Hi people,
I have a guitar amp here that sounds awesome for two minutes after I put a new master volume potentiometer in. After the two minutes the sound gets dull and the potentiometer becomes intermittent when I push the shaft.
Since the amp has three 12ax7 halfs running straight into eachother, without attenuation between them, I think the potentiometer is simply overstressed.
Measuring the signal voltage at the master volume potentiometers input only gives me 31 volts though, which seems awfully low to me ( Is this because I am not using a true RMS multimeter and the signal is severely distorted?)
Anyway, what I would like to do is cut the voltage at the potentiometer in half, the amp is 100 watt and way too loud anyway.
Would a voltage divider before the master volume pot be the best option?
What would be good values for the resistors to keep the sound as unadulterated as possible? ( Thinking about for example Miller effect of the phase inverter)
Below is the unmodded schematic of this amp. All gain stages have cathode bypasses now and all attenuators have been taken out.
Thank you in advance for your time and attention.
I have a guitar amp here that sounds awesome for two minutes after I put a new master volume potentiometer in. After the two minutes the sound gets dull and the potentiometer becomes intermittent when I push the shaft.
Since the amp has three 12ax7 halfs running straight into eachother, without attenuation between them, I think the potentiometer is simply overstressed.
Measuring the signal voltage at the master volume potentiometers input only gives me 31 volts though, which seems awfully low to me ( Is this because I am not using a true RMS multimeter and the signal is severely distorted?)
Anyway, what I would like to do is cut the voltage at the potentiometer in half, the amp is 100 watt and way too loud anyway.
Would a voltage divider before the master volume pot be the best option?
What would be good values for the resistors to keep the sound as unadulterated as possible? ( Thinking about for example Miller effect of the phase inverter)
Below is the unmodded schematic of this amp. All gain stages have cathode bypasses now and all attenuators have been taken out.
Thank you in advance for your time and attention.
Attachments
Low?? It is rare to shove more than 1 Volt at a volume control.
However, 30 Volts in 1 Meg is 0.001 Watts, in a part surely rated for 0.125 Watts.
Agree that mechanical damage is possible.
Another is poor connection so a grid (maybe not this one) is not really drained.
Thanks for all the replies.
I have found out that the dull sound was actually due to a ground that was not properly connected. After some manipulation I could just pull the ground wire out of its lug!
Tomorrow I will put the PEC pot back that I deemed broken to see if I can replicate the intermittant signal problem .
The Alpha pot that I put in after the PEC pot actually makes loud popping sounds when I manipulate the shaft too, but it is a used pot.
I will report back with my findings.
I have found out that the dull sound was actually due to a ground that was not properly connected. After some manipulation I could just pull the ground wire out of its lug!
Tomorrow I will put the PEC pot back that I deemed broken to see if I can replicate the intermittant signal problem .
The Alpha pot that I put in after the PEC pot actually makes loud popping sounds when I manipulate the shaft too, but it is a used pot.
I will report back with my findings.
If you remove all of the cathode bypass capacitors that you installed, then the gain will be at
a lower level. If the gain is still too high, reinstall the attenuators as well.
The amp must be rather noisy as configured at present.
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It´s a logarithmic control, just set it "1 point less" meaning if it was on 6 now set it on 5, etc.
Just respect original circuit, period.
Killer sounding, tested by tens of thousands for some 40 years or more, do not tweak it.
Maybe, but it´s a distorting Guitar amp, it NEEDS all that gain.
In fact, modern heirs add one or two gain stages, go figure.
The problem here was clearly caused by one tube losing its ground reference,so it would work, sort of, for a couple minutes, until space charge made its grid more and more negative and eventually cut it off.
Even grid leak bias uses a 10M or so resistor to keep leak within reasonable values.
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Thanks for all the replies.
I have swapped the PEC pot back in after reflowing a ground connection yesterday and so far it works: the amp has all the highs it could ever need.
I say so far because I have a feeling that this gremlin might pop up again. Fingers crossed.
One question I still have is this one: is in my application the power rating of the potentiometer dependent on the setting?
I ask this because when you set the pot very low there is only a very small track between the wiper and ground. Would this not affect the possible dissipation of this very small variable resistor?
I know I am wrong but would like to learn.
I have swapped the PEC pot back in after reflowing a ground connection yesterday and so far it works: the amp has all the highs it could ever need.
I say so far because I have a feeling that this gremlin might pop up again. Fingers crossed.
One question I still have is this one: is in my application the power rating of the potentiometer dependent on the setting?
I ask this because when you set the pot very low there is only a very small track between the wiper and ground. Would this not affect the possible dissipation of this very small variable resistor?
I know I am wrong but would like to learn.
A potentiometer is intended to adjust voltage, but not to dissipate significant power.
The amount of power that is actually dissipated in the pot is tiny, and of no significance.
For example, 1V across a 10k volume control would dissipate only 0.1mW of power.
Also, 30V across a 1M pot would dissipate about 1mW of power.
Only in loudspeaker systems do specially designed variable resistors dissipate much power.
These are typically wire wound.
The wiper is normally unloaded, or else loaded with a resistor much higher in value than the pot.
Adjusting the pot typically does not change the power dissipation significantly.
The amount of power that is actually dissipated in the pot is tiny, and of no significance.
For example, 1V across a 10k volume control would dissipate only 0.1mW of power.
Also, 30V across a 1M pot would dissipate about 1mW of power.
Only in loudspeaker systems do specially designed variable resistors dissipate much power.
These are typically wire wound.
The wiper is normally unloaded, or else loaded with a resistor much higher in value than the pot.
Adjusting the pot typically does not change the power dissipation significantly.
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It is true that when the current only flows through a small part of a potmeter, it can't be used at its full rated power. I usually use the somewhat conservative assumption that the current that makes it dissipate its rated power when it flows through the entire track is the maximum current it can safely handle when it flows through only a small part of the potmeter.
In your circuit, most of the current just flows from one end to the other and not through the wiper and the current is very small anyway, so I wouldn't worry about it.
In your circuit, most of the current just flows from one end to the other and not through the wiper and the current is very small anyway, so I wouldn't worry about it.
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