Radiophile.com - Zenith 6P417

Zenith 6P417, 1940

Tube complement: 12A8G mixer/oscillator, 12K7G IF, 12Q7G detector, 35L6G audio output, 50Z7G rectifier, Zenith 100-79 ballast.

Approximately 10-1/4 inches / 26 cm wide. Original green metallic paint over Bakelite.

The rear view. The one-piece Bakelite cabinet is molded like an upside-down bowl, and upside-down Bakelite chassis (see below) installs through the bottom.

A closeup showing the front-panel controls. The tuning knob is above; below are two slide switches, band (broadcast/police) on the left and tone (bass/treble) on the right.

About the Zenith upside-down Bakelite chassis
No doubt an attempt at manufacturing labor and parts cost reduction, the notorious upside-down Bakelite chassis was used in several 1940 Zenith models. The design is remarkable for several reasons:

The chassis is made of Bakelite instead of metal.
Several parts (volume-control potentiometer, tube sockets, trimmer-condenser bases, mechanical support for one of the IF transformers) are molded right into the one-piece Bakelite chassis in one step.
The chassis is mounted upside down (tube bases are on top; glass envelopes hang down) in the cabinets.

Above, a top view of an extra upside-down Bakelite chassis from my junque box. This is its normal orientation inside a cabinet, with the components on top and the tubes below. You can see the extremely cramped arrangement of parts. On top just behind the speaker, the audio output transformer nestles into a chassis cutout. The rectifier tube's socket is buried under wiring at the rear center. I pity the poor repairmen who needed to replace the AC line cord. The on-off switch and volume control, which are part of the Bakelite chassis assembly, are visible on top, centered just behind the dial.

[6D410 chassis]

Above is another view—this time the chassis is inverted so the molded-in tube sockets, and a trimmer condenser, are visible. (The tubes have been removed.) At the left, notice how the speaker frame is an integral part of the chassis mechanical support. At the right you can see the Zenith shielded wavemagnet antenna, which is nothing more than the to-become-common AM loop antenna, surrounded by an electrostatic shield in the form of a grounded wire grid. All the Bakelite-chassis Zeniths had 'em. The idea was that the desired radio signals propagate to the receiver in the form of electromagnetic waves, while some interference signals are electrostatic in nature. Eliminating the electrostatic component of the received signals should therefore result in an improved signal-to-noise ratio. This idea has merit; to a degree, it works. But the inevitable slide into mediocrity of manufactured products, as companies determine what consumers are really willing to pay for. Advanced features disappear and technological brilliance is replaced by low cost as the chief design goal, resulted in the near-elimination of this feature in later years.

The myth of the upside-down Bakelite chassis recall
Radio-collector lore tells the tale that this chassis was beset from the get-go by problems, notably poor serviceability and poor reliability. The poor reliability was ostensibly due to heat buildup in the upside-down design and the fact that the Bakelite chassis material itself is a poor conductor and radiator of heat. As the legend continues, Zenith, realizing its mistake, recalled and destroyed much of its 1940 Bakelite-chassis radio production. I've heard too that somewhere there exists a photograph showing Zenith's Commander McDonald standing over a ditch filled with scrapped Bakelite-chassis sets awaiting burial.

It's quite a story, but I don't believe it. There are many reasons for my skepticism:

These sets aren't rare. With minimal effort I've been able to collect seven different models from this series in just a few years. I have two copies of two models. I've passed up many other examples offered for sale. Had I decided to collect just table sets from just one model year of any other single manufacturer, I would have faced a task of similar difficulty.
These sets aren't unreliable. I've restored most of my Bakelite-chassis sets and I play them often. They perform well and don't seem to suffer problems of excessive heat buildup. Having the tubes inverted and the components above was not a particularly unusual configuration among designs from this time period. I have several other sets with this inverted chassis configuration (Setchell Carlson 416; Silvertone 3251, 6320, 6405, 6407).
I agree that these sets are difficult to work on due to the cramped arrangement of parts. It's also difficult to get the chassis in and out of the cabinets due to the way the two parts fit together. But no manufacturer before or since has cared about the repairability of its products. While I do think that complaints from repairmen were a factor in Zenith's decision not to continue this design in subsequent years, I find it difficult to believe that any such complaints would cause a big company to recall and destroy months worth of valuable production.
One would think that if Zenith had recalled these sets, either from end-users, from dealers, or both, that there would be some indication to that effect in Zenith service literature. There isn't. Bakelite-chassis sets are fully covered in the service data just like all the others.
I found and purchased a new-old-stock model 6D411 cabinet, in its original box. I can think of one likely reason that this original part existed for me to find and buy it: It was ordered by a Zenith dealer as a repair part for a customer who already owned a 6D411 whose cabinet became damaged. So repair parts existed and were distributed to dealers for the purpose of repairing these sets, just like other regular radios.
In the radio-collecting domain, the paradigm of scholarly research has been the books written by Harold Cones and John H. Bryant, who are aptly known as the "Radio Professors." One of the superb reference books they've written for radio hobbyists and historians, Zenith, The Glory Years 1936-1945, History and Products (additional credited author Martin Blankinship), contains a wealth of information about Zenith products and the Zenith company of the named time period. But on the subject of the upside-down Bakelite chassis, the book disappoints. It has little to add, despite the fact that the authors had access to the Zenith corporate archives. They only repeat the apocryphal tale, stating that most records from this era were lost. Essentially, we're told that the story must be true simply because so many radio collectors say it is.

There's something else unusual about these upside down Bakelite chassis sets—they came with ST-shaped (shouldered tubular) 35L6G and 35Z5G (not GT) tubes. As far as I know these tube types were used only in certain 1940- and 1941-model-year Zeniths. Almost immediately, tube production shifted over to the newer, smaller, "bantam" GT (glass tubular) style of envelope, and the 35L6GT (later to become the 50L6GT) and 35Z5GT took their place as part of the ubiquitous All American Five (AA5) tube set. I believe the 35L6G and 35Z5G were not sold on the replacement market—I've never seen one new in a box, and if one has been replaced in a Bakelite-chassis Zenith, the replacement is a GT-style tube. The only 35L6G and 35Z5G tubes I've seen are branded Zenith, and are either in place in a Zenith chassis or loose in a batch of used tubes. Zenith must have ordered and used the full production of 35L6G and 35Z5G in new Bakelite-chassis set production, and that was that.

Also, evidently several five- and six-tube 1941 Zenith models were originally equipped with 12SA7G (not GT) tubes. Zenith service literature says so, and I've heard of a model 6D512 in a collector's hands that has the original Zenith-branded 12SA7G still in place. So, yet a third common AA5 tube was produced with ST glass at the start.