Some Notes And Comments on "The 1L6 Issue"

An Overview & Summary based on "In Search of the Elusive 1L6 - An Obsessive Odyssey" by A. Padgett Peterson, P.E and used with his permission

1L6...The Tube Made from 'Unobtainium'. Introduced by Sylvania in 1949, it only ever had one application - the American, battery operated, short-wave, portable radio receiver. It was always a relatively scarce and expensive tube - which Zenith used from the G500 model of 1950 up to the end of the tube T/O era with the B600 of 1962. Although mainly used by Zenith in the Trans-Oceanic, it also saw service with the Hallicrafters and RCA 'clones' of the T/O. Surprisingly, it may also be found in some lower end portables that have continuous coverage up to 30MHz.

Padgett Peterson has noted that:

"...supplies of 1L6s began to dry up as early as late 1994. Asking prices were generally still in the $10 range but supply was becoming erratic. By late 1995, prices had hit $15, early 1996 $20, and 1997 is in the $24-$30 range. Surprisingly, Antique Electronic Supply unearthed a number of JAN (military specification and generally a bit better than commercial) 1L6s made by Sylvania in 1968 in early 1997 which they sold for a comparatively reasonable $30. And some people complained."

Why The Zenith T/O?

By 1938, the head of Zenith, Eugene F. McDonald, foresaw the market for a portable shortwave receiver - based, largely, on his desire to keep in touch while on his yacht and when at his Canadian fishing camp. The new 1.4v 50ma Loctal-based tubes had just become available and while true "portables" would not be a part of the Zenith line until 1940, a design team was tasked with solving the problem of maintaining oscillation over a very wide range of frequencies - something which had presented difficulty in 'low-voltage' shortwave receivers.

The first examples had a single band to cover all frequencies and McDonald complained that tuning with the small knob and dial required "micrometer fingers" - he insisted that another means be found. This was achieved by selecting five fairly narrow shortwave bands (most bands were only about 400 kc wide) focusing on those allocated to commercial stations. This form of 'electronic bandspread' became a hallmark of all Zenith T/O's.

When production of the new 8G005 "Trans-Oceanic" began after World War Two,  oscillation was a function of the pentagrid converter, the 1LC6. The RF tower coil assembly was similar to pre-war designs with three coils (Antenna, Detector, Oscillaor) but performance was not as good and, in 1947, production reverted to the superior 1LA6 for revision TZ1 - the slightly lower tranconductance being offset by the 15% greater plate resistance.

Enter the 1L6

By the time the new 1950 models were on the drawing board, Zenith - a 'risk-averse' company - had two problems with its flagship product. The T/O circuit worked very well but was considered 'fragile' - changes had not improved it - and, furthermore, there was a considerable investment in the design of the coil tower and bandswitch arrangament. Although the new (1940) miniature tubes had proven themselves, in size, power consumption, and longevity, Zenith did not appear to have any desire to update the circuit to take advantage of these tubes. In fact, close examination of the 5G40 (G-500) chassis shows that not only was there room for a 1LA6 socket, the chassis was actually punched for a loctal/octal sized socket and a filler had to be used for the 1L6 even though it is obvious that all of the other positions were for miniature tubes.

The question arises, at this point, as to why Zenith appeared to have no intention of risking the circuit design change required for the 1R5, the only listed minature pentagrid converter, or accepting the loss of shortwave sensitivity that would entail. 

However, it was at this late stage that Sylvania, as Zenith's major tube supplier, offered a 1LA6 in a miniature tube form factor - the 1L6. So, in 1949, the G-500 became an all-miniature-tube shortwave portable, the first of its kind, and 200 hours use from the new combined A/B "long-life" batteries were a selling point.

From this point onwards, the introduction of the G-500 model in 1949, to the demise of the tubed Trans-Oceanic with the 1963 B600 model, the basic oscillator/converter circuit and coil tower remained unchanged - apart from the small rearrangment of bands in 1951 when the H500 replaced the narrow 49m band with two "marine" bands to offer continuous coverage from 2MHz - 8MHz.

So Now What? Some Alternatives to the 1L6

So, we have a unique (and prized) vintage radio. It uses a tube that is rare, expensive and relatively fragile. For this reason, many 'old hands' will insist on the temporary replacement with a 1R5 during restoration and testing. This makes good sense and leads us to:

The substitution most commonly used in Trans-Oceanics is simply to replace the 1L6 with a 1R5. This is possible, despite the seemingly totally different design, because while the fifth grid in the 1R5 is tied to the filament internally and to B+ in the circuit - the B+ line contains a 68k resistor which drops the current to a negligable level. Although this is the 'wrong' tube it will actually work well up to about 10MHz, the sensitivity suffers a marked decrease from there. An accurate alignment can restore some of the lost sensitivity but the overall performance, when compared to a properly aligned set using a 1L6, is still inferior.

Since the Trans-Oceanic circuit was designed for the 1LA6 loctal tube, with a suitable adaptor it should be able to operate quite well in the later G500, H500 and 600 series, though a tube shield might be advisable. Indeed, close examination of the chassis for the G and H 500 will reveal that the punch-out for the 1L6 tube socket is loctal-base sized. 

A suitable Loctal-to-B7G converter can be obtained from Bill Turner at www.dialcover.com

Under the Mullard-Phillips designation for European tubes, an alternative that would appear to merit investigation is the DK92. This was a tube developed comparatively late (c.1953) and is not often seen in the USA -- where it bears the nomenclature 1AC6. It is a combination tube with a design similar to the 1R5 though with separate screen connections, each grid being handled individually instead of the 3-5 connection of the 1L6 or the 2-4 connection of the 1R5. This enables it to be used with a g1 oscillator grid, g2 oscillator plate-mixer cathode, g3 mixer grid, g4 mixer screen, and g5 supressor with exactly the same socket wiring as the 1L6

On a tube tester, the 1AC6 behaves like a supercharged 1R5. Absolute ratings are very similar to the 1L6 with 90v maximum screen voltage instead of the 67.5 v maximum of the 1R5 (80v nominal in a T/O). The only limiting factor is a 60v maximum (30v nom.) on G2, the oscillator "plate" while the T/O circuit design is for 84v. 

James Duckworth has done considerable practical work on the circuit changes required for the most effective use of this tube. The results of some of these experiments, as performed on a G500, can be seen at this mirrored page:  James Duckworth Experiments 

In the ten years since Padgett Peterson did his original research into this subject, there have been advances in the ease of circuit design and construction, as well as the more widespread use of surface-mount components. The Internet has fostered a closer-knit community of vintage radio enthusiasts and allowed for greater sharing of knowledge. In 1997, Bob Pierfelice in the USA developed  solid-state 1L6 replacement. The article describing this device, with schematics and photos has been mirrored by Padgett Peterson and may be seen on his site HERE

Now, ten years later, Edward Allison of Petersburg, PA, has developed an updated (and smaller, more compact) version of the solid-state 1L6 and is offering them on Ebay at around $US35 each as experimental types.  Ed's new 'tube' is a gold-pin, plug-in replacement using the latest JFET and dual gate MOSFET transistors. He claims it easily outperforms the 1L6 on the upper frequency bands with improved sensitivity and conversion gain and that re-alignment of the radio will not be necessary since the capacitances of the inputs and outputs of this device are made to be very close to those of a 1L6 tube.  

In the very near future, I hope to be able to acquire one of these units to offer my own impressions and pics.  Ed also plans to develop a solid-state 1U4 to complement his work on the 1L6 and his solid-state 50A1 Regulator for the 600 series T/O.

Padgett Peterson also suggests using the 1U6 as a common substitution and notes that:

"This is an identical companion to the 1L6 except for one thing, it is a 25 ma tube instead of a 50 ma.This means that it will test identically to a 1L6 in a tube tester but when placed in a radio, nothing will happen since it has double the filament resistance and the TransOceanic uses a series filament string. To use a 1U6 in place of a 1L6 a 56 ohm resistor must be added in parallel with the filament so that the other tubes in the series string will receive proper current. (Note: some sets may have already been so converted. If in doubt, check)...However, the supply of 1U6 is probably even more limited than the 1L6."

The final possibility is to return to the same mechanism used by the first TransOceanis, the 7G605 - a separate oscillator. Since the oscillator never needs to be exposed to B+, an under-chassis solid state solution might restore weak 1L6s to operation or allow full frequency use of the 1R5.

last update: 31st May 2007