Zenith 5G500 and 6G501M Universal 3-way portables
with "Wavemagnet" antennas

This Zenith 5G500 Universal and its brother, the 6G501M are broadcast-only portables from 1940. Their "airplane cloth" exteriors are often found in very rough condition. These are certainly not beautiful or highly collectible radios. As a matter of fact, I had considered them to be among the uglier radios of the era.

The beginning of a radio dynasty
However, these radios are historically significant as the forerunners of the Transoceanic series. They were designed with the Zenith movable "Wavemagnet" antenna for which E. F. McDonald, head of Zenith, was granted two patents. One patent was for the Wavemagnet antenna itself, a loop antenna which can be mounted on a window and thus pick up broadcasts in an otherwise shielded metal railroad car, ship or skyscraper. The other was for the hinge mount that allowed the Wavemagnet to be partly rotated while stuck to the window. Most later generations of the Wavemagnet are not rotatable.

The illustrations in the patent applications are unmistakable in their depiction of these radios.

Zenith models 5G500 (left) and 6G501M. Both Zenith portables use primarily loctal tubes with an octal 117Z6 rectifier. The 5G500 is a 5 tube radio with a 1LA6 as converter, 1LN5 as IF amp, 1LH4 as audio preamp/ detector, and 1LB4 as audio output. The 6G501M adds another 1LN5 as RF amplifier tube and changes the audio output tube to a 3Q5 which is an octal tube. The RF amp in the 6G501M is untuned resistance-coupled. Note that both models use the 1LA6 which is a loctal version of the expensive 1L6 as used in Transoceanics of the 1950s.

The 6G501M chassis (top in picture below) has six tubes and the 5G500 has five, but both sets only have two sections of variable cap. Schematics for both can be found on the Nostalgiaair site. See my homepage for a link.

Repairs
I've owned the 5G500 for a number of years and had repaired it and replaced many of its capacitors long ago. It came without its tubes or its Wavemagnet antenna. I located a complete set of new tubes. It worked fine with a substitute generic loop antenna.

About a year ago, I picked up a couple of "junk" radios that had the proper Wavemagnet. I thought both were the five-tube 5G500 sets. However, both turned out to be six tube 6G501M sets. One in awful cosmetic condition donated its Wavemagnet to the 5G500. The other was missing its front cover but was otherwise in fair condition. The chassis of the one in the worst condition had been repaired and new caps installed. The repair job had been done carefully with good components probably in the late 1960s. I decide to use its chassis in the better cabinet. I cleaned all the switch and tube socket contacts and swapped out a weak 117Z6GT. Very little additional work was needed.

Using my frequency counter, I determined that the IF was about 30 KHz low. (IF + tuned frequency = frequency counter reading for local oscillator.) My favorite distant weak station at 1510 KHz showed a frequency counter reading of 1933 Khz versus the expected 1965 KHz. Given the amount of work that had been done to the chassis, I was not surprised that the IF had been adjusted.

Alignment by frequency counter
Purists will likely jump at my new simplified alignment procedure. Since I had a very weak station at the high end of the broadcast band, I knew that the AVC would not be a factor. For the 6G501M chassis, I retuned both IFs and "walked" the dial until the 1510 station came in at the proper 1965 KHz oscillator frequency. I then adjusted the RF section of the variable cap to peak at that same 1510, turning the Wavemagnet in a position to maintain the weakest signal to limit AVC while adjusting the trimmer.

Not surprisingly, the set tracked properly after this treatment. The alignment instructions called for 1400 as the proper setting for peaking the RF trimmer if using an RF generator. Re-tweaking at 1400 made no discernible difference. The radio performed well with the simplified procedure.

Since I had worked on the 5G500 some years ago, I checked its alignment using the same technique. It was spot on. Surprisingly, the RF trimmer was also adjusted properly despite having been set to work with a generic loop antenna.

The handles are obviously not original. Both are just pieces of an old belt. The 5G500 has the belt ends knotted and the 6G501M has the ends sewn, something I learned to do more recently.

Performance
Both sets perform reasonably well. I like the ability to move the Wavemagnet antenna around for best signal. Both do a fine job at picking up my favorite weak-signal rock and roll station. The added broadband RF amp tube in the 6G501M increases both signal and the noise floor providing little discernible improvement in signal-to-noise versus the 5G500 without the RF amp. A tunable RF amp, as found in all the tube Transoceanics would have been a major improvement.

The Wavemagnet has connections for attaching an external antenna and ground for greater signal pickup. Those connections go to a single wire loop around the wavemagnet loop for inductive pickup.

The 5G500 radio as a museum piece.
Check out this site for a picture of the 5G500 in the Old State House Museum in Boston. The high-res picture shows a dial that is identical to that on my 6G501M (left) with the "Wavemagnet" logo in place of the "Universal" logo (right) on the dial of my 5G500. (If museum personnel would like an unbroken Zenith knob, contact me.)

Clones have no suction cups
Checking into the history of the Wavemagnet antennas helped me understand why early Transoceanic clones have loop antennas that are not removable. Zenith held the patent. The Hallicrafters clones do have a movable ferrite-rod antenna. Perhaps that was sufficiently different that they could defend a patent suit. Let me know if you have the answer.

Replacement of the cover material on a 6G501M
Rod Sievers shows a 6G501M which he has recovered with tweed tolex material. Nice job, Rod.

Headphone connections
The ads for these radios imply the ability to connect headphones. That can be done if the speaker plug is removed. HOWEVER, the connections carry full B+. That is rather dangerous especially if the radio is powered by line voltage. Zenith shows an ad illustration of the use of headphones, but the setting is obviously with the radio on battery power. While a radio battery at 90 volts will still bite, it is not as lethal as an AC-DC power line connection.

date 4-19-11