Sears Roebuck Mystery Amp

Sears Roebuck Mystery Amp

This little single-ended 'student' amp with an 8" speaker bears a very worn 'Sears Roebuck' metal tag on the back of the cabinet. It clearly pre-dates, or was separate from the famous Silvertone line of amps. The serial number (163-32) does not fit with the Silvertone system, and I have not found a matching picture of this model online.

The tube lineup was 6N7 (twin triode), 6C5 (single triode), and 6V6 (pentode), with a 6X5 rectifier.

The first two are VERY old octal tubes, dating to 1935. The 6C5 in particular was almost immediately superseded by the 6J5. The style of the tweed cabinet dates it to the late 40s or early 50s. My guess is that costs were cut by using cheap war surplus tubes.

When I got the amp, the case was falling apart (typical Sears quality), both the power and output transformers were open circuit, the non-original speaker was DOA, and the circuit had been 'got at'. Unbelievably, I found a physically identical 240v PT from a scrapped 50's AWA radio, traced the fairly simple circuit and replaced any dubious components. To say it sprang to life would be a lie. The output was pitiful, due to the very low gain of the old triodes.

Since the amp's originality had already been compromised, I decided to review the circuit to juice things up a bit. A higher gain preamp tube (6SL7) and driver (6SQ7) brought out a nice throaty rasp - not quite a Fender Champ, but certainly usable. Naturally an earthed power cord replaced the original US 2-prong cord, and I also fitted a fuse and pilot light.

Both the original and modified circuits are shown below, with more photos. I would be interested to hear from anyone who recognises this amp, or has any info.