• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Universal tube preamp PCB, ECC99/6N6P/ECC88/ECC85/6922/6DJ8 etc. MU follower or SRPP

If there is no hum. The powersupply is good enough.

You must look at the grounding between source and preamp or preamp and housing. Better is no connection between input plug and chassis and/or connection between preamp ground and housing.

How looks the source/volumecontrol you use?
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There has been lots of discussions about where to fit pots, mainly due to handling differing line level outputs from sources and minimising the loss of detail in the signal by reducing the level before it is amplified. I think the consensus was to add an attenuator to the input, if it is adjusting the line level that is the primary concern, and put the volume control on the output of the preamp, so it is the amplified signal that is controlled for loudness level. Searching for 'volume' and 'preamp' here should find the discussion.
100K log pot is the normal one one that is fitted, sometimes 50K (modern equipment) or 500K (old) depending on sources.

Something like this, Remove R1/R11. Input impedance stays about 100k.
When not remove R1,R11 input impedance varies between 50k and 100k.