The PHILCO Phorum
Does size matter? - Printable Version

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Does size matter? - fenbach - 12-10-2023

On the Rider schematic for a 37-630 [PHILCO, pg 7-57], the capacitor/resistor pair on the cathode of the 6K7G I.F. amplifier tube is 0.05uf and 400 ohms. For a 37-640, it's 0.25uf and 700 ohms. But my 37-630 itself has a 0.25uf cap and a 400-ohm resistor, both apparently original. Should I go with the schematic for the 630 or use the values in the radio? Or does it matter?
Thanks

37-640 schematic. http://www.nostalgiaair.org/PagesByModel/216/M0013216.pdf

RE: Does size matter? - Radioroslyn - 12-10-2023

Well it does. If you change the value to 700 ohms then it lowers the gain of the IF amp. I would stick to what values are in the set from the factory.

RE: Does size matter? - fenbach - 12-10-2023

What happens if I change the size of the cap from 0.25uf, which is in the radio now, to 0.05uf matching the schematic?? Mine is run 126, if that matters. Incidentally, I just discovered a Philco Service Bulletin, No.251A, that has 0.25uf and 700-ohms, both different from Riders. With different part numbers as well. The Philco bulletin also has the electrolytics [see previous thread] oriented correctly [negative to ground].

RE: Does size matter? - radiorich - 12-10-2023

Hello fenbach ,
have you looked for the original Philco Schematic I have found mistakes with riders .

Sincerely Richard

RE: Does size matter? - DaleHCook - 12-11-2023

radiorich Wrote:have you looked for the original Philco Schematic I have found mistakes with riders

Parts values sometimes changed during production runs, and those changes are often given in the Philco Service Bulletins, so you should also check those for your model. You can find all original Philco service information which this site has available for any model by using the Philco Service Index.

RE: Does size matter? - RodB - 12-11-2023

The resistor sets the bias for the tube, the capacitor is a bypass for the rf. As Terry said, the resistor value is critical to keeping the tube gain optimal. The capacitor value is more lenient. I've seen it low as 0.01mfd but I would go with what was in the radio.

RE: Does size matter? - Arran - 12-11-2023

The value of the cathode resistor does matter, the cap in parallel with it is not as critical, it's basically a bypass cap, it's to prevent degeneration in the amplifier circuit reducing the gain. A higher value cap will bypass lower frequencies, a lower value will bypass higher frequencies, if I remember correctly, but the difference between a .01, a .05, and a .25 is likely marginal in an IF amp. The purpose of the resistor is to raise the potential of the tube's cathode above ground, or more positive, to make the grid more negative since it is closer to ground potential.
With Philco there were not only different model numbers, but code numbers, and production runs as well, each code number denotes circuit variants, as well as the production run, though the latter is harder to find in most literature. I don't think there is any explanation for the difference in value of the cathode bypass caps other then Philco using what was available. I would try using a .05 uf cap if that's what you have on hand, voltage isn't that critical, you could use a 50 volt unit and it would be fine, but like most of us you probably have a bag of 400 volt or 630 volt caps.
Regards
Arran

RE: Does size matter? - DaleHCook - 12-11-2023

Arran Wrote:but like most of us you probably have a bag of 400 volt or 630 volt caps.

Some of us have more than a bag. In addition to antique radios I also work on vacuum tube test equipment, instrumentation, and broadcast electronics, so I stock significant quantities of 630 volt film caps in 20 values. Part of that stock travels with me for field repairs.

RE: Does size matter? - fenbach - 12-11-2023

Thanks Arran. For the background. And everyone for helping. I do have the Philco Service Bulletin, No.251A which I no doubt downloaded once upon a time from the Philco Library.
Go Ducks. That one's for you Dale. Icon_biggrin

RE: Does size matter? - morzh - 12-11-2023

>>Does size matter?

Not if you use Candohm.

RE: Does size matter? - RodB - 12-12-2023

Haha Mike, what service bulletin was that in?