Hi guys, I just finished building a simple 12AU7 based guitar preamp which I designed to give some good tube boost to a Peavey Transtube Studio Pro 112.
I'm very pleased with the results as I'm getting liquid like sustain and hairy metal distortion as I wanted from this solid state amp. It has very low noise even when I use the high gain input and at very high distortion settings, however I noticed that when I touch the output cable inside the amp (it is a shielded cable which is connected to a jack) the amp hums a lot (only when touched or if my hand gets close to the cable), also if I change the position of this cable (talking about the place where the cable is resting inside the aluminium chassis) it hums more or less depending on where I put the cable. The power transformer is far from the preamp and there are no motors, lamps or computer monitors near the amp nor the preamp.
The input and output jacks are grounded via the front panel (chassis), also near them I connected the star ground to chassis.
Any advice will be appreciated, thanks in advance!
I'm very pleased with the results as I'm getting liquid like sustain and hairy metal distortion as I wanted from this solid state amp. It has very low noise even when I use the high gain input and at very high distortion settings, however I noticed that when I touch the output cable inside the amp (it is a shielded cable which is connected to a jack) the amp hums a lot (only when touched or if my hand gets close to the cable), also if I change the position of this cable (talking about the place where the cable is resting inside the aluminium chassis) it hums more or less depending on where I put the cable. The power transformer is far from the preamp and there are no motors, lamps or computer monitors near the amp nor the preamp.
The input and output jacks are grounded via the front panel (chassis), also near them I connected the star ground to chassis.
Any advice will be appreciated, thanks in advance!
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I've tried removing the volume pot, and the same thing happens, so you suggest to try a better quality cable? This problem doesn't happen with the input cable which is the same type of cable (shielded instrument cable). Also if I touch the cable that goes from the preamp to the amp nothing happens (it behaves normally).
I made a mistake in the first description, the output cable is inside the preamp I built, not inside the Peavey amp.
Touching the output cable inside the preamp, moving the cable or just putting my hand near the output cable changes the hum level which is low (in the clean channel hum is not heard at all) but in certain areas inside the preamp it gets really loud, which made me think that I can get the preamp noise floor even lower while I'm using the distortion channel of the amp. As I mentioned the input and output jacks are grounded using the chassis' front panel (where the star ground is located) the other side of the cable is not grounded (as in some vintage Fender amps).
Touching the output cable inside the preamp, moving the cable or just putting my hand near the output cable changes the hum level which is low (in the clean channel hum is not heard at all) but in certain areas inside the preamp it gets really loud, which made me think that I can get the preamp noise floor even lower while I'm using the distortion channel of the amp. As I mentioned the input and output jacks are grounded using the chassis' front panel (where the star ground is located) the other side of the cable is not grounded (as in some vintage Fender amps).
Do you have an oscilloscope? I suspect your little circuit might just be oscillating (probably in the VHF region) depending on the internal layout. The audible results and behavior I have observed in such instances closely match what you have described.
I would take a look with a scope first and see what is going on, if that's not possible I would suggest making sure that the 47K resistor R26 in the grid circuit is mounted right by the socket, and I would add a small resistor in series with the output to provide some isolation between the cable capacitance and the output of your circuit. This could best be placed in series with C25, and a 220 ohm - 1K resistor would be the place to start.
Make sure that your filament wiring is tightly twisted and not looping the long way around the socket.
I would take a look with a scope first and see what is going on, if that's not possible I would suggest making sure that the 47K resistor R26 in the grid circuit is mounted right by the socket, and I would add a small resistor in series with the output to provide some isolation between the cable capacitance and the output of your circuit. This could best be placed in series with C25, and a 220 ohm - 1K resistor would be the place to start.
Make sure that your filament wiring is tightly twisted and not looping the long way around the socket.
The same happens with my amps.
I have an Aikido linestage amp. It is dead quiet and has good grounding but when I touch or place my finger near the sensitive wires from the output of the potentiometer it hums a lot.
I also have a Carver SS power amp and it does exactly the same so I'm not worrying about it.
If everything works fine and it is quiet when you are not touching it I wouldn't worry about it.
As the other members mention, just check all your grounding first.
Good luck.
I have an Aikido linestage amp. It is dead quiet and has good grounding but when I touch or place my finger near the sensitive wires from the output of the potentiometer it hums a lot.
I also have a Carver SS power amp and it does exactly the same so I'm not worrying about it.
If everything works fine and it is quiet when you are not touching it I wouldn't worry about it.
As the other members mention, just check all your grounding first.
Good luck.
I have two Fender amps - one a very nice Blues Jr. which both hum when I touch the cable without a guitar plugged into it.
Sometimes I also get a hum when the cable is coiled up on the floor with the guitar plugged in... as soon as I put my fingers on the strings - it all goes away.
It rarely bothers me - I just chalked it up to the amp amplifying exactly what it is getting - 60hz from the ambient.
Once the big human sized cap is introduced, all is better. I just wish the human cap made better noise
Sometimes I also get a hum when the cable is coiled up on the floor with the guitar plugged in... as soon as I put my fingers on the strings - it all goes away.
It rarely bothers me - I just chalked it up to the amp amplifying exactly what it is getting - 60hz from the ambient.
Once the big human sized cap is introduced, all is better. I just wish the human cap made better noise
I've tried changing jacks to plastic ones with no success, I've also tried DC heaters and I get almost the same amount of hum (as I mentioned before hum is heard only in the distortion channel not in the clean channel), I've tried different amps as well and nothing. However after some tweaking I noticed that:
If I turn the guitar volume down the hum level remains constant, the weird thing is that hum is heard as it is (sounds like pure 60 Hz hum), it doesn't seem to be affected by the distortion effect.
It is not motorboating, it sounds like 60 cycle hum.
Another weird thing I noticed is that if I touch the cable that goes from anode to the decoupling cap (C25) the hum level remains constant, but if I touch the cable after the decoupling cap (C25) the hum level is increased, and there are some portions of the cable where the hum level remains constant and some really sensitive ones (near the decoupling cap).
I've tried using un-shielded cable and the hum level remains constant.
I know that some hum is normal, but I've heard other tube based preamps with more than one gain stage that are really quiet, so I'm still not satisfied with mine. Any advice will be appreciated guys, thanks in advance.
If I turn the guitar volume down the hum level remains constant, the weird thing is that hum is heard as it is (sounds like pure 60 Hz hum), it doesn't seem to be affected by the distortion effect.
It is not motorboating, it sounds like 60 cycle hum.
Another weird thing I noticed is that if I touch the cable that goes from anode to the decoupling cap (C25) the hum level remains constant, but if I touch the cable after the decoupling cap (C25) the hum level is increased, and there are some portions of the cable where the hum level remains constant and some really sensitive ones (near the decoupling cap).
I've tried using un-shielded cable and the hum level remains constant.
I know that some hum is normal, but I've heard other tube based preamps with more than one gain stage that are really quiet, so I'm still not satisfied with mine. Any advice will be appreciated guys, thanks in advance.
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