06-18-2015, 08:00 PM
Bad news...
I did some checking tonight. Other than a weak 6A7 and a weak 43, I could find nothing amiss.
I replaced the 6A7. I do not have a spare 43 so it stayed.
No improvement. Plate voltage at the 75 and 43 tubes is normal. Placing a finger on the 75 grid cap results in a hum that is so weak, I have to put my ear up to the speaker to (barely) hear it with the volume control at max.
So, I decided to measure between B+ and B-, where the speaker field is (or should be) connected. It should read 2600 ohms. I read...infinity.
An open field coil would equal a (virtually) dead speaker. Without an energized field, there is no magnetism to make the speaker voice coil work.
I see that I never did post a schematic or a link to one, so here's one at Nostalgia Air:
http://www.nostalgiaair.org/pagesbymodel...013796.pdf
A speaker from a 602 or 37-602 would be electrically equivalent, but I do not know if the mounting would be the same. I used to have a 37-602 speaker, but I can't find it now. I must have given it away with several other speakers a few years ago (and I still have too many speakers). I seriously doubt that I would find a speaker from a 604 or 37-604, as popular as those sets are.
I could even use a PM speaker in this application, since the speaker field in this set is only there to energize the speaker, not serving any other purpose. But the mounting would be a problem. The 604 speaker uses special brackets or "ears" if you will, attached to the field "pot", to hold the speaker on the chassis.
One more thing. I'm using a 6.8 megohm resistor in place of the nonexistent bias cell, although I do not think that is a factor at this point.
I did some checking tonight. Other than a weak 6A7 and a weak 43, I could find nothing amiss.
I replaced the 6A7. I do not have a spare 43 so it stayed.
No improvement. Plate voltage at the 75 and 43 tubes is normal. Placing a finger on the 75 grid cap results in a hum that is so weak, I have to put my ear up to the speaker to (barely) hear it with the volume control at max.
So, I decided to measure between B+ and B-, where the speaker field is (or should be) connected. It should read 2600 ohms. I read...infinity.
An open field coil would equal a (virtually) dead speaker. Without an energized field, there is no magnetism to make the speaker voice coil work.I see that I never did post a schematic or a link to one, so here's one at Nostalgia Air:
http://www.nostalgiaair.org/pagesbymodel...013796.pdf
A speaker from a 602 or 37-602 would be electrically equivalent, but I do not know if the mounting would be the same. I used to have a 37-602 speaker, but I can't find it now. I must have given it away with several other speakers a few years ago (and I still have too many speakers). I seriously doubt that I would find a speaker from a 604 or 37-604, as popular as those sets are.
I could even use a PM speaker in this application, since the speaker field in this set is only there to energize the speaker, not serving any other purpose. But the mounting would be a problem. The 604 speaker uses special brackets or "ears" if you will, attached to the field "pot", to hold the speaker on the chassis.
One more thing. I'm using a 6.8 megohm resistor in place of the nonexistent bias cell, although I do not think that is a factor at this point.
--
Ron Ramirez
Ferdinand IN
