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I am on the finishing end of a repair/restoration of a '64 Ampeg B-12-XY. It was, for the most part, a very trashy mess. I discovered what may have been the origins of the real mess. I replaced all power supply decoupling caps, as a first step. This had been already done in some manner, but with nothing matching the original schematic. That was fixed up front. All electrolytics were replaced. C36, at the bias diode, was completely missing. Major hum fix there.
The oscillator for the 'Vibrato' caps were replaced along with a tube swap, bringing it back to life. The 'Echo' was not what I thought it should have been, but have since come to realize that the large speaker is overwhelming the two little speakers in the satellite cab. After replacing various components in that circuit, moving the speaker cab away from the main cab was the biggest fix. Then I could hear it.
Let me state one handicap up front. And, this may very well explain how it got so messy. My amp was built different from the typical schematic found. I hand-traced everything and found the triode assignments for V7 and V8 did not match the schematic. I found a couple other people who had the same. So, I used the original schematic as a reference, and hand-traced my amp, until I had a correct schematic. I also found that my Echo footswitch was on the cathode resistor of one of the triodes, where the drawing showed it grounding the output of the spring tank. Hand-tracing done, I CAD'ed my own drawing. It is very much the same as the original, but reflects my tube assignments and component values. A lot of caps were of different values from the original schematic. I now have a schematic that I can read. The original (from the Internet) is a very poor copy.
I moved my Echo footswitch from grounding/ungrounding the cathode resistor to that of grounding/ungrounding the spring tank output. The way mine was originally wired made a very loud pop when making or breaking the switch. On the signal path from the spring tank, it is very quiet, just like the original schematic.
I'm very satisfied with the way the amp turned out, but I'm now trying to get rid of a loud hiss on Channel 2. When the volume is above half, it starts to become obvious. On Channel 1, I have a fairly obvious hum above half volume and some hiss as well. There was quite a bit of lower frequency hum, until I moved the Hum Control pot leads from V1 to V7. I was then able to null most of it out. If I can rid it of the hiss and higher frequency hum (which is mainly on Channel 1), I may just be satisfied with it, as-is. Meaning, if there is an subtle hum at full volume, I won't worry about it. I won't be using it at full volume. Half volume at the most. But, I would like it clean all the way up, both channels.
I've done quite a bit of troubleshooting by grounding grids. So, here's where I am. Using a couple alligator jumpers, one end of each connected to signal ground, I have walked my way back through each channel, from the common tie point at the grid of V3-pin7.
Channel 1 troubleshooting, Volume at full, no input connections:
1 - Ground jumper on C2/R3c, total silence. No hum or hiss of any kind. Lifted ground
2 - Ground moved to the junction of C1/C6, the amp is very quiet with a very subtle hum. I can live with this one. I don't believe anyone would use an amp for recording at full volume. Lifted ground.
3 - Ground V7-pin9, no real change in hum other than my jumper is acting like an antenna, because I get a tremendous hum. Lifted ground.
4 - I made a grounding plug and put it in the open circuit J1. No change in original hum. Removed the plug.
I did some tube swapping. I only have the two 6D10's that came with the amp. One of them will not work in the oscillator position. It checks just as high on my B&K 707, as the other tube. I can swap the two and the Vibrato (Tremolo) works. I have quite a few 6AC10's, and in the Sylvania spec sheet, it is a higher gain tube than the 6D10. So, I put the 'good' 6D10 in V7 and a Sylvania 6AC10 in V8. I like it better, as the channel brighter than before. Not that that's what I'm looking for. But, Channel 2 has octals on the front end, and is not as bright as Channel 1. I like having one of each. I put another 6AC10 in V7. I've convinced myself the 'Vibrato' is better. So for now, the 6AC10's are in the amp.
I haven't pursued the hiss on Channel 2, yet. I will fix Channel 1 before moving on. Unless someone has something to try on it.
Anyway, I'm at sort of a stand still, for now. I don't know how much troubleshooting I'll get done over the next couple of days. But, I'll be back on it again, for sure Wednesday afternoon.
Please take a look and shed some light on something that I may be overlooking. I've included a pdf schematic of my amp. Thanks for all help.
Jack
The oscillator for the 'Vibrato' caps were replaced along with a tube swap, bringing it back to life. The 'Echo' was not what I thought it should have been, but have since come to realize that the large speaker is overwhelming the two little speakers in the satellite cab. After replacing various components in that circuit, moving the speaker cab away from the main cab was the biggest fix. Then I could hear it.
Let me state one handicap up front. And, this may very well explain how it got so messy. My amp was built different from the typical schematic found. I hand-traced everything and found the triode assignments for V7 and V8 did not match the schematic. I found a couple other people who had the same. So, I used the original schematic as a reference, and hand-traced my amp, until I had a correct schematic. I also found that my Echo footswitch was on the cathode resistor of one of the triodes, where the drawing showed it grounding the output of the spring tank. Hand-tracing done, I CAD'ed my own drawing. It is very much the same as the original, but reflects my tube assignments and component values. A lot of caps were of different values from the original schematic. I now have a schematic that I can read. The original (from the Internet) is a very poor copy.
I moved my Echo footswitch from grounding/ungrounding the cathode resistor to that of grounding/ungrounding the spring tank output. The way mine was originally wired made a very loud pop when making or breaking the switch. On the signal path from the spring tank, it is very quiet, just like the original schematic.
I'm very satisfied with the way the amp turned out, but I'm now trying to get rid of a loud hiss on Channel 2. When the volume is above half, it starts to become obvious. On Channel 1, I have a fairly obvious hum above half volume and some hiss as well. There was quite a bit of lower frequency hum, until I moved the Hum Control pot leads from V1 to V7. I was then able to null most of it out. If I can rid it of the hiss and higher frequency hum (which is mainly on Channel 1), I may just be satisfied with it, as-is. Meaning, if there is an subtle hum at full volume, I won't worry about it. I won't be using it at full volume. Half volume at the most. But, I would like it clean all the way up, both channels.
I've done quite a bit of troubleshooting by grounding grids. So, here's where I am. Using a couple alligator jumpers, one end of each connected to signal ground, I have walked my way back through each channel, from the common tie point at the grid of V3-pin7.
Channel 1 troubleshooting, Volume at full, no input connections:
1 - Ground jumper on C2/R3c, total silence. No hum or hiss of any kind. Lifted ground
2 - Ground moved to the junction of C1/C6, the amp is very quiet with a very subtle hum. I can live with this one. I don't believe anyone would use an amp for recording at full volume. Lifted ground.
3 - Ground V7-pin9, no real change in hum other than my jumper is acting like an antenna, because I get a tremendous hum. Lifted ground.
4 - I made a grounding plug and put it in the open circuit J1. No change in original hum. Removed the plug.
I did some tube swapping. I only have the two 6D10's that came with the amp. One of them will not work in the oscillator position. It checks just as high on my B&K 707, as the other tube. I can swap the two and the Vibrato (Tremolo) works. I have quite a few 6AC10's, and in the Sylvania spec sheet, it is a higher gain tube than the 6D10. So, I put the 'good' 6D10 in V7 and a Sylvania 6AC10 in V8. I like it better, as the channel brighter than before. Not that that's what I'm looking for. But, Channel 2 has octals on the front end, and is not as bright as Channel 1. I like having one of each. I put another 6AC10 in V7. I've convinced myself the 'Vibrato' is better. So for now, the 6AC10's are in the amp.
I haven't pursued the hiss on Channel 2, yet. I will fix Channel 1 before moving on. Unless someone has something to try on it.
Anyway, I'm at sort of a stand still, for now. I don't know how much troubleshooting I'll get done over the next couple of days. But, I'll be back on it again, for sure Wednesday afternoon.
Please take a look and shed some light on something that I may be overlooking. I've included a pdf schematic of my amp. Thanks for all help.
Jack
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