Interesting Filament Circuit
 

I had occasion to look at the Motorola 10VT24R schematic today. The power supply uses a transformerless voltage doubler for the B+. There are two filament transformers. One transformer was connected only to the damper tube- not uncommon at that time. The other transformer has two 6.3 volt windings. One winding went only to the 2nd video IF tube. The other winding went to all the remaining tubes.

My question is why the separate winding only for the 2nd video IF?

Not having the schematic infront of me the most likely reason would be that the cathode is at a higher voltage relative to the main heater winding and was given a separate winding so it's heater could float to that potential and avoid heater cathode shorts.


If you want to talk weird heater wiring I have a Bell/TRW tube stereo reciever where the phono/preamp stage 12AX7 heaters are used as the cathode resistor for the audio output stages thus saving parts and driving those heaters with DC. There were IIRC GE sets in the late 40s that drove one half of a center tapped tuner tube heater with DC while the other half got AC...And let's not forget Zenith pulse regulator color sets connecting the Horizontal output and HV reg heaters in series on their own heater winding so the other tube would not work with one removed...That also caused certain compatible tube substitutions to have to be done in pairs.

Don't the 35Z5, 35W4, etc have a weird setup for the tuner light?

Tom, You are exactly right. The plate voltage for the 1st video IF stage comes off of the cathode of the 2nd video IF stage. Therefore, the cathode of the 2nd stage is way above ground and would need the extra transformer.

Now that begs the question: Why did they make those two IF stages in this unusual configuration?

IIRC, the tube whose filament was driven by different voltage sources was the mixer tube in the tuner.

I had a Knightkit amplifier in the 1960s that had that exact configuration to provide DC to the filament of the first amplifier stage. Took forever to warm up.

It looked like a "smart" way of getting over the problem of dividing the B+ to run the IF amplifier. It was typical in those days for the B+ from the rectifier to be in the order of 350VDC. The signal chain would need only half that. Early RCA design featured divider resistors which got hot and burned out. Others provided +125 to +140VDC from the cathode of the audio output tube with the plate at +350VDC.

Motorola used this approach. Unusual but quaint.