description of the SSM is on the Continental Electronics SSM website. It is an interesting read, including the description of the 12 phase transformer setup.

Finally, a video of the VOA transmitter site in Greenville, NC.

This is part 4 of 5, if one wanted to, one could click through to Youtube and watch the rest of them. The VOA stuff is, as the transmitter engineer notes, 1950’s technology. No solid state modulators in these rigs. Those are some old transmitters, still in service and likely to remain that way until the VOA closes that site down, some point in the future.

Like their FM counterparts, Continental HF transmitters are the gold standard when it comes to high power tube transmitters. Sadly, they no longer make transmitters for Standard Broadcast (AM MW).

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.