I am pretty well along in re-purposing a tube phono amplifier for guitar use. It is a push-pull 6v6 design with a 12at7 phase splitter and a 12ax7 preamplifier tube. I gutted the phono stage and I'm currently using one half of the 12ax7 for the input section (using both sections provided unnecessary gain and noise). But for the lack of a series grid resistor, it is configured similarly to a standard Fender input stage.
Everything works as expected, but for considerable power supply hum that varies with the guitar's volume knob. The hum is fine at maximum volume or minimum volume, but it gets obnoxiously loud as you approach the 50% volume setting.
My wiring is pretty much a rat's nest at this stage. Is this the sort of thing that is likely to improve with better layout and shielding? Any thoughts would be appreciated.
-Nick
Everything works as expected, but for considerable power supply hum that varies with the guitar's volume knob. The hum is fine at maximum volume or minimum volume, but it gets obnoxiously loud as you approach the 50% volume setting.
My wiring is pretty much a rat's nest at this stage. Is this the sort of thing that is likely to improve with better layout and shielding? Any thoughts would be appreciated.
-Nick
An externally hosted image should be here but it was not working when we last tested it.
around like this
But you said the hum varies with the GUITAR's volume control? SO if you turn the guitar control to zero, you get zero hum?
If so, that tells me the guitar is the source of the hum. If turning the guitar to zero kills the hum, then the input jack and cord and everything back to the volume control seems to be working. Iif the input jack on the amp had a bad ground, it would hum regardless of guitar control setting.
With the guitar up and hum happening, step back away from the amp as far as possible. ANy effect on hum? Also, turn side to side each way. Does "aiming" the guitar different directions make a difference in the hum? Got flourescent lights? Turn them off, any difference.
If so, that tells me the guitar is the source of the hum. If turning the guitar to zero kills the hum, then the input jack and cord and everything back to the volume control seems to be working. Iif the input jack on the amp had a bad ground, it would hum regardless of guitar control setting.
With the guitar up and hum happening, step back away from the amp as far as possible. ANy effect on hum? Also, turn side to side each way. Does "aiming" the guitar different directions make a difference in the hum? Got flourescent lights? Turn them off, any difference.
Don't feel alone. This is what all amp builders face: "optimal brightness" sound versus volume control noise between rotation extremes. I'm no electronics expert but I have messed around in this area a lot.
What's the guitar's volume pot value? This is often worst when the volume pot resistance is much more than the pickup impedance and the amp's input impedance. You can often just throw in a 250K pot in the guitar and it may go away, but you may lose treble too as the pickup's inductance becomes more relevant to driving into the lower impedance.
It has to do with the input impedance of the amp and the value of the guitar's volume control. You can reduce the value of the 1m resistor and add a series grid resistor and install a lower-value volume pot in the guitar and the problem will usually disappear. You can add a resistor from the volume pot wiper to ground; you can even add a series resistor in the guitar. But you will lose some treble "sparkle" from your pickups (from adding resistance to ground and changing the load the pickups see) and/or some drive pushing that first tube (from adding series resistance). Pickups with a lot of turns of thin wire change sound a lot depending on the load they're pushing into.
It's a tradeoff. Stock guitars come with 250K or 500K pots, but people install 1M or even 2M pots to get the pickups brighter...and live with this volume control problem...after all you can always cut the treble later if it's too bright and some people just hate to lose any treble in the first stage. But when the hot wire to the amp is not loaded by either the pickups or the 1M to ground or the guitar's volume pot, the cable and guitar wiring and pickups become like a big antenna and it seems like the gain of the amp also goes up. Make sure the pickup coils are wired with polarity so that the outside of the coil is the ground side (or closer to ground in the second coil of a series humbucker), make sure all wiring is shielded, shield the wiring cavity. In desperation with single-coil hum, you can try the pickup coils electrically out of phase so they cancel hum and then you have to decide which way you want the magnets. But all that might reduce hum when the volume is full but not affect this off-full problem.
I'd try another 1m resistor from the volume wiper to ground in the guitar and see if it's any better. You can also add hidden screwdriver-pots for that 1M to ground and a hidden screwdriver pot for a series grid resistor, and tune your amp's input to match the individual guitar. That's my favorite fix. You have it wired like some Mesa amps; those had the same off-full volume problem.
You have only one input jack, and no network for mixing two inputs, and no series grid resistor, and a large 1m value for setting the input impedance . Your input will be really bright and not load the pickups, which probably sound great, but it will hum when the guitar volume pot has the input floating far from the ground and far from the pickup load.
Another fix is a push-push switching pot on the guitar, wired to add a resistor from wiper to ground. Some people really like push-push pots on the volume and tone to completely disconnect them from the circuit, then they put a volume pedal in the effects loop.
Good luck; the best thing you can do is experiment, and also try different guitars. This is an area where experimentation or adjustability makes a big difference, and where the guitar and amp have to work together, but individual guitars are very different from each other.
What's the guitar's volume pot value? This is often worst when the volume pot resistance is much more than the pickup impedance and the amp's input impedance. You can often just throw in a 250K pot in the guitar and it may go away, but you may lose treble too as the pickup's inductance becomes more relevant to driving into the lower impedance.
It has to do with the input impedance of the amp and the value of the guitar's volume control. You can reduce the value of the 1m resistor and add a series grid resistor and install a lower-value volume pot in the guitar and the problem will usually disappear. You can add a resistor from the volume pot wiper to ground; you can even add a series resistor in the guitar. But you will lose some treble "sparkle" from your pickups (from adding resistance to ground and changing the load the pickups see) and/or some drive pushing that first tube (from adding series resistance). Pickups with a lot of turns of thin wire change sound a lot depending on the load they're pushing into.
It's a tradeoff. Stock guitars come with 250K or 500K pots, but people install 1M or even 2M pots to get the pickups brighter...and live with this volume control problem...after all you can always cut the treble later if it's too bright and some people just hate to lose any treble in the first stage. But when the hot wire to the amp is not loaded by either the pickups or the 1M to ground or the guitar's volume pot, the cable and guitar wiring and pickups become like a big antenna and it seems like the gain of the amp also goes up. Make sure the pickup coils are wired with polarity so that the outside of the coil is the ground side (or closer to ground in the second coil of a series humbucker), make sure all wiring is shielded, shield the wiring cavity. In desperation with single-coil hum, you can try the pickup coils electrically out of phase so they cancel hum and then you have to decide which way you want the magnets. But all that might reduce hum when the volume is full but not affect this off-full problem.
I'd try another 1m resistor from the volume wiper to ground in the guitar and see if it's any better. You can also add hidden screwdriver-pots for that 1M to ground and a hidden screwdriver pot for a series grid resistor, and tune your amp's input to match the individual guitar. That's my favorite fix. You have it wired like some Mesa amps; those had the same off-full volume problem.
You have only one input jack, and no network for mixing two inputs, and no series grid resistor, and a large 1m value for setting the input impedance . Your input will be really bright and not load the pickups, which probably sound great, but it will hum when the guitar volume pot has the input floating far from the ground and far from the pickup load.
Another fix is a push-push switching pot on the guitar, wired to add a resistor from wiper to ground. Some people really like push-push pots on the volume and tone to completely disconnect them from the circuit, then they put a volume pedal in the effects loop.
Good luck; the best thing you can do is experiment, and also try different guitars. This is an area where experimentation or adjustability makes a big difference, and where the guitar and amp have to work together, but individual guitars are very different from each other.
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Yes...the symptom is at the GUITAR volume control, not the amp volume control. So you're talking about the instrument cable which is always shielded, but even more the resistors at the first grid. And thinking more about my recommendation to try a resistor from the pot wiper to ground, that's not as good as the same resistor at the amp, and you'd have to experiment with values as my recommended starting point might not do anything at all.
I'm not even sure I fully understand it; the gain seems to go way up when there's no load on the input. Adjusting the grid series resistor and parallel resistor to ground make the problem go away but often at the expense of pickup brightness, or lowering the value of the guitar's volume pot will make the problem go away but again often at the expense of pickup brightness.
First off, I'd like to thank everyone for the great discussion! There are a lot suggestions, so it may take a while for me to work through the different implementations.
To clarify, the amplifier - as it stands - has no controls of any kind. The only adjustable parameter is the volume knob of the guitar (stock Squier 60s Telecaster with a 250k potentiometer). As the amplifier is currently undergoing surgery, the guitar cable simply plugs into a free-floating input jack with 12" alligator clip leads connecting signal and ground to the rest of the circuit (there is obviously a lot of room for improvement here with regard to layout). The amp is effectively hum-free at maximum volume and minimum volume, but hums very loudly as the 50% volume position is approached.
To address a few of the suggestions:
- as is to be expected, paralleling the cathode resistor with a large capacitor increases overall gain, but doesn't otherwise affect the action of the volume control
- orienting the guitar differently doesn't substantively make any difference and there is no fluorescent lighting in the vicinity to turn off
- replacing the 1m resistor to ground with a 100k one didn't make a noticeable change of any kind, but it did make it easier to add a series capacitor between the guitar and grid to help make the low-end less muddy
I'll keep everyone posted on any progress. Thanks again for the healthy discussion and suggestions!
-Nick
To clarify, the amplifier - as it stands - has no controls of any kind. The only adjustable parameter is the volume knob of the guitar (stock Squier 60s Telecaster with a 250k potentiometer). As the amplifier is currently undergoing surgery, the guitar cable simply plugs into a free-floating input jack with 12" alligator clip leads connecting signal and ground to the rest of the circuit (there is obviously a lot of room for improvement here with regard to layout). The amp is effectively hum-free at maximum volume and minimum volume, but hums very loudly as the 50% volume position is approached.
To address a few of the suggestions:
- as is to be expected, paralleling the cathode resistor with a large capacitor increases overall gain, but doesn't otherwise affect the action of the volume control
- orienting the guitar differently doesn't substantively make any difference and there is no fluorescent lighting in the vicinity to turn off
- replacing the 1m resistor to ground with a 100k one didn't make a noticeable change of any kind, but it did make it easier to add a series capacitor between the guitar and grid to help make the low-end less muddy
I'll keep everyone posted on any progress. Thanks again for the healthy discussion and suggestions!
-Nick
You eliminated the series grid resistor. That has some advantages for tone. But it's the combination of that missing 68K grid resistor in the amp and the 1M to ground that can make problems with the guitar volume pot value and pickup impedance.
Sooo...did you try a series grid resistor with that series capacitor?
Sooo...did you try a series grid resistor with that series capacitor?
Exactly, it's a trade-off and he's at the "good tone" extreme. He has very nice bright tone and plenty of gain even potential for a bit of dirt at the first stage because he has no series resistor. He has no tone problem and the amp is not a difficult load for the pickups to drive, mostly just the capacitance of the grid and into a load like that the pickup inductance does not cut the treble and he has plenty of gain. But now the volume pot is the major load on the input, way out there at the end of the cord. When he turns the guitar's volume way down, it short the input, and there's no noise. When he turns the volume pot on the guitar way up the pickups are an impedance to ground, and there's little noise. If you watched impedance of the guitar, or even just the resistance of the guitar at its jack, it goes down at the extreme positions of the volume pot and up in the middle. That difference is amplified. If he limits the gain and lowers the input impedance at the amp, it's harder for the guitar to drive and might affect tone, but the guitar volume pot works without hum when turned just less than full.
Yes, but that puts it at the end of a long cord. You want it in the amp instead.
And...what is the value of your volume pot in the guitar? It usually says right on them.
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