My Marantz PM-50 transistor amp, has this problem. The left and right channels are of different loudness, about 6dB, even when the balance is at centre. This happens in all inputs and with any speaker. The difference can also be understood through headphones, but not as much, as with speakers. I appreciate your help, thank you!

6 dB means double the loudness of the output signals. To check where the problem originates from :

1 - open up the amplifier and find the volume control (a double deck potentiometer)
2 - the volume controls are at one side interconnected to earth. With a wire or cable clips interconnect the other side of the controls (making the signals equal for each channel).
3 - if the difference disappears, the cause is in the input section, the pre-amplifier, or the feeding signals. Check all feeding cables (check by reversing input L - R cables to see the effect).
4 - if the difference still exists, the problem is in the output stage. Most probably one of the drivers or power transistors is gone. If that is the case you need professional help : find a proper repair man.

This is all based on your message that the problem is not based on a specific speaker. Did you reverse the L - R speaker wires to test that? Different speaker impedances could also cause this.

Succes!

Thank you Credendovidis, I reversed the speaker wires without any change, so the issue is indeed in the amp. I'll see what can be done inside the unit.

In many cases the problem can be tracked to a bad coupling capacitor and distortion typically occurs at different loudness levels for each channel. Professional help is warrented.