05-26-2016, 09:18 PM
Well I finally got this thing put back together and everything seems to be "mostly" working. No explosions, no blown fuses or tripped breakers and and it didn't make any smoke...pretty much a miracle after rewiring it from the ground up (is that a pun??). Plugged in a guitar today for the first time and with the exception of some hum it sounded great, true Ampeg mojo. Almost brought a tear to me one good eye.
The excessive hum issue remains. All tubes and filter caps have been replaced. I have looked over the grounding several times, tried moving things around and can't seem to locate the cause. I tried star grounding, running everything to one single point common with the chassis and this did not help. I tried floating the shield on one end of the inputs and the pre/pwr amp cables from the pcb's to the jacks (with the star grounding intact) to avoid possible ground loops with no success.
Here is what I have determined:
1. The problem seems to be in the preamp section. I can plug a shorted cable into the power amp input and all goes quiet. I can plug a guitar with a stomp box directly into this jack and the hum is minimal.
2. Assuming that all tone controls are neutral at the noon position, the bass control acts normally. I can cut or boost the bass and there is no appreciable difference in the hum.
3. The midrange control is a different story. I get considerable hum when I use any boost, maybe starting at 2:00. It gets completely overwhelming fairly quickly. The frequency select appears to be working as the noise around the fundamental hum frequency seems to follow the selected frequency.
4. The treble is similar to the midrange, I get significant additional noise as I start to boost the treble control.
5. The reverb is dead, as I turn it up I get increasing noise until it starts to have a high frequency oscillation.
6. The hum balance seems strange to me. I would expect the sweet spot to be somewhere close to the middle of the sweep. Not so, it's quietest at one end of the rotation and picks up some treble noise as I rotate it the other direction.
7. All heater wiring is tightly twisted and routed away from all other wiring as much as possible.
8. I moved the 47k power tube control grid resistors to the tube sockets as previously suggested.
Question: The midrange inductors are all original. Could these possibly be part of the problem? I have no clue if the opto-isolators in channel one are good. If these were defective, what would be the result and how would I test them?
That's about all of it. Apologies if I wrote a book here, just trying to give some detailed info for others to digest. Any suggestions from the forum would be welcomed and much appreciated.
The excessive hum issue remains. All tubes and filter caps have been replaced. I have looked over the grounding several times, tried moving things around and can't seem to locate the cause. I tried star grounding, running everything to one single point common with the chassis and this did not help. I tried floating the shield on one end of the inputs and the pre/pwr amp cables from the pcb's to the jacks (with the star grounding intact) to avoid possible ground loops with no success.
Here is what I have determined:
1. The problem seems to be in the preamp section. I can plug a shorted cable into the power amp input and all goes quiet. I can plug a guitar with a stomp box directly into this jack and the hum is minimal.
2. Assuming that all tone controls are neutral at the noon position, the bass control acts normally. I can cut or boost the bass and there is no appreciable difference in the hum.
3. The midrange control is a different story. I get considerable hum when I use any boost, maybe starting at 2:00. It gets completely overwhelming fairly quickly. The frequency select appears to be working as the noise around the fundamental hum frequency seems to follow the selected frequency.
4. The treble is similar to the midrange, I get significant additional noise as I start to boost the treble control.
5. The reverb is dead, as I turn it up I get increasing noise until it starts to have a high frequency oscillation.
6. The hum balance seems strange to me. I would expect the sweet spot to be somewhere close to the middle of the sweep. Not so, it's quietest at one end of the rotation and picks up some treble noise as I rotate it the other direction.
7. All heater wiring is tightly twisted and routed away from all other wiring as much as possible.
8. I moved the 47k power tube control grid resistors to the tube sockets as previously suggested.
Question: The midrange inductors are all original. Could these possibly be part of the problem? I have no clue if the opto-isolators in channel one are good. If these were defective, what would be the result and how would I test them?
That's about all of it. Apologies if I wrote a book here, just trying to give some detailed info for others to digest. Any suggestions from the forum would be welcomed and much appreciated.