PR100 Ampl/Mixer Bogen, David Co., Inc.; New York NY, build
I can't find anything on this preamp. I have the one where the chassis splits in two and the tubes are mounted horizontal on the back where the I/O's are . I am trying to find a schematic or service manual. I got a nasty 60Hz hum on the outputs. Volume doesn't effect the hum but the 'Lo Filter' increases and decreases the amplitude. It uses bridge rectified DC for the filaments and a 6X4 full wave for the B+. I have moved anything with 60Hz around and it doesn't effect the hum, things like the On/Off switch wires and the bulb wires. I was thinking it was the power transformer but I only measure 6uA of leakage current. Kinda stumped with this one.
Thanks in advance,
-bird
I can't find anything on this preamp. I have the one where the chassis splits in two and the tubes are mounted horizontal on the back where the I/O's are . I am trying to find a schematic or service manual. I got a nasty 60Hz hum on the outputs. Volume doesn't effect the hum but the 'Lo Filter' increases and decreases the amplitude. It uses bridge rectified DC for the filaments and a 6X4 full wave for the B+. I have moved anything with 60Hz around and it doesn't effect the hum, things like the On/Off switch wires and the bulb wires. I was thinking it was the power transformer but I only measure 6uA of leakage current. Kinda stumped with this one.
Thanks in advance,
-bird
LOL. I looked at the ripple on the filter caps and sure enough it's 60Hz instead of 120Hz. A full wave rectifier should be 120Hz. It would help if the person that assembled the amp soldered one of the legs of the high voltage AC secondaries to pin 6 instead of pin 5. The solder looks the same as all the other connections so I think it's original and probably never worked correctly.
Easy fix.
Easy fix.
Coming back to this project to which I find myself pulling my hair out.
So originally I had over 100mV of 60Hz on the output. I checked the power supply caps and found there was 60Hz on them and the full wave rectifier was wired wrong. After fixing the wiring issue (moved the high voltage secondary lead from pin 5 to pin 6 on the 6X4) I still had a lot of 120Hz ripple on the cans and output, over 100mV. So I then removed the cans and re-stuffed them with modern electrolytics which got rid of the 120Hz ripple on the B+. BUT, I still had ~10mV of 60Hz ripple on the output. I changed some of the wiring coming from the controls to the gain stages to shielded wire which got me down to ~4mV. I noticed that they run the AC switch wire to the front which goes right next to the controls, I twisted the switch wires together (bypassing the switch) and kept them away from the controls and it dropped the hum down further, 1mV is what I can read on my scope. The switch also carries the current for the AC receptacle in the back of the preamp so when you have a power amp plugged in there is more current going through the switch and more hum. I tried running the neutral with the hot to the switch as a twisted pair and it helped a little but not enough. The switch itself is sitting right next to the Bass control and other low signal switches. The switch is also rated for 3A and looks to me unobtainium. I decided to use a relay and mount it on the back panel next to the power transformer, that way there is only about 15mA of AC current going through the switch at all times. The relay is rated for 10A and powers the receptacles on the back and the preamp transformer. It works great but I am still getting 1mV of AC hum on the output and it looks like maybe a 50kHz oscillation. When hooked up to a power amp and an efficient speaker I hear enough hum to be annoying. I want the output dead clean.
I will try and draw up a schematic because I can't find one online. I will also take some pictures so folks can take a look inside. Basically there are two 12AT7's and three of the triodes are gain stages, if I shunt the grid of one of the gains stages coming from the controls the output looks perfect on my scope. There are still lots of wires that maybe should be shielded. Or maybe I should make a metal cage around the power supply and relay area. Or maybe there is something unstable with this thing. The guy I got it from told me it had issues and that it had hum and an oscillation he couldn't figure out. I have got the thing much better but I want it to be quiet. I find it hard to believe that Bogen was okay with this much hum on the output but I have never run into another one of these so I have no experience if it's normal for these units. I ran into an issue a long time ago with an old guitar amp where the power transformer was putting out a lot of EMF for some reason, no matter what I did I couldn't get the thing to have no hum until I removed the power transformer and situated it far from the unit in isolation. I am starting to think I have a similar issue here. I ended up replacing the power transformer on that guitar amp it fixed the issue.
In what direction would any of you go from here? Move on to isolating the transformer? Keep removing wiring on the gain stages and make them shielded? I think if shielding was an issue to begin with Bogen would have done it originally though so I am leaning to the power transformer. Maybe the amp running half the secondary winding for so long damaged it? The DC readings look the same for both halves and they both put out the same amount of AC. I just want to enjoy it without hum :|
Any and all advice is welcome, I always get great ideas from you guys that I wouldn't have thought of
So originally I had over 100mV of 60Hz on the output. I checked the power supply caps and found there was 60Hz on them and the full wave rectifier was wired wrong. After fixing the wiring issue (moved the high voltage secondary lead from pin 5 to pin 6 on the 6X4) I still had a lot of 120Hz ripple on the cans and output, over 100mV. So I then removed the cans and re-stuffed them with modern electrolytics which got rid of the 120Hz ripple on the B+. BUT, I still had ~10mV of 60Hz ripple on the output. I changed some of the wiring coming from the controls to the gain stages to shielded wire which got me down to ~4mV. I noticed that they run the AC switch wire to the front which goes right next to the controls, I twisted the switch wires together (bypassing the switch) and kept them away from the controls and it dropped the hum down further, 1mV is what I can read on my scope. The switch also carries the current for the AC receptacle in the back of the preamp so when you have a power amp plugged in there is more current going through the switch and more hum. I tried running the neutral with the hot to the switch as a twisted pair and it helped a little but not enough. The switch itself is sitting right next to the Bass control and other low signal switches. The switch is also rated for 3A and looks to me unobtainium. I decided to use a relay and mount it on the back panel next to the power transformer, that way there is only about 15mA of AC current going through the switch at all times. The relay is rated for 10A and powers the receptacles on the back and the preamp transformer. It works great but I am still getting 1mV of AC hum on the output and it looks like maybe a 50kHz oscillation. When hooked up to a power amp and an efficient speaker I hear enough hum to be annoying. I want the output dead clean.
I will try and draw up a schematic because I can't find one online. I will also take some pictures so folks can take a look inside. Basically there are two 12AT7's and three of the triodes are gain stages, if I shunt the grid of one of the gains stages coming from the controls the output looks perfect on my scope. There are still lots of wires that maybe should be shielded. Or maybe I should make a metal cage around the power supply and relay area. Or maybe there is something unstable with this thing. The guy I got it from told me it had issues and that it had hum and an oscillation he couldn't figure out. I have got the thing much better but I want it to be quiet. I find it hard to believe that Bogen was okay with this much hum on the output but I have never run into another one of these so I have no experience if it's normal for these units. I ran into an issue a long time ago with an old guitar amp where the power transformer was putting out a lot of EMF for some reason, no matter what I did I couldn't get the thing to have no hum until I removed the power transformer and situated it far from the unit in isolation. I am starting to think I have a similar issue here. I ended up replacing the power transformer on that guitar amp it fixed the issue.
In what direction would any of you go from here? Move on to isolating the transformer? Keep removing wiring on the gain stages and make them shielded? I think if shielding was an issue to begin with Bogen would have done it originally though so I am leaning to the power transformer. Maybe the amp running half the secondary winding for so long damaged it? The DC readings look the same for both halves and they both put out the same amount of AC. I just want to enjoy it without hum :|
Any and all advice is welcome, I always get great ideas from you guys that I wouldn't have thought of
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