Tag Archives: vacuum tube

Alpine, New Jersey–My Recent Visit…


W2XMN–Armstrong’s original transmitter building built in 1937, 2010 November 6
W2XMN–Interior of building, 2010 November 6


I had the great pleasure of venturing out to Alpine, New Jersey this past Saturday. I was honored to receive an invitation from Dave Amundsen, the Director of Engineering for CSC Management (the organization who presently owns the Alpine site), to celebrate the 75th anniversary of Major Armstrong’s public demonstration of FM in 1935, through a live radio broadcast.

To be able to stand in close proximity to the original Armstrong tower was truly an incredible experience, and the engaging conversations with the many radio hams in attendance was priceless.

Steve Hemphill was kind enough to serve as a tour guide through the multitude of apparatus, including the one used to transmit the anniversary broadcast. Steve personally built a replica of Armstrong’s 42.8 Mhz FM transmitter, and has lent his helping hand to restore a variety of other instruments on the Alpine site.

The GE BT-1-B 250 watt FM broadcast transmitter that resembles an old refrigerator (photographs immediately below) is Steve’s handiwork. He purchased the GE 250 watt transmitter off Ebay back in August 2002 and refurbished it back to its original shiny and functioning glory.

GE 250 watt transmitter, 2010 November 6
Steve Hemphill with the GE 250 watt transmitter, 2010 November 6








I posted some photographs of the back interior of the GE transmitter to illuminate the amount of work involved in this project. You will see all new wiring–really quite incredible!

GE 250 watt transmitter–backside interior view, 2010 November 6
GE 250 watt transmitter–backside interior view, 2010 November 6










This particular GE transmitter originally belonged to WAVE-FM in the late 1940s and was subsequently sold to the Louisville Free Public Library for use as their transmitter at WFPL-FM, sometime in the 1950s. In 1950, public radio made its debut with WFPL when the Louisville Free Public Library became the first library in the country to obtain a radio license. They started broadcasting using a GE model BT-11-B transmitter (10 watt unit), later upgrading to the model seen here.

GE 250 watt transmitter–backside with "City of Louisville" tag, 2010 November 6


Armstrong’s tower stands as it did decades ago. The bottom of the tower has changed with the construction of a building that serves to house most of the apparatus.

Armstrong Tower, 2010 November 6
Armstrong Tower, 2010 November 6


Armstrong Tower, 2010 November 6







During World War II, Armstrong carried out contract work for the Signal Corps. The tower built for his work on radar still stands (photographs below), albeit with some modifications.

Radar Tower, 2010 November 6
Radar Tower, 2010 November 6







The W2XMN building contains a small museum housing various communications equipment. The photographs below are all from the museum.

Prototype of 30 KHz FM side-channel Multiplex Receiver, 2010 November 6
Prototype of 30KHz FM side-channel Multiplex Receiver, 2010 November 6






The 30 KHz FM side-channel multiplex receiver (photographs above), is a prototype of the receiver designed by John Bose in 1953. The receiver is described in detail in US Patent 2,835,803, issued to Bose in 1958 May 20.

Vacuum Tubes, 2010 November 6
Sample component wiring board–Part of Pulse Generator, Armstrong Experimental Radar, 2010 November 6



David Terwilliger–Radio Ham and textbook of knowledge, 2010 November 6


Double Super-het single knob, 2010 November 6















James O’Neal was kind enough to explain the meaning of Conelrad (CONtrol of ELectronic RADiation) to me. I have seen the CD triangle in a circle emblem on the back of radio apparatus and wondered what it signifies. Mr. O’Neal explained that during the Cold War Conelrad was set up to provide warnings to the public. The system was introduced in 1951, and if an alert was given, radio stations were to go off the air. Chosen stations were to move to either 640 or 1240 KHz, continually alternating so as to avoid enemy direction finding equipment capable of locking into US locations by using radio stations as beacons. In 1963, the Emergency Broadcast System (EMS) replaced Conelrad. Since 1997, the Emergency Alert System (EAS) has been used.

Also in attendance was John Nashmy, who in the aftermath of 9-11, assisted in moving a number of New York television stations to the Alpine site so they could continue broadcasting.



The Regenerative Circuit

Listening to the radio now, as I do quite often, I never would have imagined the difficulties that plagued it in the past. Although not yet around to experience early radio listening first hand, I have read enough to understand just how close one would have had to have been to the speaker. In fact, headphones were generally required. It was here at Columbia University during his undergraduate study that Armstrong made his first momentous discovery, the regenerative circuit, eliminating the need for headphones and providing the foundation of many radio receivers that followed.


Improved Audion Receiver, 1914 January 14

Improved Audion Receiver, 1914 January 14
In 1906, Lee de Forest created the "audion," an early vacuum tube. He had added a third element, the grid, to the Edison-Fleming diode. Subsequently, in 1912, he accidentally connected the output circuit of one audion to its own input circuit and obtained a loud howling sound,which he later identified as regeneration. But instead of attempting to understand this hissing or howling sound, he tried to abolish it.  A modern day example of this phenomenon results when a microphone is placed too close to its accompanying speaker, resulting in an uncomfortable noise (feedback).
When Armstrong began experimenting with the audion, he took numerous measurements in order to figure out how this tube functioned, eventually devising a circuit that would operate as a powerful amplifier of  incoming radio waves (E. H. Armstrong drawing above on left). He figured out that if part of the plates output current was fed back to the grid in a controlled manner, the incoming signals were remarkably strengthened . In addition, he discovered that when feedback was increased sufficiently the circuit could be used as a transmitter by generating high frequency oscillations, a required element for radio communications (E. H. Armstrong drawing below).

Circuits for Using the Audion as a Generator of High Frequency Oscillations, 1914 March 13

Circuits for Using the Audion as a Generator of High Frequency Oscillation, 1914 March 13
Armstrong’s invention led to a nearly twenty-year legal battle over patent rights. Between 1914 and 1934, he and de Forest fought in court both individually and through the corporations (Westinghouse Electric and American Telephone and Telegraph, respectively) who had purchased rights to their patents. Long and complicated, the battle began when de Forest sat in the audience listening to one of Armstrong’s lectures on the transmitting and receiving properties of his regenerative circuit. Although de Forest had given lectures in 1913 on the qualities of his audion invention, he never claimed it retained any of the capacities that Armstrong had discovered.  de Forest went on to submit a claim to the same invention in 1914, but the courts upheld Armstrong as the patent owner for the next ten years. It ended in 1934 when the Supreme Court decided in de Forest’s favor.
Regardless of the Supreme Court’s decision, the engineering community still recognizes Armstrong as the rightful inventor and has given him multiple awards for it. In 1917, the Institute of Radio Engineers (IRE) awarded him their Medal of Honor and in 1942, the American Institute of Electrical Engineers (AIEE) awarded him the Edison Medal, the highest award of the Institute (In 1963, the IRE and the AIEE merged to form the IEEE (Institute of Electrical and Electronics Engineers). These prestigious awards still stand today.
The original early drawings seen here, along with extensive litigation files, can be found within the Edwin H. Armstrong collection.

Vacuum Tube?

A vacuum tube from the Armstrong collection

Most people today have heard of Moore’s Law and the explosion of the number of transistors in modern technology, but if you are of the younger generation, the word “vacuum tube” sparks only faint recognition.  In fact, some younger readers may not recognize the term at all.  But this device was the forerunner of the modern transistor, which is used in nearly all electronics today.

Edison discovered the principle that would lead to the vacuum tube while developing a carbon-filament electric lamp.  In 1883, during the course of his experiments he observed a dark film on the inside of the glass which caused the carbon filament to burn out. Curious as to the cause, Edison hung a tiny metal plate inside the bulb and found that a small current passed from the hot filament to the metal plate, but not in the other direction.

This “Edison phenomenon” was regarded as an academic curiosity for many years, until it was picked up by John Fleming in 1904.  Fleming used this discovery to create the “Fleming valve,” a device that would induce current flow in one direction and not the other.  This is called the “diode” today (a term that many will remember from Physics class).

Closeup of the inside of a vacuum tube

In 1906 Lee De Forest found that he could place a fine wire mesh between the filament and the metal plate and control the current flow through the circuit.  In effect, he made a “faucet”; if the voltage on the grid was high, current would flow.  If the voltage on the grid was low, the current would be blocked.

Think of this as a faucet: by using a little bit of your own energy to turn the knob, you can control a large flow of water.  Similarly, the De Forest “audion,” or vacuum tube used a small voltage from an incoming radio signal to shape a large amount of current provided locally.

You can see a picture of a vacuum tube from the E.H. Armstrong collection at the right.  The wire running through the center is the cathode, the wire spiral is the mesh, and the thick black outer shell is the anode.  This vacuum tube was originally used in the litigation case Armstrong vs. Motorola.

Next, I’ve included a picture of an interesting device below.  Do you recognize it?

Fundamental electronic element


This is a air variable capacitor.  By rotating the plates outwards, the surface area is decreased and the capacitance drops.