Notes: This receiver is pretty uncommon so a few comments are in order. With 0 dB front end attenuation and maximum i-f gain this receiver has extraordinary gain and sensitivity - especially at the lower frequencies. Designed mainly as a shipboard receiver it is likely this receiver would have performed well on 500 kHz with the ships antenna fully encrusted in ice and laying on the deck! Words can not adequately describe the unbridled gain of this receiver - you'll just have to try one for yourself.

The receiver is extremely well built on a cast aluminum foundation in which individual stages occupy their own compartments. DC and bias voltages enter through feedthrough capacitors and signal openings from compartment to compartment are kept as small as possible. One wishes all receivers were built this way!

Unfortunately, the blocking and two-tone dynamic range numbers tested disappointingly low - even with the i-f gain of the receiver cut way way back. Considerable time was spent to insure that the receiver was functioning, as best as one can tell, to factory specifications. All voltages were correct, as was LO injection levels when compared with the figures given in the manual. The rf circuitry used is somewhat unconventional. The receiver covers from 70 kHz to 30 MHz and uses up conversion to a first i-f of 38 MHz. The main signal path includes a manually switched front end attenuator, manually switched front end bandpass filters that feeds a single stage 7788 tube rf amplifier. Signal from the plate of the rf amplifier passes through a 14 section 30 Mhz low pass filter. This is applied to a 6C4 cathode follower that feeds the first mixer - a quad of 1N82A 'VHF' diodes in a balanced design. The signal is amplified and filtered by two tuned circuits, a single 6688 amplifier tube and two more tuned circuits. A 6BL8 triode section forms another cathode follower which feeds the grid of the 6BL8 pentode section as the 2nd mixer with an output at 5.94 MHz. A somewhat unusual arrangement has the LO injection in series with the rf signal to the grid. The plate of the 2nd mixer passes through a 6 kHz wide crystal lattice filter. From there it's on to the third mixer - a 6BE6 converter stage - for the final i-f of 455 kHz. Here, the signal encounters the Collins mechanical filters for either SSB or CW. No additional filter is used here for AM - just the 6 kHz filter in the previous i-f. The remainder of the circuitry is pretty straightforward. One nice feature of this receiver is that it does have a product detector.

This receiver was on loan so it was only possible to observe its performance - not modify the receiver to try to improve it. In order to locate the source of the poor dynamic range a high impedance probe / spectrum analyzer setup was used to 'sniff' IMD stage by stage. The culprit turned out to be the 6BL8 second mixer stage. With simple tests and limited time I was unable to determine the exact cause of the poor IMD performance but would suspect the 6BL8 biasing, or more likely the crystal filter in the plate circuit or the VFO amplifier which has it's output in series with the rf signal. Even if the IMD could be improved in the second mixer, next in line is the third mixer - a 6BE6 converter - generally a dynamic range 'show stopper' in receivers where it's used. This would be one fun receiver to modify for truly high performance!

The AM distortion numbers show the somewhat typical increase at lower audio frequencies - most often caused by the low audio frequencies riding on the AGC line. Also there is the usual increase in audio distortion as the modulation percentage increases. This is due to the detector's inability to cleanly demodulate the higher amplitude levels although this receiver does pretty well.