03-31-2018, 09:32 AM
Hi, Arran:
Thanks for taking the time to think about my problem. Someone here did send me a schematic for a Philco 38-14, where they threw in a couple of resistors in the "power supply" kind of sort of in parallel with the ballast resistor, to bias the cathode. I considered it, but the way it was done looks like a major rewiring job. I fooled with this for 3 hours last night and decided I will just modify it for an LR44 cell and move on.
I read the part about replacing the ballast resistor with diodes or capacitors. Did I read that correctly? I've never heard of that. Can you elaborate a little more on that? Maybe post a sketch?
I'm having trouble with my audio in this set as well, which was one of the reasons I have been trying so hard to get this bias thing right. My audio is not clean. It is a little garbled, sounding like there is 60 cycle hum mixed in with it. I replaced all the caps and any resistor outside 10%. I've swapped out all the tubes. There is about 2v of ripple on the B- relative to chassis, and I see it on the 75 tube grid and plate. Not so much on the 43 plate. I don't know if this ever happens to you, but I have become so frustrated with this set that I am going to set it aside and cool off for a while. I've got a model 20 to start on, but first I need to repair the transmission in my brother's tractor/backhoe, which should keep my mind off of this for a couple of weeks. If I don't get inspired beforehand, I will try to post more details about the audio problem in a separate thread. I am beginning to suspect a problem with my bench setup, as I have had this problem on 2 other sets and never could get it satisfactorily resolved.
Again, Thanks!
Mike
Thanks for taking the time to think about my problem. Someone here did send me a schematic for a Philco 38-14, where they threw in a couple of resistors in the "power supply" kind of sort of in parallel with the ballast resistor, to bias the cathode. I considered it, but the way it was done looks like a major rewiring job. I fooled with this for 3 hours last night and decided I will just modify it for an LR44 cell and move on.
I read the part about replacing the ballast resistor with diodes or capacitors. Did I read that correctly? I've never heard of that. Can you elaborate a little more on that? Maybe post a sketch?
I'm having trouble with my audio in this set as well, which was one of the reasons I have been trying so hard to get this bias thing right. My audio is not clean. It is a little garbled, sounding like there is 60 cycle hum mixed in with it. I replaced all the caps and any resistor outside 10%. I've swapped out all the tubes. There is about 2v of ripple on the B- relative to chassis, and I see it on the 75 tube grid and plate. Not so much on the 43 plate. I don't know if this ever happens to you, but I have become so frustrated with this set that I am going to set it aside and cool off for a while. I've got a model 20 to start on, but first I need to repair the transmission in my brother's tractor/backhoe, which should keep my mind off of this for a couple of weeks. If I don't get inspired beforehand, I will try to post more details about the audio problem in a separate thread. I am beginning to suspect a problem with my bench setup, as I have had this problem on 2 other sets and never could get it satisfactorily resolved.
Again, Thanks!
Mike
