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Philco 635 Hum - Printable Version

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RE: Philco 635 Hum - morzh - 03-01-2026

OK, how about you try to change the 6A7 tube? I never saw this effect but 6A8 are known for going very weak on their triode section (I even had a thread here many years ago). Not sure this could affect anything. But worth checking.

RE: Philco 635 Hum - RodB - 03-01-2026

Yes Mike, that's the spot. It just looks like a connection point on the schematic I was viewing and wanted to point it out.

RE: Philco 635 Hum - morzh - 03-01-2026

OK. Now we know this conenction is not present in Dconant's radio.
So gotta be something else.

RE: Philco 635 Hum - morzh - 03-01-2026

OK, there is something in your wording that I do not fully understand.

So,

>>Volts on strong station tuned in is 4 volts when move off station I see the voltage slowly climb to 9 volts.
Current is .002 on station to .004 off station. Takes same amount of time for current to increase as volts increase.

Are you measuring V/I across / through the shadograph or something else? The shadograph acts as regular resistor, so the Ohms law applies. If the V increases, so does I, and vice versa.

>>>Going from off station to on station the results of both readings is immediate.

Does that mean that slow change is only when you de-tune from the station?



ALso, "off station" means "not tuned to the station" and "on station" means "tuned"?

RE: Philco 635 Hum - dconant - 03-02-2026

Mike just to clarify what I was saying on my readings. First voltage readings were across the shadowgraph wires. The stronger the station is tuned in the less voltage there is same for current. As I move the dial off the station the higher the voltage goes. Same for current. Current is read reming one wire from the shadowgraph and connecting the amp meter between those points.

To answer your questions.
>>>Does that mean that slow change is only when you de-tune from the station? Yes that is what I was saying.
>>>ALso, "off station" means "not tuned to the station" and "on station" means "tuned"? Yes this is what I was saying.

As for changing the 6A7 tube I had tried another one previously with no change. So I robbed one from a Philco 60 and that has made the difference. Seems to be responding in a timely manner now. Strange thing is the other tubes test about the same but slightly weaker but still reading good on my tester. The one using now is slightly better. So I guess we can say the problem is solved.

RE: Philco 635 Hum - RodB - 03-02-2026

Mike, Dan - The thought occurred to me that being the meter is in the plate circuit of the 6A7 maybe if the first IF transformer primary were tuned as sharp as possible, the meter would be more responsive.

RE: Philco 635 Hum - morzh - 03-02-2026

Rod,

Very well could be.




Dan,

Has the radio been aligned yet?

RE: Philco 635 Hum - dconant - 03-02-2026

Mike and Rod, I did another alignment and found to be off since I had last done it before 6A7 tube change. Really did not make any difference in the shadowgraph. This radio seems to be pretty picky with what tubes are in it. Anyway I am please with the shadowgraph response now. The swing is 1/4" to 7/8" now with response being immediate.

The next phase of this radio is lots of cabinet work. I need to pretty much rebuild the face. That will be a warmer weather project.

Thank you both for sticking with me.
Dan

RE: Philco 635 Hum - morzh - 03-02-2026

Rod, Dan


Actually the shadograph is driven by the plate currents of both 6A7 and the RF stage 78.
But as long as you are pleased with it, it should not matter a whole lot.