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Philco 118 B+ on IF and RF tubes?
NV3G
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#1

07-10-2017, 10:35 AM
I am having a problem seeing the dot that connects to the B+  buss in order to get B+ to pin 3 of the two 78 tubes ( RF and IF amplifier ). Please help me locate the how the B+ gets to the two 78 tubes on the schematic? My 82 year old eyes with cataracts do not work well.

Thank you,
Oscar
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nv3g
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Radioroslyn
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#2

07-10-2017, 10:56 AM
Hi Oscar,
B+ comes off of the left side wire from the spkr's field coil. Also feeds HV to the driver transformer. The RF amp and mixer  HV passes thru the shadow meter, it has a parallel resistor so if the coil is open you'll still have HV there.
http://www.theoldradiofixerupperguy.com/...del118.pdf

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(This post was last modified: 07-10-2017, 12:21 PM by Radioroslyn.)
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morzh
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#3

07-10-2017, 11:00 AM
They connect at the top side of the wire-wound 6.5kOhm resistor in parallel with that capacitor at the rightmost of the schematic, where it meets the Field coil. This point then goes to the shadowmeter, and the IF 78's load goes to that same point, and the RF 78 as well as the 6A7 loads, go to the right side of the shadowmeter.
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Arran
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#4

07-10-2017, 05:14 PM
Don't attempt to power up one of these without the field coil, or a substitute choke in place, or bad things will happen to some of the resistors around those tubes. Pulling the speaker does NOT kill the B+ to the entire radio, so whatever is still connected gets almost the full B+ from the power supply with the load of the power outputs removed. It's a design flaw really that Philco did not add a jumper on the speaker plug and socket to take care of this.
Regards
Arran
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