Magnavox big Imperial, 1959 to 1962
Posted: Sun Oct 24, 2021 3:02 am
I posted a couple pictures of Imperials for sale, and one video with an Imperial playing Al Hirt music. I have the schematic for my 1960 Imperial model 1ST233L and for Bills' 1962 Imperial model 1ST664. To be fair, a 1962 big Imperial is not supposed to exist, yet here we have one, documented and in the flesh, so to speak. We know for sure it exists. The schematic that I have for mine is dated 1-62. The schematic I have for Bill's is dated 3-63. Since my Imperial is a 1960 model, documented by its eye tube, plus by codes on speakers and tubes, the Conquest record changer and built in the 42nd week of 1959, I'd have to bet there is an earlier version of the schematic, or two, for the 81 series amp and the 64 series tuner. As to the differences I have found between my schematic and Bill's, mine shows an optional balance control across the input to the amplifier. The balance control is not shown at all on Bill's schematic. The balance control that is actually on mine is in the tuner, R2. It is ganged with the treble switch. It is accessible thru the hollow shaft of the treble switch. This component on Bill's schematic is also R2 and is also ganged with the treble switch.
The differences between the earlier Imperial and the later one lies in the input to the amplifier. Mine has an .068 cap, C59 across the 2700 ohm resistor in the input of the amplifier. The input is fed directly to the treble level control on the amp chassis in both versions with the above exception. The tone chokes, L18, L19, L 20 and L21 on my chassis is listed at 420 ohms, the ones on Bill's 480 ohms. There are a couple other components drawn opposite on the two schematics, but the components are the same and do the same job. So, what difference will the changes I see here make in how the two instruments sound? The tone chokes are built two in a component and are used as a crossover network for the treble channel. Both amplifiers run the 6V6's at 320 volts on the plate. Both amplifiers use a 12AX7 for the treble AF amp with 120 volts on the plates. The bass AF amp and the phase inverter are both 12AT7's with 75 volts on the plate. Are they purposely driving the treble amps harder for some reason? There are no bass level controls on the amp chassis in either version.
The differences between the earlier Imperial and the later one lies in the input to the amplifier. Mine has an .068 cap, C59 across the 2700 ohm resistor in the input of the amplifier. The input is fed directly to the treble level control on the amp chassis in both versions with the above exception. The tone chokes, L18, L19, L 20 and L21 on my chassis is listed at 420 ohms, the ones on Bill's 480 ohms. There are a couple other components drawn opposite on the two schematics, but the components are the same and do the same job. So, what difference will the changes I see here make in how the two instruments sound? The tone chokes are built two in a component and are used as a crossover network for the treble channel. Both amplifiers run the 6V6's at 320 volts on the plate. Both amplifiers use a 12AX7 for the treble AF amp with 120 volts on the plates. The bass AF amp and the phase inverter are both 12AT7's with 75 volts on the plate. Are they purposely driving the treble amps harder for some reason? There are no bass level controls on the amp chassis in either version.