The KO-6 provides secure FDX voice communications over landlines or HF radio links. It can also multiplex up to five FDX teletype channels or one facsimile channel  It supports 45.5 or 75 baud teletype or facsimile from the AN/TXC-2. Voice input comes from a special handset of which there are six in total.

Training required: Formal training at Lackland AFB.
Quantity procured: 26 for the USN.   Procurement for other branches unknown.
Cost: $50,000 each
Reference: CSP 6620A

For more detailed info on the KO-6, please select this link.

KO-6 image (production model) courtesy National Cryptologic Museum
A recollection by Marvin C. Cruzan, ex-USAF......

On September 10, 1958, I was granted my top secret clearance. That month we also began training on current equipment. The TSEC/KO-6 was a monster the size of three large refrigerators. It had over 1,500 vacuum tubes and required constant attention and calibration. It was in SAC's inventory as the primary device for secure communications and was supposed to be used in airborne command aircraft. It was installed at various ground locations throughout the United States, Europe and Asia, but I'm not sure that it ever became airborne.

Until now, all of the earlier machines were electro-mechanical such as the first one we had been trained on. The TSEC/KO-6 was all electronic and represented a new generation of encryption devices. It was capable of encrypting voice, facsimile and teletype simultaneously. Actually, it was three separate machines in a single cabinet using a common power supply. You could almost say it was six different machines because each function had independent send and receive sides. It could be doing six operations at once.

These machines, and others in development, were becoming too bulky and heavy to move quickly. Since it was impossible to evacuate them if the need arose, they were installed with thermite devices mounted on top, for quick destruction. These thermite devices could reduce a TSEC/KO-6 to a puddle of molten slag in just a couple of minutes. Our feeling was that the TSEC/KO-6 was going to be short lived because of its complexity. It was a Model-T but it was a sign of things to come.

The unbreakable rule for the TSEC/KO-6 was to never reduce the power below the level needed to keep the filaments warm. Experience showed that the pulse driving circuitry, consisting mostly of 5814 dual triode power amplifiers, was sensitive to the characteristics of the individual vacuum tubes. Powering the tubes on and off caused them to age faster. It also caused them to explode.

The TSEC/KO-6 power supply also generated a considerable amount of heat. Normally, each of the equipment bay drawers would be left ajar to relieve as much heat as possible. The exception was that drawers were closed and locked during fire drills or other very rare occasions when the classrooms might have to be left unattended and unlocked.

Each of the three class rooms in the two buildings dedicated to the TSEC/KO-6 had four of these monsters. During a routine fire drill one day, someone threw off the circuit breaker to one of the buildings. I'm glad my classroom wasn't in that building. When the drill was over and the power restored, it sounded like popping corn as the tubes exploded inside their cabinet drawers. We spent two days on extra detail cleaning glass shards out of the bay drawers, replacing tubes and re-calibrating the equipment. Just before Thanksgiving, we graduated from the TSEC/KO-6 course.

My KO-6 Experience by  James R. Hartle....

We had a KO-6 in Kunia Tunnel,  Hawaii in 1966.  As the new troop on site, I got the job of breaking it down for shipping it back to the Crypto Depot at Kelly Air Force Base, Texas.  It was a huge machine and comparing it to three refrigerators is accurate.  It broke down into six cubes each, about 3 feet square.  To pack it up required the installation of cover panels on the front, sides and back of the cabinets.  These covers had screw holes every inch.  There were literally hundreds of screws which had to be put in.  Since I was the new Airman, everyone figured I could do no harm to pack it up.  The important part of this story is that the screws had Allen heads, not Phillips or slots.

As an enterprising young Airman I soon tired of using the little Allen wrench and looked to automate the process.  This was my downfall!  I found an 1/4 inch electric drill (this was before the days of variable speed and slip clutches, and reverse), modified an Allen wrench to use as a "bit", inserted it in the drill and started going to town.  Those screws just flew in.  If you were not real careful when the screw tightened up, it stripped out the inside of the Allen head. This posed no problem because the machine was going to the dump anyway. Wrong!

After three days or so, I got all the cabinets sealed then told the boss, MSgt Tommy Holland, it was ready to go.  He called the Crypto Account Custodian to arrange shipping.  The custodian told MSgt Holland that the keying material was still in the machine (a type of geared rotors).  As a result, Tommy told me to remove the front panels and get the rotors out.  Reluctantly I went to try and get the screws out.  No joy - most were stripped. I told him about the problem with the screws being stripped, took my tongue lashing, the reference to my parentage, my future in the Air Force and started devising a plan to get the screws out.  I had to use a pair of vice grips and worked two weeks removing these screws.

Finally... all was done. Rotors were removed and everything packed up again.  To get from Kunia Tunnel to Hickam AFB, an Armed Convoy was required. The driver and the sidekick were each armed with a 45 sidearm and I sat in the back of the "duce-and-a-half" with an M1 carbine.  We got to the ramp at Hickam AFB and held off the ramp tramps at gunpoint until we got the KO-6 aboard a  C-130 Hercules for the flight back to Kelly AFB.

A few days after the shipment departed, we got a teletype message from the Crypto Depot asking us not to return the KO-6, just dump it into the Pacific, 200 miles off shore with all panels off so it would sink. That would have been a lot more fun!!!  That is my story on the KO-6. You never know what a young Airman will do to get out of work.  I should have stuck with the Allen wrench.

James R. Hartle, SMSgt (Ret), USAF
Crypto Maintenance  1965-1969

George Mace also contributes the following:

"I was never trained as a technician on the KO-6, however, from 1960-66 I was the final inspector of all repaired US Army Crypto Equipment at the COMSEC Regional Issuing Office (CRIO) at Lexington Army Depot.

The KO-6 had two geared timing mechanismís (GTM) which were about twelve inches cubed and could be disconnected from the bay that housed them.  This was the electro-mechanical part of the KO-6 and had magnificent machined gears. About 1963, the bar magnets and coils were replaced with lighted pickups. I remember this well as there were hundreds of bar magnets which became surplus so the technicians gave them to their children as toys. There were (red or orange) plastic discs with slots in them like IBM cards that were placed behind each of the six aluminum spindles. The spindles pulled off (ball detent) and you can see the round index pin on the gear to position the plastic discs. I believe those discs were the keying elements and changed just like an IBM key card.

The designator for this sub-assembly is KO6-1A/TSEC
Alternate view
Close up of one of the gear assemblies which generate a pulse. Here, the magnet is in very close proximity to the pickup coil.
All photos in this table by Ralph Simpson.
The KO-6 is the only US crypto device I know that came with it's own tools. The toolbox was the oak Kennedy machinist's toolbox, with green felt slide-out drawers. Later models used steel Kennedy toolboxes, which weren't so bad either.

The KO-6 like some other US crypto devices was a "white elephant" that showed up AFTER the technological parade had passed!  The KY-8/28/38 was in the same category, as no one was going to wait for synchronization of key before calling in a fire mission.

As only the US Army could do after modifying all in-house KO-6's, we destroyed them! "


1) Marvin C. Cruzan
2) James R. Hartle  <Hartle1(at)aol.com>
3) George Mace <gmace8(at)comcast.net>
4)  Ralph Simpson <ralphenator(at)gmail.com>
5) Nationaa; Cryptologic Museum
6) Nick England <navy.radio(at)gmail.com>

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Feb 3/15