Supro Black Magick 1695T -- Repair Shop --

A new Supro Black Magick 1695T on the bench today.
This one came in with a loud annoying 50 Hz hum ( 60Hz in US ), which most of times means that the heaters(filaments) are responsible for that.
A "common" mistake is reproducing old vintage circuitry without thinking the modern safety standards, modern tube quality/tolerances and recording needs...
In this expensive cathode-biased amp the filaments are powered directly by AC from the PT... When a heater gets hot can source electrons just like the cathode. If the heater is negative with respect to the cathode, electrons will be jumping from the heater to cathode(leakage current) having this loud hum as a result. Also, even the tubes with the same part number by the same manufacturer can be completely different, if they are not matched the results in terms of hum and noise are even worst.