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Hello all...
I have a Crate BV-2612 that I am tracking down a nasty hum problem. I found some old posts where Enzo (and other forum members) had some wonderful feedback/suggestions specifically around removing the hum balance pot (which if memory serves, corrected the problem i am going to be describing below). I really did not understand how hum balance for the entire amp would have an impact to just the clean channel, but in any case, I did speak with Enzo outside of this forum and he recommended I post this to rekindle the conversation. So bear with me as i try to be as complete as i can in conveying the full history i have with this amp.
First...some background history on this unit. A Customer of mine brought this amp to me a little over a year ago saying that they are getting no sound out of it. One of the two output tubes were shot and the 12AU7 tube was gone. The remaining 12AX7's were also very weak. Opening up the amp, I found that the unit had already been worked on by someone else prior to it getting to me. I'll get to that part later. I found both fuses (F2 and F3) were blown, and also found R90, D9 and D10 were fried. Research at the time indicated that these amps were somewhat known for having cold solder joints. After testing the rest of the board and performing a close inspection of the solder pads, i found that the daughter board for the output tube have a number of loose solder joints. I went through and repaired all the bad parts and re-flowed the solder pads on the daughter and main boards....basically anything that looked suspicious....which was pretty much everything
. After putting it all back together with a new set of JJ tubes, all was well with the unit. I will be honest, i recall that this amp did have some ground noise (not ground loop, but just background noise...some hiss and light hum)...but nothing i would have considered at the time out of the ordinary.
About the previous work that was done. I noticed at the time I originally repaired this item, some power supply filter caps and power resistors were replaced. Also someone had install a biasing pot....which based on my evaluation did not seem to be very effective. I believe this either had a fixed or self adjusting bias....cant recall, but if it is self adjusting, then i would not expect the pot to really make any difference....and now i am thinking about it, i will probably remove it and put it back to 'stock' configuration.
Now, let me explain what is specifically going on with it. My Customer brought this amp back to me complaining about the 'clean' channel (1) having a lot of hum while the effect channel (2) is much quieter. I wont say completely quiet, but significantly less than channel 1. This hum is reproducible without anything plugged into it (no guitar, cables, pedals, etc.) It happens with or without the reverb tank connected to the main board. This is (what i am told) a rev 1 system with the hum balance pot....which by the way is adjusted to minimize the hum...but comes no where near making it go away. The channel 1 volume does affect the overall volume of the hum....as you turn it up, it increases, but seems to max out when you get it mid way up. If you turn it all the way down, the hum is low, but does not go away and is still much louder than channel 2 if it was turned all the way down. If i pull the input tube (V1), the amp is dead quiet....no hum at all...but this is kind of expected. I have replaced V1 with a different 12AX7, no change. All tubes in the system were tested and check out. I even rotated them out with other known good tubes; again, no change. Channel 1's bass, mid, treble do not really affect the hum at all....in fact, they only seem to attenuate it...iow....turning the bass all the way down, make the hum less 'bassy' if you take my meaning.
So, with all this said, my gut is telling me ground loop. Everything seems perfectly tight in regards to wiring. Chassis screws are solid and i seem no evidence of a bad solder joint in and around the channel 1/input section (which is where i am focusing on.) I pull a couple of caps and found C39 to be out of range and the installed part does not match the schematic parts spec (which may or may not be an issue.) It is supposed to be a 22uF, 50V per the parts list, but a 35V is installed. I also measure a little over 40uF instead of 22uF...so this one is going to get replaced. I am in process of continuing to evaluate pretty much all the components in this area of the board, but while i am doing that, i wanted to get this information out here for you guys to consider and provide input. Any feedback you can provide would be most appreciated.
Thanks in advance...
Nabeel Sayegh
I have a Crate BV-2612 that I am tracking down a nasty hum problem. I found some old posts where Enzo (and other forum members) had some wonderful feedback/suggestions specifically around removing the hum balance pot (which if memory serves, corrected the problem i am going to be describing below). I really did not understand how hum balance for the entire amp would have an impact to just the clean channel, but in any case, I did speak with Enzo outside of this forum and he recommended I post this to rekindle the conversation. So bear with me as i try to be as complete as i can in conveying the full history i have with this amp.
First...some background history on this unit. A Customer of mine brought this amp to me a little over a year ago saying that they are getting no sound out of it. One of the two output tubes were shot and the 12AU7 tube was gone. The remaining 12AX7's were also very weak. Opening up the amp, I found that the unit had already been worked on by someone else prior to it getting to me. I'll get to that part later. I found both fuses (F2 and F3) were blown, and also found R90, D9 and D10 were fried. Research at the time indicated that these amps were somewhat known for having cold solder joints. After testing the rest of the board and performing a close inspection of the solder pads, i found that the daughter board for the output tube have a number of loose solder joints. I went through and repaired all the bad parts and re-flowed the solder pads on the daughter and main boards....basically anything that looked suspicious....which was pretty much everything
. After putting it all back together with a new set of JJ tubes, all was well with the unit. I will be honest, i recall that this amp did have some ground noise (not ground loop, but just background noise...some hiss and light hum)...but nothing i would have considered at the time out of the ordinary. About the previous work that was done. I noticed at the time I originally repaired this item, some power supply filter caps and power resistors were replaced. Also someone had install a biasing pot....which based on my evaluation did not seem to be very effective. I believe this either had a fixed or self adjusting bias....cant recall, but if it is self adjusting, then i would not expect the pot to really make any difference....and now i am thinking about it, i will probably remove it and put it back to 'stock' configuration.
Now, let me explain what is specifically going on with it. My Customer brought this amp back to me complaining about the 'clean' channel (1) having a lot of hum while the effect channel (2) is much quieter. I wont say completely quiet, but significantly less than channel 1. This hum is reproducible without anything plugged into it (no guitar, cables, pedals, etc.) It happens with or without the reverb tank connected to the main board. This is (what i am told) a rev 1 system with the hum balance pot....which by the way is adjusted to minimize the hum...but comes no where near making it go away. The channel 1 volume does affect the overall volume of the hum....as you turn it up, it increases, but seems to max out when you get it mid way up. If you turn it all the way down, the hum is low, but does not go away and is still much louder than channel 2 if it was turned all the way down. If i pull the input tube (V1), the amp is dead quiet....no hum at all...but this is kind of expected. I have replaced V1 with a different 12AX7, no change. All tubes in the system were tested and check out. I even rotated them out with other known good tubes; again, no change. Channel 1's bass, mid, treble do not really affect the hum at all....in fact, they only seem to attenuate it...iow....turning the bass all the way down, make the hum less 'bassy' if you take my meaning.
So, with all this said, my gut is telling me ground loop. Everything seems perfectly tight in regards to wiring. Chassis screws are solid and i seem no evidence of a bad solder joint in and around the channel 1/input section (which is where i am focusing on.) I pull a couple of caps and found C39 to be out of range and the installed part does not match the schematic parts spec (which may or may not be an issue.) It is supposed to be a 22uF, 50V per the parts list, but a 35V is installed. I also measure a little over 40uF instead of 22uF...so this one is going to get replaced. I am in process of continuing to evaluate pretty much all the components in this area of the board, but while i am doing that, i wanted to get this information out here for you guys to consider and provide input. Any feedback you can provide would be most appreciated.
Thanks in advance...
Nabeel Sayegh
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