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by D.K. Owens
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Substitutes
for the WD-11
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Westinghouse
introduced the WD-11 tube in 1922 for their Aeriola, Sr.
(Model RF). They needed a tube which could be heated with
dry cells instead of storage batteries. The WD-11 was also
used in the RS, Regenoflex, Radiola X, Radiola III and
Radiola III-A, sets made for RCA by Westinghouse. The
development of this tube is well documented [1].
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General Electric
developed the UV199 tube independently, with the same
objective of economical dry cell operation. The UV199 was
introduced in 1923 with the Radiola IV. Its characteristics
were similar to the WD-11 except for the filament demand of
3.3V/.06A vs. 1.1V/.25A for the WD-11. Besides its filament
economy, the UV199 was also more robust and less microphonic.
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The filament
structure of the WD-11 is notoriously delicate. When a
filament burns out, it may flop against the plate applying
B+ to the filament circuit with disastrous results [2]. RCA
chose the UV or UX199 for subsequent superheterodyne and
regenerative receivers. No apparatus using the WD-11 was
designed after 1924, and it became obsolete so quickly that
RCA issued Service Bulletin #6 (Dec. 11, 1925) showing how
to convert the Radiolas III and III-A to UX199 and UX120
tubes [3].
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Westinghouse issued
the WD-12, a UV-based version of the WD-11, in 1923. It was
replaced by the UX-based WX-12 in 1925 for converting
storage battery sets to dry cell operation. The Crosley
"Pup" appears to be the only radio
factory-designed for the WD- or WX-12. The WD-11 and WX-12
were dropped from the official RCA product line in 1933 [4].
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Those who want to
play these early sets have a tube problem. RCA quit stocking
WD-11 tubes 70 years ago. Today they are prohibitively
expensive, being worth more than the radios using them.
Because of their value, those who have WD-11s are reluctant
to put them into service.
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UX199s can be used
per the RCA bulletin. Adapters are still around, but both
are quite expensive. The 864 (VT-24), developed for aircraft
use, is a popular substitute because it has the same
filament requirements and similar characteristics, but is
also now quite expensive.
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The Type 30 has been
used as a substitute, but is both expensive and requires an
oddball filament voltage. Another substitute is the Loctal
3D6, a beam pentode, connected as a triode and used with an
adapter. Its filament requirements are similar to the WD-11.
Miniature pentodes (1L4, 1S4, 3S4, 3Q4, 3V4) used in the
same manner have also been described [5].
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Another elegant
solution is to use a miniature, wire-lead 5676 in a WD-11
base. A silvered test tube can be fastened over it over it
to give the appearance of a WD-11.
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Testing Possible
Substitutes
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I decided to carry
out some tests on possible substitutes that would be cheap,
plentiful, functional and resemble the WD-11. A look at the
tube manual uncovered the following possibilities:
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Tube Fil. V/A Bias V Plate mA Gm µ Base Price-$
WD-11 1.1/.25 -4.5 2.8 400 5.6 Unique 100+
864 1.1/.25 -4.5 2.9 610 8.2 4-Pin 25-35
1G4 1.4/.05 -6.0 2.3 825 8.8 Octal 10-12
30 2.0/.06 -4.5 2.5 850 9.3 4-Pin 16-20
1LE3 1.4/.05 -3.0 1.3 760 14.5 Loctal 4-5
1Q5 1.4/.10 -4.5 5.8* 1075* 7.8* Octal 3-4
1A5 1.4/.05 -4.5 5.6* 1070* 8.0* Octal 3-4
1LA4 1.4/.05 -4.5 5.5* 1065* 8.1* Loctal 4-6
* Triode-connected (screen tied to plate)
All values with Ep = 90V
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The first four tubes
are shown for comparison purposes only. Types 1G4, 30 and
864 are not good candidates due to their high cost. The last
three turned out to be good possibilities. Characteristics
of the triode-connected 1Q5, 1A5 and 1LA4 are not listed in
tube manuals and had to be measured. The 1Q5 is a beam power
tube and the 1A5 a power pentode. The 1LA4 is a 1A5 with a
Loctal base. The characteristics of the other tubes were
taken from the tube manual.
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RCA recommends that
the Radiola III-A be operated with 22.5V on the detector and
90V with -4.5V bias on the three amplifier tubes.
Accordingly, the transfer characteristics of the WD-11, 864
and the possible substitutes were examined at Ep = 90V.
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Fig. 1 shows the
results. The WD-11, 1Q5 and 1A5 have long, linear transfer
curves. The 864 and 1LE3 have a much narrower operating
range. The steeper slope of the 1Q5 and 1A5/1LA4 reflect
their higher transconductance. All the tubes have similar
amplification factors (µ) except for the 1LE3 which is
nearly twice that of the others.
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Making a WD-11
Base Template
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Because of the unique
WD-11 base, substitutes must either be re-based or provided
with adapters. Re-basing yields good looking results, but
when it's time to replace the tube, you have to do
everything all over again. Adapters are easier to make and
use and involve less risk to the tubes. When the tube is
worn out, just plug in a new one. There are some adapters
sold commercially, but they are expensive.
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You can make your own
adapters at very little cost using old 4- or 5-pin bases
from dud tubes. These should be the small, 1-1/8"
diameter bases. It is easier to drill new holes in 4-pin
bases, but 5-pin bases can also be used. A template will
simplify your work.
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The WD-11 pin layout
is shown in Fig. 2. The template was made from a scrap of
1/4" thick Plexiglas about 2" square from the
local glass shop and a similar piece of Bakelite from an old
radio panel. Lay out the pin locations on the Plexiglas as
shown. Use a sharp scribe and be accurate. Drill one
3/16" hole and three 1/8" holes where indicated.
If you spot the holes first with a center drill, the main
drill will run true. A drill press is essential for
precision. Also drill a 1/8" hole at the center of the
pin circle for later use.
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Now,
clamp the Plexiglas and Bakelite pieces together and drill
two holes near opposite corners for bolts to hold the two
parts together. Bolt the parts together and, using the
center hole as a guide, drill through the Bakelite with the
1/8" drill.
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This hole in the
Bakelite is the pilot hole for a 1-1/8" hole saw which
you will next use to make a hole in the Bakelite to fit the
tube base to be adapted. This hole may need a little filing
or sanding to get a snug fit. Fig. 3 shows the template and
Bakelite bolted together.
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Making a WD-11
Base
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Remove each of the
existing pins on your tube base by holding it with pliers so
it can't rotate and drilling away the flange from the
inside. When the flange is removed, the pin will pull out
easily. Use a drill of the correct diameter to remove the
flange without removing any Bakelite. Scrape all the old
cement out of the base. Insert the base into the template so
the bottom is against the Plexiglas. You can now easily see
how to position the base so that the new holes will lie
between the old holes.
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Drill a 3/16"
hole in the base using the hole in the template to guide it.
Insert a small piece of 3/16" rod through both template
and base to prevent slippage and drill the three 1/8"
holes and one 3/16" hole. The new pins are made from
thin-wall brass tubing in 3/16" and 1/8" diameters
from a hobby shop. This tubing is full diameter, while the
holes will be slightly undersize, therefore the new pins
will fit tightly and need no anchoring.
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Cut the tubing to
1-3/4" length. I use a miniature tubing cutter from the
hardware store. It gives a rolled edge at the cut which
makes insertion and soldering easy. The best way to insert
the pins in the base is to chuck them in your drill press
and use the press to push them in. Make two 9/16-long
collars to use as depth gauges. Drill one to slip over the
3/16" pin and one to slip over the 1/8" pins. I
made these from tubular spacers.
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Start the brass
tubing into the base by lightly tapping it with a small
hammer. Then slip on the collar and press the tubing into
the base until its end is flush with the end of the collar.
Watch it closely to be sure it goes in straight. Now your
pin will protrude the correct distance from the base.
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Fabricating
Adapters
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The sockets I used to
make adapters for the substitute tubes came from the junk
box. They were the molded Bakelite kind with molded-in
mounting plates. The diameter below the plate is about one
inch so they fit snugly into the bases with just a little
filing. Cut off the mounting ears with tin snips, but leave
the rest of the mounting plate in place. Wiring of the
adapter will depend on the type of tube you are using as a
substitute. Diagrams for the 1LA4/1LE3 and 1A5/1Q5 are shown
in Fig. 4. The resistor across the filament connections
needs explanation.
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Volume in
these old sets is controlled by varying the filament
voltage through a rheostat. It is sized for use with 0.25A
tubes. Because the substitutes draw only 0.05 to 0.1A, the
rheostat will have little effect on filament voltage and
the set will run wide open with no volume control action.
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For
substitutes to behave like WD-11s, they must draw similar
filament currents. The 1/4-watt resistor, R, shunting the
filament increases the total circuit current to about that
of a WD-11. This will give good volume control action. R
should be 6.8 ohm for the 1Q5 and 5.1 ohm for all other
tubes.
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Connect the resistor
between the filament lugs on the socket and solder wires to
them and to the grid and plate lugs. The wires can be bare
except the one from the plate (or grid depending on how you
orient the socket with respect to the base). This wire needs
to be insulated where it crosses over the socket. Guide the
wires through the pins in your WD-11 base, seat the socket
firmly in the base and solder the wires to the pins.
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Drill two small holes
on opposite sides of the base into the edge of the socket.
Make sure that the holes go between socket pins. The
diameter of the hole will depend on the size of the
self-tapping screws you use to fasten the socket to the
base. It is a good idea to experiment on some scrap Bakelite
before drilling the sockets.
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I use #0 x 1/4"
or #1 x 1/4" screws. Using a larger drill, enlarge the
hole in the base shell so that the screw passes freely. Do
not drill into the hole in the socket edge. Insert the
screws to hold the base and socket together. Don't over
tighten them or you may crack the shell.
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Fig. 5
shows (A) a WD-11 tube, (B) a re-based 864, (C) a 1LE3 in
its adapter, and (D) a 1A5 in its adapter. The superficial
resemblance of the substitutes to the WD-11 is obvious.
Details of the adapter construction are also visible.
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Testing the
Substitutes
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A Radiola III-A was
equipped with WD-11 tubes and its operation studied. This
set has 2 sections: a regenerative detector and an AF
amplifier followed by a push-pull second audio output stage.
There is provision for headphones after the first audio
stage and each section has its own filament rheostat.
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Because of the
various interacting adjustments to be made for proper
reception, there could be no "standard" setting
for comparing tubes. so the test was subjective. In each
case, a weak station was tuned in and the positions of the
filament and regeneration controls noted at the settings for
loudest volume and best sound quality. The position of the
regeneration control at which oscillation occurred was also
noted.
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The set was operated
with 22.5V on the detector and 90V on the audio amplifiers
with -4.5V bias. The filament supply was 1.5V. A voltmeter
was connected across the filament pins so as not to exceed
1.1V for the WD-11s. At first, only the "front"
half of the set was operated (detector + first audio) with
headphones.
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Using WD-11s, the set
was adjusted for best volume and sound. Then the WD-11s were
replaced first with 1LE3s, then with 1A5s, and the set
re-adjusted for optimum reception in each case. You'll
recall that the 1LA4 is electronically identical with the
1A5.
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The 1LE3s were rather
touchy. Oscillation onset occurred at a much lower setting
of the regeneration control and the filament control had to
be reduced to avoid distortion. Nevertheless, performance
was satisfactory and quality was acceptable. The volume was
louder because of the higher µ of the 1LE3.
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The 1A5s also
oscillated at a lower setting than WD-11s and required
reduction of the filament voltage, but overall performance
was much smoother than the 1LE3 and distortion was less at
high volume.
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A horn speaker was
then connected to the second push-pull audio stage and the
filament rheostat turned on. There was a noticeable tendency
for the 1LE3s to distort at high volume levels, but
satisfactory room-level sound could be achieved with
filament adjustment. As before, the 1A5s were smoother and
more like WD-11s in operation. Distortion was less;
room-level sound was easily achieved. In all cases, volume
control action was as good as with WD-11s.
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After similar tests
were done with 1Q5s, it was found that there was little
perceptible difference in the performance of 1A5s, 1Q5s and
1LA4s. In conclusion, I prefer the 1A5 or 1Q5 for a
substitute. They are cheap and available and perform very
much like the WD-11. The 1LA4 works just as well, but is a
bit more expensive. If you happen to have some 1LE3s on
hand, however, they are also functional and will do the job.
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