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Old 26th September 2015, 06:33 AM   #1
ygg-it is offline ygg-it  Italy
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Default I finally won my fight to kill the hum, but not clear why

Hello, after months and several tempts my tube amplifier is now so quite that even I put my ear close to the speaker I cannot hear hum.

Before with source connected I had maximum hum (not too much, but anyway I had) when my volume potentiometer was in middle position, the dial in which I normally use to listen.

Without anything at input hum was directionally proportioned to volume knob (max hum at max volume...).

Running tubes' filament on DC

Hum if volume control between midpoint and max; No hum if volume knob at max or zero


So this (great) forum said it was where the resistance from slider to common is highest and the cause could be a bad capacitive coupling to the circuit from pot slider to valve grid.

How I killed it: by chance and for different reason yesterday I changed my input potentiometer from 100K to 10K and then hum completely disappeared!

Why? I don't know, but I would like: faulty potentiometer (but I didn't have symtoms), oscillations, RF ...

My input cable is long (more than 2 meters) and my source is a Cd player.
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Old 26th September 2015, 09:14 AM   #2
DF96 is offline DF96  England
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More likely that your hum is now 20dB down so likely to be inaudible. As we said, all the symptoms are consistent with capacitive coupling to the grid circuit. Nothing wrong with your old pot. Something wrong with your physical layout.
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Old 26th September 2015, 09:58 AM   #3
M Gregg is offline M Gregg  England
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You have,

increased the loading on the input cable and connected the input closer to Gnd.
This might be OK or it might change the sound depending on the drive of the source.

Regards
M. Gregg

Last edited by M Gregg; 26th September 2015 at 10:02 AM.
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Old 26th September 2015, 01:54 PM   #4
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Now the input circuit is not so "High Z", less sensitive to swing of the input voltage, and associated noise. Good luck & enjoy.

Doug
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Old 26th September 2015, 07:26 PM   #5
DF96 is offline DF96  England
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No, it is just as sensitive to the input voltage. It is less sensitive to the hum current which comes in via stray capacitance.
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Old 26th September 2015, 10:43 PM   #6
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Depening on the circuit, you might change the frequency response. If your bass becomes boomy then you'll know why..

6n2j looks like similar to the 6ax7 so I would expect the bass to get boomy if you with this low input impedance.

If I look at your original post, my thought is this - did you ever try to ground the case of the volume POT to the chassis? Both Channels?

But maybe since you didn't post a picture, nobody took it seriously enough to comment further.

Of course some people like boomy bass.
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Old 26th September 2015, 10:44 PM   #7
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btw - try placing your cd player closer to your amp. use a nice cable too . you woun't regret it.
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Old 27th September 2015, 04:53 PM   #8
DF96 is offline DF96  England
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Nothing to do with interconnect cable length. The hum was being picked up from the pot slider to the valve grid.
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