This is a symptom of defective heater cathode insulation in the triode, or that the heater wiring is coupling into the grid. Try changing the tube. Check that the heater wiring does not pass close to the grid, and that the heater wiring does not forma loop around the tube socket. Hum is always a more difficult problem with unbypassed cathode resistors. If the heater supply is not balanced wrt earth, you may need to make it so.
When you say no hum at max, do you ground the input or leaving it open. I don't think the hum goes away at max if you leave the max end of the pot open.
You might have ground noise induced into the tube. The reason you have low noise at min is because it's like shorting the input to the ground through the 10K resistor.
It does not seems like grounding of the pot is the issue because it's quiet when you turn it down. You did not give enough information to go any further, you can try the filament as suggested.
You might have ground noise induced into the tube. The reason you have low noise at min is because it's like shorting the input to the ground through the 10K resistor.
It does not seems like grounding of the pot is the issue because it's quiet when you turn it down. You did not give enough information to go any further, you can try the filament as suggested.
Could be ground connection/loop?
The amplifier uses a 3 wires power cable, and earth is connected to the chassis.
The chassis, the trafo, pot chassis, are also connected to the circuit ground L+R.
But why if volume is max (inputs are connected to CD player in mute condition), hum disappear?
The amplifier uses a 3 wires power cable, and earth is connected to the chassis.
The chassis, the trafo, pot chassis, are also connected to the circuit ground L+R.
But why if volume is max (inputs are connected to CD player in mute condition), hum disappear?
I don't think it's ground loop between CD player and the preamp. If it's the problem, you will have hum at max volume. Change the tube as suggested. Take a picture of the circuit. The schematic you show has no issue, it has to be something else.
My assumption still hold as you do have the max connect to the CD player that is assumed to have low output impedance. That it is quiet at both ends because the input is connected to low impedance. In the middle, the input is at higher impedance because it is connected to the middle of the pot.
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Try disconnect the cable from the CD player. You should have loudest hum when the pot is at max. If so, you don't have ground problem between CD player and preamp.
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Isolation washers in rca input jack use for best ground loop .this is most important point in amp .
Might isolate on the chassis, it has to connect to the ground inside the preamp.
Isolation washers in rca input jack use for best ground loop .this is most important point in amp .
Anyway, I don't think it's the ground loop here.
ok ....but ground loop is different .best Condition of ground means Silent speakers .
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Must be induced hum, from any of possible sources, such as heater wiring, power wiring or other external hum fields. Don´t think that the problem can be accounted to groundloops. The grid is seeing the greatest impedance when the volume pot is in the mid between max and minimum volume, and thus most sensitive to external humfields. This is true when the input is connected to external equipment, or grounded.
Usually ground loop between CD player and preamp will cause hum regardless of volume setting. Also that's the reason I did say disconnect the connection and check. If it is ground loop between them, it will be quiet!!!! If it is within the preamp, opening the cable should make the hum loudest when crank to max.
The fact it quiet down when max tell me it's not grounding of the CD.
This is a common problem with high-gain guitar amps and especially with the volume control in the guitar itself. Using a lower-value pot sometimes affects the sound of the pickup, but helps reduce the noise in the middle; that's part of the traditional characteristics of Fender versus Gibson. If you think about it, it's not surprising. Maybe the pot should ideally be an L-pad instead? But at min the input is shorted, at max it's loaded by the output impedance of whatever's driving it (the guitar pickup in my example) but in a guitar amp, when the guitar volume is just off full the input to the first tube might be pretty high-impedance without much of a resistance to ground, making the cord etc. just a big antenna without much resistance to ground.
In other words, in desperation you might try a lower-value pot or a resistor across wiper to ground (which will affect the taper).
If the schemo in the 1st post is accurate, that 10K grid stopper is in the wrong place. Stopper resistors need to be connected directly to the grid connection with nothing else connected to the grid.
Can you o'scope what the amp's doing? Bursts of noise/hum at some volume control positions and not others can be a sign of high frequency instability coming from either a gNFB loop, or a gain stage that takes off with oscillation if it has dirty admittances reflected from the plate back into the grid circuit by Miller Effect. This can cause negative resistances, and oscillation if any parasitic LC tuners have high enough Q-factors. That's why you want grid stoppers: to load these parasitic components. Connecting the 470K grid return directly to the grid pin could introduce such parasitic reactances, why you want it behind the stopper, not ahead of it.
Can you o'scope what the amp's doing? Bursts of noise/hum at some volume control positions and not others can be a sign of high frequency instability coming from either a gNFB loop, or a gain stage that takes off with oscillation if it has dirty admittances reflected from the plate back into the grid circuit by Miller Effect. This can cause negative resistances, and oscillation if any parasitic LC tuners have high enough Q-factors. That's why you want grid stoppers: to load these parasitic components. Connecting the 470K grid return directly to the grid pin could introduce such parasitic reactances, why you want it behind the stopper, not ahead of it.
OP needs to post pictures of the wiring. I bet people can tell a lot more. For these higher impedance circuit, wiring is everything. I think we don't have enough to talk about and OP has not been back.
I just tested without anything at input: now hum is directionally proportioned to volume knob (max hum at max volume...!).
The filament wiring is on prited circuit so I cannot change anything.
The two L+R tubes' filaments, that cover four triodes in total, (6,3V each for every tube) are in series (12,6 V). May be I can solve with resistors to ground.
The filament wiring is on prited circuit so I cannot change anything.
The two L+R tubes' filaments, that cover four triodes in total, (6,3V each for every tube) are in series (12,6 V). May be I can solve with resistors to ground.
That's what I suspect, that ground loop is not your problem. That's why I want you to disconnect the cable and try again. In fact, it's the low output impedance of the CD player that quiet it down when you turn to max.
Another question is whether you have hum in both L and R?
Are both channel using the same tube ( half a tube for each channel)?
Is the preamp a store bought unit that you have not done any modification or is it your own build or modification of an existing preamp?
You have not said whether the preamp was humming before, or is the humming just appeared lately. This is very important, if the preamp hums all along, you have a design problem. If it just appear without you doing anything, stop looking for where the noise being pick up and concentrate on a defect component.
Another question is whether you have hum in both L and R?
Are both channel using the same tube ( half a tube for each channel)?
Is the preamp a store bought unit that you have not done any modification or is it your own build or modification of an existing preamp?
You have not said whether the preamp was humming before, or is the humming just appeared lately. This is very important, if the preamp hums all along, you have a design problem. If it just appear without you doing anything, stop looking for where the noise being pick up and concentrate on a defect component.
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All good comments by Alan0354.
It is not an earth loop problem, you have heater voltage coupling into the grid circuit. The most likely cause is deffective layout of wiring. Either in the pirinted circuit (assuming you laid it out yourself), or connections to it. Or possibly the magnetic field from a nearby transformer.
From the point of view of hum, running the heaters in series is not real smart. If you have them in parallel, you can wire a low value pot across the heater supply, with the the wiper earthed (remove any existing earth on the heater supply. The heater supply IS earthed I take it? If not, it should be) Usually you can adjust the pot to that hum is balanced out. If it balances out at the worst volume cointrol setting, it will usually be good at other settings. But with heaters in series this trick will not work - you will only be able to balance out the hum in one tube at a time.
It is possible that you have two causes of hum - eg faulty heater-cathode insulation in one tube AND magetic coupling from a transformer. Two causes of hum can cancel each other out under certain conditions. However, your tests showing worst hum at pot centre with input connected, and worst hum at pot top with no input, together strongly indicates this isn't the case.
So, I'd be looking at how the heater wiring is laid out vis-a-vis the grid circuit.
It is not an earth loop problem, you have heater voltage coupling into the grid circuit. The most likely cause is deffective layout of wiring. Either in the pirinted circuit (assuming you laid it out yourself), or connections to it. Or possibly the magnetic field from a nearby transformer.
From the point of view of hum, running the heaters in series is not real smart. If you have them in parallel, you can wire a low value pot across the heater supply, with the the wiper earthed (remove any existing earth on the heater supply. The heater supply IS earthed I take it? If not, it should be) Usually you can adjust the pot to that hum is balanced out. If it balances out at the worst volume cointrol setting, it will usually be good at other settings. But with heaters in series this trick will not work - you will only be able to balance out the hum in one tube at a time.
It is possible that you have two causes of hum - eg faulty heater-cathode insulation in one tube AND magetic coupling from a transformer. Two causes of hum can cancel each other out under certain conditions. However, your tests showing worst hum at pot centre with input connected, and worst hum at pot top with no input, together strongly indicates this isn't the case.
So, I'd be looking at how the heater wiring is laid out vis-a-vis the grid circuit.
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