Sentinel Model 333W
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:
|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|
For additional information on the FreModyne, I highly suggest the reader visit The Hazeltine FreModyne FM Receiver.
Riders Volume 20, Sentinel pages 20-21 to 20-25.
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