Jim:
A lot of us got the radio bug from a Hallicrafters S-38 series or a Zenith Trans-Oceanic. I have both and still enjoy them, but when I need to tease out an obscure signal my old National HRO [1] usually beats about any other radio in the shack (new or old) hands down.
The S-38 is a good choice since outside of the band-switching arrangement it has the guts of an All-American Five radio. This makes it a lot more repairable, most of the components are “generic”, i.e. tubes, capacitors, IF [2] transformers, et cetera.
The only downside to a [transformerless] AC-DC radio is that the tube life is not as long [as with most transformer] AC designs. It is almost always the high voltage heaters (the rectifier and audio output tube) that fail.
One very important thing to keep in mind about the S-38 (or any metal-cased AC-DC radio) is they can be deadly if not properly checked and maintained. Most AC-DC radios use the chassis as a ground return; that means one side of the power line is hard wired to the chassis. In the S-38 the chassis is isolated by rubber grommets, cardboard bottom and back and plastic knobs. Crumbling grommets, a shifted chassis, too long a screw, et cetera can put the case at line voltage — being at a minimum a nasty shock hazard. This is pretty easily remedied on most radios; Make sure the knobs are all non-conductive, the chassis is still floating in rubber grommets. (Replace them all). The backs and sometimes the bottoms are missing. Make new ones with perforated Masonite (pegboard). JWR Adds: For those who are collectors, some very nice replica back covers and bottoms are available from Retro-Tronics [3].
Add a 3-wire cord grounded to the case (not the chassis). Failing that, then add polarized plug so that the chassis is always at ground potential (the larger prong of the plug). On wood cased AC-DC radios tape over the screws that hold the chassis to to case. Never use an AC-DC radio that is missing any knobs. Make sure (or have someone who knows how) make sure the chassis is isolated from any metal you can touch.
Other recommendations:
- Add a [soft start] in-rush suppressor [4] in the power circuit. This lengthens warm-up time but makes the tubes last almost forever. The resistance of the filaments is rather low when cold and the in-rush current is rather high for a moment, this is why the dial light on these radios is momentarily very bright when turned on stone cold. At this moment the filaments (espescially in the 35 and 50 volt tubes) are under maximum stress. Limiting the in-rush allows for more gradual warm up.
- I’d add a fuse to both legs of the power line, the 1 amp glass pigtail fuses work nicely in most AC-DC sets. Encase them in vinyl or heat-shrink tubing.
- Keep spare antenna materials, I lost several antennas during Hurricane Ike: I actually lost them all!
- Consider using an isolation transformer when operating on AC. These are de rigueur for servicing tube radios.
- Go to NostalgiaAir.org [5] and print out a schematic of your radio and keep it with the radio (inside the case in an envelope if space and safety permit). It’ll help you or someone who knows how to fix your radio, down the road.
- Keep a spare set of tubes. Make sure to test that they all actually work in the set. Most radios will operate sucessfuly with weak tubes except the converter tube. A weak one characterized by reception pooping out as you increase frequency. A really weak one will only operate on the bottom 1/2 of the broadcast band, forget about shortwave! The usual converter tubes (AC-DC) are 12SA7 and 12BE6.
As a side note, I own an example of the greatest AC-DC radio made: a Scott SLRM [6], it was made for the U.S. Navy during WWII. It is deadly by design. It is AC-DC but was primarily designed for use on the 120 Volt DC common to existing ships of the era. One side of the power line is tied to the case by design. This is okay for a ship with floating or polarized DC power, but deadly elsewhere. Mine has a permanent isolation transformer [, which with this design is a must for safety]. Regards, – “Tired Tubes”
JWR Replies: Thanks for those suggestions, particularly regarding grounding isolation transformers. By coincidence, I have been looking for a Scott SLRM for my family’s use here at the ranch. If anyone out there has a spare that is gathering dust, I’m willing to pay the going retail price, or work a trade from the JASBORR [7] inventory, for some goodies of slightly greater value. I’m not looking for gem. I just need a decent SLRM that is working and complete to start with, for restoration. The only real “must” is that the volume potentiometer isn’t scratchy. The speaker can be blown, since I can replace that.) The tubes (other than the tuning eye) can be weak. Again, I can replace those. The capacitors can be original. I can re-cap it, and replace any resistors that are outside of their value specifications. I’ll have who is a wizard with an oscilloscope a friend re-align it. It can even have a tobacco smoke-stained front, but the dial must be nice and legible. I’d prefer one with a civilian ID plate, but a Navy ID plate is okay if there is no corrosion. I’d prefer one that is already set up with an isolation transformer of the appropriate size, but that isn’t a must. Does this sound one that somebody out there has available? If so, please e-mail [8] me. Thanks!