Philco Model 90 Baby Grand (Type 2) Cathedral Radio (1931)

The Philco 90 Baby Grand was introduced in June of
1931 along with the model 70. It continued the line of
highly successful Philco compacts that had begun with
the model 20, almost one year earlier in August of
1930. The model 90 is today one of the most famous
and widely recognized of all vintage radios. It uses the
cabinet designed and patented in February of 1931 by
Edward L. Combs. With an initial selling price of
$69.50, compared to 49.95 for the model 70, fewer
model 90s were sold and as a consequence it is harder
to find today.

Although very similar, the 90's cabinet is larger than
that of the 70 and 21 and it also has a different style of
moulding around its base. In fact this feature renders
the 90 immediately distinguishable from the 70 and
other models, since its moulding is of a "raised design"
with a central void area, whereas the other sets all
have a continuous front moulding.

The model 90 uses the Philco 9-tube Balanced Super-
heterodyne chassis, which is the mid-unit of the 7, 9 and
11-tube line of Balanced Superheterodynes introduced
for 1931. The schematic may be found here.

There was an evolution of the 90's chassis through its
lifetime. At the time of its introduction the circuit used
push-pull 45s in the output stage and provided no AVC.

Mid-life, Philco revised the design to provide AVC using their "multiplex detector"* circuit. This required two tubes and so the push-pull
output was replaced with a single ended 47 pentode in order to keep the tube count at 9. The final chassis design, introduced in Jan of
1932, combined the LO and mixer (1st detector) tubes into one using the autodyne arrangement (just as for the type II model 70 chassis),
allowing the single ended 47 output stage to be upgraded to use push-pull 47s. At the same time, operation of the AVC was improved by
adopting the latest type 35 variable mu tubes.

The chassis in my model 90 is the second version, employing a single type 47 tube. The complete tube line-up is 24 (RF), 24 (mixer), 27
(LO), 24 (IF), 27 (detector rectifier), 27 (detector amplifier), 27 (1st AF/driver), 47 (output pentode) and 80 (rectifier). The radio covers the
standard broadcast band from 550-1500kc, uses a 4-point tone control ("brilliant", "bright", "mellow" "deep") and provides automatic volume
control.

* For a description of the Philco "multiplex detector" see the bottom of my Philco 111 page.