04-05-2014, 10:58 PM
Rich,
Connecting the plate of the 42 to the wiper of the volume control by all rights should set up a feedback loop, and it's encouraging that you hear that in your speaker. This suggest that you have at least some portion of your amplifier working. However, if that works, you really ought to be able to inject some audio and hear it through the speaker.
How are you injecting audio? Since the chassis of the Philco 60 is grounded, I would attach a ground lead to the chassis, and attach the negative lead from the audio output of your signal generator to the chassis as well. Hook the positive lead from your audio output to one side of a 0.01 uF or 0.015 uF capacitor, and a second test lead to the other side of the cap. Now you can probe around your chassis with some audio frequency to see what happens.
If I'm reading the schematic correctly, all the volume control does is to attenuate the audio signal before it is fed into the triode section of the 75 tube (1st audio amp). That means that the volume control should have no effect on the audio you feed in from your signal generator anywhere from the control grid of the 75 tube onward.
I can't claim to be the most experienced radio guy, and I hope I'm not leading you astray. Some of the more experienced forum members may have some better suggestions. I'm still probably a week or two away from being able to play around with my set at the stage where you are. Right now I'm just looking at the schematic and thinking through how I might try to diagnose the problem. I may be more helpful in a week or two when I have worked through these issues on my own set.
As long as you don't give up, you will eventually get it working.
Roger
Connecting the plate of the 42 to the wiper of the volume control by all rights should set up a feedback loop, and it's encouraging that you hear that in your speaker. This suggest that you have at least some portion of your amplifier working. However, if that works, you really ought to be able to inject some audio and hear it through the speaker.
How are you injecting audio? Since the chassis of the Philco 60 is grounded, I would attach a ground lead to the chassis, and attach the negative lead from the audio output of your signal generator to the chassis as well. Hook the positive lead from your audio output to one side of a 0.01 uF or 0.015 uF capacitor, and a second test lead to the other side of the cap. Now you can probe around your chassis with some audio frequency to see what happens.
If I'm reading the schematic correctly, all the volume control does is to attenuate the audio signal before it is fed into the triode section of the 75 tube (1st audio amp). That means that the volume control should have no effect on the audio you feed in from your signal generator anywhere from the control grid of the 75 tube onward.
I can't claim to be the most experienced radio guy, and I hope I'm not leading you astray. Some of the more experienced forum members may have some better suggestions. I'm still probably a week or two away from being able to play around with my set at the stage where you are. Right now I'm just looking at the schematic and thinking through how I might try to diagnose the problem. I may be more helpful in a week or two when I have worked through these issues on my own set.
As long as you don't give up, you will eventually get it working.
Roger
