Only Spoiler in todays World - is the fact that a lot of 'Modern' ECC81, ECC82 and ECC83 (goes for the US numbered12AT7, 12AU7 and 12AX7 as well) do NOT have a 300mA Heater !!!!
Most of the Russian 'Renamed Models' use 350-400mA => Plugging one of those in a Serial Heater System, and you get a lowered Voltage on those Heaters in that Tube ...!!!!
A good number of Chinese has the same problem ...!!!
Beside being a problem (of variable size - depending on ...) - it can also be a problem in Parallel Heater Circuits if the Transformer can't supply the extra Juice (Amperes) required ....
Per
