Masterpiece VI (6)

The 1938 Masterpiece VI (MPVI) does not include a cabinet.  It is only the receiver, amplifier, and 18-inch McMurdo Silver Super Giant speaker.

 

In the summer of 1937 for the 1938 model year, a revised Masterpiece appeared as the Masterpiece VI. Outwardly very similar to the Masterpiece V (MPV), yet there are numerous design changes. The most visible is that on the front panel, the MPVI has only 2 oblong escutcheons behind each of the left and right groups of 3 knobs, where the MPV had 6 individual escutcheons.  On the receiver, there are 2 more tubes than MPV chassis.  The MPVI employs all-glass G-style tubes with full chrome shields for the receiver.

 

The Intermediate Freq. (IF) and Audio (AF) sections are significantly revised from the MPV, including the addition of the 6J7 microphone pre-amp and the use of 6J5s for the 6C5s of the MPV. A single 6J5 on the MPVI receiver chassis replaces the push-pull 6C5 2nd-AF amp tubes found on the amp chassis of the MPV.  The AVC section is all new, replacing the 6K7-6H6 pair of the MPV with two 6B8 tubes in the MPVI. One 6B8 implements an IF AVC circuit, and the other 6B8 implements an RF AGC circuit.  The IF AVC circuit is the same as the MPV circuit, acting in the IF stages to hold the output volume constant.  The RF AGC circuit holds the RF gain just below saturation, so that on weak signals RF gain is automatically increased to maximum.

 

The MPVI IF section is much more complicated than the MPV's.  It provides 4 bandwidth choices including 32kHz, 12kHz, 8kHz, and 4kHz, by using IF transformers which are switched in and out depending on the IF bandwidth choice.  On the narrowest bandwidth of 4kHz, three IF stages are used including a narrow-band 1st IF amp.  On wider bandwidth settings, the 1st IF amp is excluded and only the 2nd and 3rd IF stages are used, along with IF transformer taps that widen the bandwidth of the IF in 3 stages.  The IF switch is a massive 6-deck, 11-pole, 6-throw switch to switch the IF transformers in and out.

 

The Masterpiece VI has 5 bands, 2 RF stages, volume expander, 4-bandwidth Fidelity control, separate bass and treble controls, two-speed tuning knob, magic eye tuning indicator and terrific deco styling.  The separate amp chassis has a power supply with 2 x 5Z3 tubes, and an audio amp with 2 x 6L6G tubes.  In later production radios, the amp chassis was amended to include only one choke in the power supply (L2), while L1 was moved outboard and mounted on the base of the Jensen Super Giant speaker.  This necessitated a 7-wire harnes between the amp chassis and the speaker, compared to 5 wires in the early production MPVIs.

Band coverage is as follows:

A	150	400	kHz	Longwave
B	550	1800	kHz	AM broadcast
C	1800	5500	kHz	Shortwave 1
D	5.75	19	MHz	Shortwave 2
E	17	70	MHz	Shortwave 3