Owners of the classic Zenith Transoceanic radios from the early-mid 50's will probably be aware of the pain involved if they have to buy a 1L6 tube (or "valve") for their beloved radio: A "good" 1L6 - seemingly the tube that goes bad most often in these radios - can fetch up to $60 today, a significant fraction of what one might have paid for a second-hand radio to restore.
The original 1L6 - a "not-too-common" tube even during its
heyday. A "good" one like this is even rarer today!
Click on the image for a larger version.
There are other tubes that will plug in, but these simply don't work on the higher shortwave bands (e.g. the 1R5, intended for AM broadcast band battery portables and not for the higher shortwave frequencies) or, in the case of the European 1AC6, requires a bit of modification and has issues with radio alignment. There is, of course, the electrically equivalent and comparatively easy-to-find 1LA6, but it's in a completely different form factor (e.g. a loctal tube rather than a 7-pin miniature) and requires either an adapter or a different tube socket. Finally, there are the solid-state replacement options which are roughly comparable to the cost of a known-good 1L6 and while some work fairly well, they definitely lack that "tube" aura.
One of the sticking points is that the 1L6 serves both as the local oscillator and frequency converter: One of the internal grids of this hexode is used as sort of the "plate" of the oscillator while a grid closer to the anode takes the signal from the RF amplifier stage and modulates the electron stream to mix it with the local oscillator to produce the 455 kHz IF - and it does this all with a filament that consumes just 50 milliamps at about 1.25-1.4 volts. Without significant rewiring, this kind of rules out the use of pretty much any tube other than one that takes just 50 milliamps at 1.4 volts for its filament!
Having established that there really aren't any other 7-pin miniature tubes that are "close enough", what about broadening the scope to include something entirely different?
"A simple push-pull amplifier using Russian Rod tubes and power transformers" - link.
While that article discusses the use of a 1Ж18Б (usually translated to "1J18B" or "1Zh18B") pentode, there is another member of that family, the 1Ж37Б (a.k.a. 1J37B or 1Zh37B) that is also a pentode rated for operation to at least 60 MHz. One property in its favor is that its filament voltage and current are "pretty close" to that of the 1L6: Anything between 0.9 and 1.4 volts will work and the rated filament current is around 57 milliamps - a tad higher than the 1L6, but something that we can probably live with.
Doing a quick finger-count of the number of elements of a pentode and comparing that with the number of tube elements that one would need to simulate a 1L6 hexode immediately reveals a problem: How would one use a pentode as a pentagrid converter when we are an element short?
The 1J37B to the rescue?
As it turns out, the 1J37B is a unique animal: As a result of its construction using metal rods to form and modulate sheets of electrons rather having the grid-like structures of "conventional" tubes, it actually has TWO "first" grids that are pretty much identical - a construct that is often likened to that of a dual-gate MOSFET.
The internal mechanical layout of the 1J37B is also quite interesting in that it is essentially two tubes in parallel, sharing the same cathode, screen "grid" suppressor "grid" and plate connections. In the middle, the identical sheets of electrons from the cathode go in two directions, each controlled by its very own "C1" control rod (e.g. C1' and C1"). Beyond C1' and C1", the structures of the screen, suppressor and plate elements are physically mirrored and connected together.
In comparing the specifications of the 1L6 and the 1J37B, the important specifications (e.g. transconductance, capacitance, filament voltage and current) weren't terribly far off. Some of the voltage ratings for the 1J37B - particularly that of the screen, rated for 60 volts maximum - are below that which one would see when used as a 1L6, but those may be dealt with later.
What if we could use one of these two "first grids" and the "screen grid" as the basis of the local oscillator section and simply apply the input signal to be amplified and converted to the other "first grid"? Because it was more like two tubes in parallel than one tube with multiple control grids I wondered if there was enough isolation to allow both oscillating and signal mixing functions to occur simultaneously. I was a bit skeptical if this idea, even though I was the one that thought of it (as far as I know.)
I decided to try it.
Making the base
Rather than mess with the Zenith TransOceanic for the first attempt at this, a friend of mine (Glen, WA7X) rummaged through his collection of old radios and produced an old Motorola battery/AC radio that used 1 volt tubes - including the 1R5 which is (sort of) "pin compatible" with the 1L6. Being a broadcast band radio I figured that if the concept was usable at all, the simple, nearly foolproof low-frequency circuits of such a radio would be the place to try it first: If it worked there, there may be some hope that it would work in the ZTO.
I needed to make a fake tube base, but not having a dud 7-pin miniature tube immediately at hand - and remembering from my past how difficult it is to solder to the "bloody stumps" of the dumel-like wires on the carcass of a deceased tube' base - I set about making one. I first covered the 7 pin socket in the radio with two layers of masking tape and then poking through this tape and into the socket seven lengths of bare, 17 or 18 AWG copper wire. A ring of masking tape was then placed around the outside of these pins and some "5-minute" epoxy was dripped into the middle, carefully avoiding the copper "pins": No doubt a small piece of plastic tubing or a taped-together ring of a sheet of plastic from a discarded "blister pack" would have made a nicer form than a floppy piece of masking tape, but it did the job.
Working the copper pins back and forth to make sure that they were surrounded with epoxy I allowed the requisite "5 minutes" for the "fast curing" epoxy to (somewhat) set. I then heated the contrivance with an SMD hot-air rework gun on its lowest heat (212F, 100C) for several minutes which immediately caused the epoxy to set hard enough to work once it had again cooled.
Carefully removing the "base" from the socket and peeling away some of the masking tape I trimmed the seven wires underneath to lengths comparable to that of a typical tube and did similar to the top side. I then had my 7-pin, solderable "tube base".
From this point on the wiring of the 1J37B to the base seemed pretty straightforward..
Wiring it up:
For the initial stab at replicating the function of a 1R5 the 1J37B was wired to the 7-pin base as follows:
1J37B Pin [7 pin base connection for the 1R5]
1 - Filament (-) [Pins 1] Filament and suppressor grid
2 - "Grid" 1' [Pin 4] "Oscillator Grid" (G1)
3 - Grid 3 (suppressor) [Pins 1] Filament and suppressor grid
4 - Filament (+) [Pin 7] Filament
5 - "Grid" 1" [Pin 6] "Signal Grid" (G4)
6 - "Grid" 2 (Screen) [Pin 3] "Oscillator plate/grid" (G2)
Plate wire (top) [Pin 2] Plate
Or, put another way:
7 Pin base connection for the 1R5 [1J37B Pin connection]
1 - Filament (-) and Suppressor Grid [1 - Filament (-) and 3 - Suppressor Grid]
2 - Plate [Top plate wire]
3 - 1L6 "G2" [6 - Screen Grid]
4 - Oscillator Grid (1L6 "G1") [2 - Grid 1']
5 - No connect (see text) N/A
6 - Signal Grid (1L6 "G4") [6 - Grid 1"]
7 - Filament (+) [4 - Filament (+)]
Again, note that applying the word "grid" to the 1J37B, while descriptive of the function, is not accurate: These "grids" operate more as control rods to deflect/direct the sheet of electrons being emitted from the cathode.
For replacing a 1R5:
Right at home, the completed 7pin miniature tube base in the Motorola
"test" radio in the 1R5's position.
Click on the image for a larger version.
A bit of explanation about pins 1 and 5 is in order at this point. For the 1L6, pin 5 connects to a pair of grids that surround the "Signal" grid (1L6 pin 6), but on the 1R5 the suppressor grid is internally connected to the "low" side of the filament using pins 1 and 5. Because the 1J37B is a pentode, the suppressor grid must be grounded which means that it would be connected to the filament low side as well.
Whoever made the radio could, in theory, use pin 1 and/or pin 5 for this connection and there is no real way of knowing without visually inspecting the socket. Because of this it would be a good idea to connect both pins together when emulating a 1R5 unless you know for certain how this connection is made in the radio with which you are testing.
For replacing a 1L6:
When using a 1R5 as a "pinch hit" replacement for the 1L6 the voltage applied to pin 5, which is nominally at about 85 volts, is effectively shorted to "ground". In the Zenith TransOceanic H-500 there is a 68k resistor in series with that line which means that the current will be around 1 milliamp or so, dropping the "85 volt" line - also used on the screen of the RF amplifier - by 3-5 volts, an amount likely not enough to be noticed. If the intent is to never use this replacement in lieu of a 1R5 we would just leave pin 5 disconnected.
Trying it out as a 1R5:
For testing it out in the "1R5" configuration (e.g. 1R5 pins 1 and 5 connected together) in the Motorola radio I inserted a 10k resistor in series with the anode lead in order to monitor its current, but despite this inserted loss the faux 1R5 worked the first time. The filament voltage across the 1J37B was 1.0-1.1 volts, well within its operational specifications and indicating that the tube in series with it across its 3 volt "A" battery (a 1S5) was probably seeing an extra 0.25 volts or so across its filament.
There was a minor problem, however: At some frequencies the radio would start squealing - something that it did not do with the 1R5. It is possible that there is a failing component in this radio somewhere, or it may also be that this faux 1R5 has enough extra gain to cause circuit instability, or a combination of both. Despite this minor quirk, the results were encouraging as it is usually easier to dispose of extra gain than obtain it in the first place.
As a 1L6:
I then decided to try this faux 1R5 in my Zenith TransOceanic H500 with pins 1 and 5 still connected together. While it seemed to work fine on the AM broadcast band, the radio got increasingly deaf with each higher band. A quick peek with a spectrum analyzer on a service monitor showed that the oscillator was working on all bands, but it was always a low in frequency, causing mis-tracking of the RF filtering with the error increasing as one went up being low by about 600 kHz on the highest (16 meter) band.
There was another problem: On 19 meters the radio started to oscillate, behaving like a regenerative receiver on the verge of oscillation and on 16 there was just solid hash, indicative of instability - likely because of excess gain. Referring back to the 1J37B specifications, I'd noted before that the noted maximum indicated screen voltage was on the order of 60 volts - but nearly 90 volts was being applied in the TransOceanic. Because of the rather low current pulled by the screen grid (being used as the "plate" for the local oscillator) and the still-within-specs amount of plate current (around 3 milliamps). I wasn't particularly worried about violating this voltage rating as there is no actual delicate "grid" that can be damaged, but it occurred to me that the gain could be reduced a bit by lowering the screen potential. With a bit of experimentation I determined that a 33k resistor paralleled with a 1000pF capacitor in series with pin 3 of the 1L6 socket reduced the screen voltage to around 65 volts - still a bit above its specifications - but this change resulted in unconditionally stable operation.
Disconnecting the now-unnecessary pin 5 and wielding an alignment tool I went to work re-tweaking the radio. For all but the 16 meter band, the local oscillator adjustment was well within the range of the various coils and capacitors, but for 16 meters, removal of the local oscillator's slug only brought it to within about -400 kHz of where it should have been.
On the lower bands, particularly AM Broadcast, 2-4 MHz, 4-8 MHz and 31 meters, the radio's sensitivity was reasonably good - not quite up to that of the 1L6 on 31 meters, but perfectly usable nonetheless. For the higher bands, 25 and 19 meters, I could still hear a bit of ambient atmospheric noise and those radio stations for which propagation was extant, but like 31 meters, the receive sensitivity was still a bit low indicating the need for yet more tweaking.
More tweaking and testing:
I later did a bit more experimentation, adjusting bias and re-dressing the leads, but I could not affect the 16 meter tuning range significantly enough to bring it back into dial calibration, nor could I make a "dramatic" improvement in the high-band sensitivity. If I'd replaced the core in the 16 meter oscillator coil with an aluminum or brass slug I may have been able to drag it up to frequency, but I didn't try it.
I did prepare another 1J37B tube and wired it in an identical manner to the first shown in the previous pictures (but with shorter leads) but interestingly it behaved remarkably different than the first: It seemed to be much more prone to bouts of spurious oscillation (e.g. broadband noise) and fitful, intermittent local oscillator operation - a state not dramatically affected by swapping the two "first grids" C1' and C1". Otherwise, the tube seemed to be behaving about the same in terms of DC current as the first.
What this told me is that my initial configuration of using the "screen grid" as the oscillator plate and applying the RF signal to be mixed to the other "first grid" may not be the best approach, as was my initial hunch - particularly in light of the fact that two seemingly identical tubes, both with fairly similar DC characteristics, seemed to behave radically different in this circuit - a strong indicator of a "non optimal" circuit topology that required on "quirks" of each, individual tube!
In the future I may reconfigure the circuit a bit to see if configuring the tube in some sort of "Gammatron" configuration may yield better results - but that will have to wait until I get more free time...
* * *
Additional information about the 1J37B and the "Gammatron" mode of tube operation:
- The 1J37B at the Radiomuseum - link (Includes discussions about operating the tube as a Gammatron.)
- Russian rod tubes at "Radicalvalves" - link (Information about the 1J37B and other "rod" tubes.)
This page stolen from ka7oei.blogspot.com