Using the same recording, and same track on the same source component...
Swap your amplifier interconnects at the preamp. Left IC to right output, and Right IC to left output.
Does the scratch follow the swap? If so, it’s not the amplifier. It’s upstream, or an interconnect cable. If the scratchy sound remains in the right channel, then the cause is your amplifier (or a bad tube, or something else).
Swap the amplifier interconnects, so you have the right IC on the left channel preamp output, and the left channel amplifier input. Connect the left interconnect to the right channel as above. Play the track. Does the scratchy sound follow the swap? If so, it’s the cable, or a poor connection. If not, then the scratchy sound is caused upstream.
Restore the interconnects to normal.
Swap the source interconnect at the preamp. Left IC to right input. Right IC to left input.
Play the same track. Does the scratch move to the other channel? If so, it’s either the source component, or it’s the interconnect cable, or it’s on the recording.
Restore the interconnects to normal.
Try a different recording. Does the scratchy sound still occur? If so, then it’s your source component, or the interconnect cable. If not, it points to the recording.
Swap the interconnect cables. Put the right IC on the left channel. Put the left IC on the right channel.
Play the track. Do you hear the scratchy sound? Which channel?
When you hear the scratchy sound, are you adjusting volume using a volume pot on the preamp, or with a remote control?
