I will try that, but the preamp does not hum. Only the tape repro without any tape rolling so its between the headblock and the output, or maybe the ground is messed with somewhere. I'll try your method !
It is not modified, except for updated power supply (modern diodes instead of the old selenium rectifier) and also I have wired one of the input straight to the volume pot ( "A77 mod" style, to avoid the first preamp stage), it doesn't seem to be the source of my problem but I'll desolder this just to double check.