Hammarlund had carved out a considerable niche in the commercial and government markets with their ground breaking Comet-Pro introduced in 1932.  However, by the end of 1935 the design was looking distinctly dated in the face of competition such as the National HRO.  The boys a Hammarlund had not been resting on their laurels.  Rather, they were accepting input from their major customers, including the U.S. Army Signal Corps, and designing a new receiver that would take the communications receiver to a distinctly higher level.  Careful examination reveals the name to be more that marketing hyperbole.

The basic Super-Pro design remained in production through the war years.  There were three distinct families with a number of variations in each family to cover different frequency ranges.  The original SP-10's were based on big-pin glass tubes such as the 6C6 and  6D6.  The SP-100's, introduced in 1937, migrated to metal tubes in the front end for improved performance.  The SP-200's, introduced in 1939, used metal tubes throughout.   Further information on model identification is located HERE.

The vast majority of surviving Super-Pro's are SP-200's.  Many were military production.  The SP-10's and SP-100's can be quickly identified by the "doubled-up" toggle switchs on the front panel.

Technical Features:
  • Single-conversion superhet - 465 KHz IF
  • 545 KHz - 20 MHz*  Band-switched
  • 2 RF stage
  • 2 IF stages
  • Crystal filter - optional*
  • Frequency dial -  Calibrated
  • Bandspread:  Separate knob and dial - 0-100
  • External power supply
* See Model Identification

Introduced to the public in early 1936, the Super-Pro is a 16-tube super-heterodyne receiver covering 545 KHz to 20 MHz in five bands.  The circuit includes 2 RF stages, 3 IF stages with variable bandwidth, and a 12-watt push-pull audio amplifier.  Other features include a signal-strength meter and a calibrated beat-frequency oscillator.  There are fourteen tubes on the main chassis and two more in the external power supply.

The RF Front End

The heart of any receiver lies in it's tuned circuits, and the Super-Pro is no exception.  The main tuning dial, on the left, is directly calibrated in frequency, and offers better than 1/2% accuracy.  The black slotted mask, geared to the band switch, uncovers the correct scale for the band in use.  The dial on the right, is connected to the twelve-gang band-spread capacitor.  The bandswitch connects the appropriate number of sections to provide an optimal spread range for the band that's in use.  As a result, the major ham bands each cover the most of the 0-100 scale.  Tuning is by way of a simple-but-effective friction drive that engages the edge of the dial scales.  Tuning ratio is a pleasing 12:1 with no backlash.

Here's a bottom view of the tuning assembly with the shielding removed.  Each coil assembly is mounted on a ceramic base along with it's associated trimmer capacitor.  The input coils (left) have Faraday shields between the primaries and secondaries to maintain proper performance with balanced antenna connections.  The 12-section band-spread capacitor is clearly visible.

Careful design and through shielding provide excellent rejection of unwanted signals.  Read this testamonial from an engineer at radio station WOR.

Up until this time, most designers of high-performance receivers had eschewed band-switched designs because the performance of the available rotary switches was not reliable.  Hammarlund's solution was this custom-designed cam-actuated band switch.  The contacts are silver plated, and the action is make-before-break to eliminate arcing in current carrying circuits.  This is just one of the five sections.  Sixty years later, most of these switchs still work just fine with no attention whatsoever.

Variable Bandwidth                
Three of the IF transformers in the Super-Pro have adjustable coupling between their primary and secondary windings.  The lower coils are moved by cam-actuated push rods.  This gives a continuously variable pass band of 3 to 12 KHz.  The front-panel bandwidth control is calibrated in kilohertz.

The SP-10 and SP-100 models could be ordered with an optional crystal filter for CW reception.  The crystal filter was standard on the SP-200 sets.

These curves may seem upside-down to 21st century eyes.  They reflect the proceedure used to generate them:  The receiver, with AVC off, is connected to a calibrated signal generator and output level meter.  The generator output is set for a low level, e.g., 10 microvolts, and is tuned for a peak on the output meter.  The frequency is recorded as the center of the passband, and the level is noted as a reference.  As the generator is tuned away from the center frequency, the question becomes:  How much must the generator output be increased to reach the reference level on the output meter?

Here is a family of selectivity curves for an SP-200 receiver.  ("Right Click" and "View Image" for a clearer look.)  They show the selectivity for four positions of the "BAND WIDTH" control.  The front-panel bandwidth scale indicates the approximate half-voltage (-6 dB) bandwidth of the IF amplifier.  The fifth curve, "A", is for the 3 KC bandwidth plus the crystal filter in it's widest position (1).

This chart, not only indicates the broad range of selectivity available to the operator, but also the filter's steep "skirts" that reject nearby unwanted signals.  In modern practice we consider the ratio of a filters bandwidth at 6 and 60db attenuation.  The Super-Pro's 3:1 "shape factor" is still very respectable today.

High Fidelity

Most communications receivers have a fixed IF bandwith of approximately 4-6 KHz to provide maximum intelligibility of speech, especially under difficult conditions.  This limits the high-frequency audio response to 2-3 KHz. Because one of the target markets for the Super-Pro was broadcast monitoring and network rebroadcast, it was provided with this variable bandwith feature.  When conditions are good, the passband can be opened up to enjoy the music.   Conversely, the IF can be tightened up to a very restriced 3 KHz to "dig out" voice signals when the going gets tough..

This hi-fi capability is reinforced by a very clean sounding  14-watt audio amplifier using push-pull triode-connected tubes.  It will, indeed, provide room-filling volume.  A Super-Pro, connected to a good speaker system, will outperform virtually any AM broadcast receiver ever built.