The Hammarlund Comet Pro
Copyright 2003, Alan R. Klase. All rights reserved.

Hammarlund had been in the radio components business since 1910.  They were a significant player in the variable capacitor arena since 1916.  With the advent of the broadcast boom they introduced the Hammarlund-Roberts line of sophisticated broadcast receiver kits.  These were cleverly engineered tuned-radio-frequency sets that maintained a nearly constant RF passband while being tuned across the nearly three-to-one frequency range of the AM broadcast band.

As a result of years of development of high-performance broadcast radios, Hammarlund engineers were acutely aware of the importance of  optimum selectivity in receivers, and that they would be unable to acheive such selectivity at short-wave frequencies using the TRF arcitecture.  Late 1931 saw the introduction of the Comet All-Wave Superheterodyne.  This radio was targeted at the emerging short-wave broadcast market.

An early Comet, with it's Hammarlund supplied  electro-dynamic speaker.  It's an eight-tube superhet with on-board AC power supply.  The radio tuned 545KHz to 20MHz using five sets of plug-in coils.  The variable capacitors, plug-in ceramic coil forms, and double tuned IF transformers were all Hammarlund products.

Technical Features
  • Single-conversion superhet - 465 KHz IF
  • 545 KHz - 20  MHz - plug-in coils
  • No RF stage
  • 2 IF stages
  • Frequency dial - 0-100
  • Bandspread:  front-panel trimmers
  • Internal power supply

There are two 465KHz IF stages with a total of six tuned circuits.  The middle knob and drum dial are the main tuning control.  The two controls to the right and left provide "vernier" tuning of the mixer and local oscillator.  The toggle switch actuates a fixed-tuned BFO for CW reception.  List price was $130, less tubes and cabinet.

Comet Pro Prototype

The initial Comet "Pro" appears in Hammarlund's ad in April 1932 QST.  "An eight-tube custom-built super-heterodyne which will do all that the professional operator demands between 14 and 200 meters."   The center control with the illuminated drum dial is now band-spread, while the two knobs with circular scales are "band-set" (main tuning).  The type 47 audio-output power pentode has been eliminated in favor of a type 27 triode, and a phone jack has been added to the front panel.

NYC Radio Row retailer Leeds offered the Comet Pro for an introductory price of  $88.20 while mentioning a $150 list price.

Technical Features
  • Single-conversion superhet - 465 KHz IF
  • 545 KHz - 33  MHz - plug-in coils
  • No RF stage
  • 2 IF stages
  • Frequency dial - 0-100
  • Bandspread:  single knob + WL & OSC band-set
  • Internal power supply

The mixer coil socket is labled "W.L" (Wave Length) and the oscillator coil socket "OSC."  Note that the ganged bandspread variable capacitor has four sections.  The two extra sections are connected by jumpers in the lower frequency coils to provide a wider band spread range.

Production Comet Pro

The early 1930's were a period of extremely rapid development of shortwave receivers.  The Comet Pro underwent a series of changes throughout it's life time.  The schematic for the first major revision is dated September 1932.  The calibrated band-set dials have been recessed behind the panel, and are now actuated by friction-type reduction drives acting on the edge of the dials.   Later Comet "Pro's" were housed in a wrinkle-finished steel case.   The crystal filter model is shown at the right.

Improved Comet Pro

The screen-grid tubes have been replaced by the latest RF pentodes, sharp-cutoff 57's in the mixer and detector and remote-cutoff 58's in the other locations. "A-A-G" antenna terminals have been added to accomodate balanced feed lines.  The oscillator is now an electron-coupled design for improved stability.   The high-power 47 audio output of the Comet has been reinstated along with  an audio output transformet to properly feed both head phones and speaker.  A second filter choke has been added to the on-board power supply to allow operation independent of the speaker.  An 8-inch permanent-magnet speaker in a matching case was available as an option.

Inside:  A tuning handle has been incorporated into the BFO coil (lower left) to allow operator adjustment.   The plug-in coils are covered by cylindrical shields to limit unwanted coupling.  New IF transformers, tuned by air-dielectric trimmers to acheive better stability, were introduced in late 1932.

The "Standard" model Comet Pro is a reasonably good performer exhibiting good sensitivity, selectivity, and large signal handling capability.  Performance on AM is quite good, and while a properly designed TRF-autodyne (regen) might of had a slight edge for CW on 160 and maybe 80 meters, the "Pro" was probably the best available receiver for the higher frequencies when it was introduced. 

Further Improvements

As further developments occured in the industry they were incorporated into the Comet Pro.  A Lamb-style crystal filter was made available, for true sigle-signal CW selectivity, in September 1933.  A ten-meter coil set was anounced in October, and automatic volume control in November.  The lack of an RF stage was, perhaps, the major shortcoming of the Comet Pro, and after market products, such as the Peak preselector, in a matching cabinet with identical knobs, appeared to remedy the situation.

The claims and counter claims expounded in period advertising often make interesting reading.  For example, in October 1933 QST, McMurdo Silver touts that Admiral Byrd has chosen his 5A receiver for his next Antarctic expedition.  Two months later Hammarlund published a testimonial radiogram from an Arctic expedition complete with pictures of a walrus and an Eskimo.

The Comet Pro enjoyed considerable success in the commercial market.  Here's a DC Comet Pro, one of several, modified by AT&T for use as frequency-selective voltmeters to service multiplex telephone circuits.  The added meters read signal level in dB and battery voltage.  Custom coils tuned the LF band.  (N3FRQ Collection)

Hammarlund was justifyably proud of there penetration of the market.  This list of Comet-Pro users appears in their ad in the June 1933 issue of QST:

American Airways
Eastern Air Transport
Northwest Airways
Transcontinental & Western Air,Inc.
Inter-American Aero Travel and Supplies
44th Division Aviation,N.J.
War Department, Air Corp., Wright Field

Columbia Broadcasting System
Station WOR
Station WOWO
Station WAIU
Station WINS
Station WCAO
Icelandic State Broadcasting Service, Reykjarik, Iceland
Short Wave Station VE9GW, Canada
Station CMKA, Cuba

American Radio News Service
U. S. Embassy Santiago, Chile
Argentine Embassy, Paris, France

Georgia Agricultural Dept.
Police Dept., Youngstown, O.
Police Dept., Klamath Falls, Ore.
Albany Junior Chamber or Commerce
Mississippi State Penitentiary
Hotel New Yorker

Naval Research Labs., Washington
U. S. S. Woodcock
Radio Material School, Washington
9th Naval District
U. S. Navy Yard, Charleston, S.C.
Marine Corp., Signal Section

Message Center, Washington
Headquarters, 8th Corp Area, Fort Sam Houston
Signal Section, San Frdncisco
Signal Section, Honolulu, T. H.
Signal Section, Manila, P. I.
Signal Section, Corozal, C. Z.
Headquarters, 4th Corp Area, Fort McPherson
Headquarters, 9th Corp Area

Department of National Defense, Canada
Director of Signals, Canada
Director of Naval Stores
Royal Administration of Telegraph, Stockholm
Radio Nautica, Italy
First Brigade Exchange, Haiti

Radio Electric, Switzerland
Technish Bureau, Kolomen, Hoiland
Prometheus, Greece
Rothermel Corp, Belgium
Eltone Ceritte, Italy
Muvaffak Bey, Ankarg, Turkey
Standard Oil Co. of Venezuela
Harry Pegram, South Africa
W. F. Greene, Dutch West Indies
Pedro Perez, Saintz, Spain
Post Exchange, Canal Zone