I recently added a line input to a Philco 118 console, then I felt that the MP3s and CDs sound just like AM radio - no base or treble. So I did a quick check on the frequency response between the line-in and the speaker output with a signal generator and a scope. At the lower end, the output level starts drop at around 400Hz, and down to -19 dB at 100Hz. The upper end is better but with a single 10" speaker, I don't expect too much. I have replaced AF coupling capacitors to be at least 2X the original values. So what else could be robbing the bass? Transformer? Speaker coil?

:I recently added a line input to a Philco 118 console, then I felt that the MP3s and CDs sound just like AM radio - no base or treble. So I did a quick check on the frequency response between the line-in and the speaker output with a signal generator and a scope. At the lower end, the output level starts drop at around 400Hz, and down to -19 dB at 100Hz. The upper end is better but with a single 10" speaker, I don't expect too much. I have replaced AF coupling capacitors to be at least 2X the original values. So what else could be robbing the bass? Transformer? Speaker coil?
:
Lewis

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:Is the speaker in an enclosure? A speakeer in free air will have just about zero bass.
:Lewis
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http://picasaweb.google.com/lh/photo/uaQSMMKO63FcPrP-9TPXjg?feat=directlink

Significantly increasing the resistance of 47, the plate resistor of the 1st AF tube, will significantly increase bass. I did this in my Philco 60. Try something between 500K and 1Meg. The junction between the original .1Meg resistor and your additional resistor to the 1st AF plate is also a nice place to inject negative feedback, should you want this. For proper phase relation, take it off of the '42 driver plate, and send through a .047 MFD cap and suitable series resistance to adjust to your taste.

Though you can also increase the resistance of the '42 driver tube's grid to increase bass response, there is a chance of the grid being driven positive, with subsequent grid current being drawn, which can be a bad situation. Still, I have seen resistances as high as 1 meg on these tubes with favorable audio results. It's better to increase the plate resistance of the 1st AF tube.

Increasing the grid and plate resistances will also tend to decrease the upper register, especially when the plate resistance of the 1st AF tube is increased, but you probably won't notice this much. This is due to the time constant being a bit too sloppy for high frequency response. You might also notice a slight increase in the upper middle register, though, again, probably not. Negative feedback, if you use it, will take care of this.

T.

Sorry...didn't mean to skip everyone else's replies. Somehow mine tacked onto the top post instead of the bottom one. ...Not trying to be rude, though.

T.

Thanks Thomas. I'll try a larger plate resistor. Next time, I'll also measure the frequency response with a 4 ohm resistor in place of the speaker.
G.

It probably won't be that big of a deal...just something you observe slightly in the speaker--regarding treble response. The bass response will change notably, though.

T.