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Notes on Model 21 Philco's Model 21 is a rather unusual set. Introduced around March 1931, very early production sets used Model 20 chassis. At least one example is known to exist. However, the majority of Model 21 sets use the 21 chassis, which in reality is a slightly modified Model 20 chassis.
Model 21 was the first cathedral radio to use the classic cabinet designed by
Edward
L. Combs The 21 was changed to use the more powerful Type 45 audio output tubes in place of the 71A outputs used in Model 20. Also, the 21 used one of the then new Mershon (electrolytic) condensers as the input filter. Its value was a whopping 14 microfarads! The Mershon mounted in place of the large multi-section filter condenser can used on Model 20. The remainder of the filter condensers were mounted in a smaller can under the chassis in Model 21. Having recently gone through a Model 21, I now understand why the same schematic was referenced for Models 20 and 21 in the early Philco Service Manuals - the basic circuitry is the same! The differences lie in the power output tubes used (Type 45 instead of Type 71A), and a different power transformer and B-C resistor to handle the different filament voltage and biasing requirements of the Type 45 tubes. Also, as mentioned above, the input filter condenser is a MUCH higher value than that used in Model 20.
MAJOR PARTS SPECIFICATIONS
Push-Pull
Input Transformer (Part No. 4232) - 1:4 ratio (1:3 ratio may be used as a
replacement)
Power Transformer (Part No. 4813) - High Voltage 350-0-350V, 110 mA; Filament 1 -
2.5VCT, 7.45A; Filament 2 - 2.5VCT, 3A; Rectifier Filament - 5V, 2A.
B-C Resistor (Part No. 4824) - From terminals 1 to 7, in ohms: 800, 263, 75, open,
1800, 1430
Filter Condenser - The part number, and the values of the internal components, are not
known for this part. The values given on the Model 21
schematic are educated guesses, based on the values of those used in Model 20
save for the .09 uF condenser across the filter choke. The filter choke in the 21
is the same one used in some Model 70 receivers, which also had a .09 uF condenser
connected across it. I used 4.7 uF electrolytics in place of the 1.0 and 1.5 uF units
in the Model 21 I serviced, leaving out the .09 uF section entirely. The set works
well without it.
Filter Choke (Part No. 4819) - 7.5 H, 130 mA, 285 ohms DC
Electrolytic Condenser (Part No. 4848) - 14 uF, 400 volts, wet. (There is a discrepancy in
the value of this unit, as the 1932 Philco parts catalog specifies 14 uF, while
both 1934 and 1935 Philco parts catalogs specify 12 uF. In the Model 21 I
serviced, I found that 10 uF was sufficient as a replacement with no audible
hum.)
TOP VIEW OF MODEL 21 CHASSIS
Notice
the strong resemblance to a Model 20. The major difference is the blanking plate
placed over the hole where a large rectangular multi-section can condenser was
used in Model 20, and an electrolytic capacitor mounted on this plate. And, of
course, those output tubes are Type 45 instead of Type 71A.
The power transformer in this particular set was bad, and was replaced with a
transformer from a Philco Model 96 console. The hookup was similar, but it did
require alteration of the hole through which the terminals of the transformer
are accessible under the chassis.
Yes, this chassis is rusty and needs a good cleaning.
BOTTOM VIEW OF MODEL 21 CHASSIS
Again,
the view is very similar. The multi-section filter condenser has been moved
UNDER the chassis in Model 21, as shown here, save for the input section which
is a large electrolytic in Model 21.
View the Model 21
schematic. Return to the Tech Section Index. © 1997-2006, Ron Ramirez. All
rights reserved. Unauthorized duplication prohibited.
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, a style later used in the easier to find Model 70 and 90 Baby
Grand sets.
