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Field coil question/general - Printable Version

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Field coil question/general - bridkarl - 07-24-2025

Not specifically a Philco question, but I expect to see this again so maybe someone can explain.
In this schematic I see an .005 cap going back to the speaker coil - red line. I assume the purpose is to complete the audio circuit by biasing the grid screen on the 6V6 audio output tube.

This radio hasn’t had this cap installed so I have a severe motorboating problem. Given this is a field coil speaker setup that means  as it’s part of the power supply this could be the source of the B+ issue causing the motorboating???  Basically the 6V6 hasn’t been biased correctly.

Or am  I totally misunderstanding ??

RE: Field coil question/general - Vlad95 - 07-24-2025

This capacitor is usually present in all circuits of vacuum tube power amplifiers. It is connected in parallel to the primary winding of the audio transformer and has nothing to do with the power supply or second grid bias, but is designed to block high-frequency resonances in transformer and excite the output stage of the audio amplifier.

RE: Field coil question/general - RodB - 07-24-2025

In addition to what Vlad wrote, the 6F6 is biased by cathode bias resistor R27. The value of this resistor is critical to controlling the current flow through the 6F6.

RE: Field coil question/general - MrFixr55 - 07-25-2025

Hi Bridkarl,

+1 on the others. C39, the cap across the output transformer primary is to cut high frequency oscillation and tailor high frequency response and is in almost all output stages. C40 is to capacitively couple the output tube plate to high impedance "can" type headphones. Do not use a modern pair of headphones with this unit.

Still having the motorboating problem? I don't know if you saw my previous post. The 6F5 1st audio amp has a grid cap. Make sure that this wire is either shielded or very close to the shield. If this a metal tube, make sure that there is continuity between the metal case of the tube and chassis ground.

Try unplugging the 6H6 detector tube then powering back up and turning up the volume control. You may hear some hum but should not hear motorboating. Pulling the detector tube divides the radio into 2 sections: the amp and power supply, and the RF / IF section. This is an example of "divide and conquer" troubleshooting. If the motorboating continues with the detector unplugged, then troubleshoot the amp and power supply. If the motorboating stopped, power down, reinstall the detector, pull the IF tube and retest. If no motorboating, reinstall the IF tube and pull the mixer and oscillator tubes. Continue in this manner until the offending stage is discovered and troubleshoot that stage.

RE: Field coil question/general - bridkarl - 07-25-2025

Thanks. I’ll try this procedure. I replaced some out of spec parts but haven’t tested it yet.

RE: Field coil question/general - MrFixr55 - 08-14-2025

Hi Bridkarl,

How did you make out with the motorboating problem with your Hallicrafters?

RE: Field coil question/general - bridkarl - 08-14-2025

I just set the whole thing aside for a while. I’ll take a fresh look at it later.

RE: Field coil question/general - MrFixr55 - 08-14-2025

Can'tsay I blame you. Clear your had with a different project, then pickup where you left off. Don'[t forget the "Divide and Conquer troubleshooting.

RE: Field coil question/general - bridkarl - 08-14-2025

Yea. I’m digging into a 41-226 right now. Mostly recapped. Having to figure out bandswitch wiring. See other posting