Sentinel Model 333W

Circa 1949

When I first saw this set in an antique shop I almost passed it by. An AM-FM radio this small surely must be a reproduction, I thought. I have never seen a reproduction of this style of radio, however, let alone one of a Sentinel, so curiosity got to me and I examined the set. Sure enough, it was the real deal and I picked it up. A look at the circuit reveals it to be an AA5 AM radio with a single 9-pin miniature for the FM band.

Above, the Sentinel model 333W. It is a Bakelite radio with a nice, big, slide-rule dial. As tube radios go, this is small: 10-1/2" wide, 7" tall, and 5-1/2" deep.

Below, a close-up of the dial. It is unusual to see an AM-FM radio of this size. Sentinel also made an AM-only radio using this same cabinet. The one-tube circuit used for FM reception in this radio is called the FreModyne circuit, designed by engineers at the Hazeltine Corporation. It is a form of super-regenerative circuit, and combines local oscillator, mixer, and detector into a single tube of the dual-triode type. Because of the FreModyne circuit, it was possible to build an AM and FM receiver of a small size, and to get around the Armstrong patents for the FM system used in larger, and higher tube-count receivers.

Above and below, before and after views of the chassis. The dial light was inoperative and been disconnected by a previous owner when the insulation became brittle and fell off. The wiring around the rectifier had also been changed. I of course, corrected all these problems, and the before photo was taken after I had restored the pilot light wiring. Restoration was pretty straight forward on this set.

Above and below, views of the chassis. This is a hot chassis set, and the chassis is connected directly to the power line thru the on-off switch.

The FM reception with the FreModyne circuit is not that great. It is certainly better than the normal AM-band reception, but the sound is not static-free.

.The tube line-up of the Sentinel 333W:

ID Type Function
V1 12AT7 27.75Mc oscillator-IF mixer-and FM detector
V2 12BE6 AM Converter-1st Detector
V3 12SK7 AM Intermediate Frequency Amplifier
V4 12SQ7 AM Detector-AVC-1st Audio Amplifier
V5 35L6-GT Beam Power Output
V6 35Z5-GT Rectifier

For additional information on the FreModyne, I highly suggest the reader visit The Hazeltine FreModyne FM Receiver.

Service Data:

Riders Volume 20, Sentinel pages 20-21 to 20-25.

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