Adding to this thread.....6L6 metal tubes and 6L6G, GA and GB all have a maximum screen voltage of 270V to 300V, depends on what data sheet you are reading. Max plate is 400V
The ST70 has a screen voltage that is over 400V. I tried original 6L6s in my ST70. I had to turn the bias pots much more counterclockwise to get the 6L6s down to 40mA than with EL34s. I measured the bias at -37V. The 6L6 data says that at 300V on the screen and 400V on the plate, the bias should be around -25V. I think the tube simply isn't designed to have such a high voltage on the screen.
Bottom line is that only 6L6GC tubes are designed to have 400+ volts on the screen.
Tim
The ST70 has a screen voltage that is over 400V. I tried original 6L6s in my ST70. I had to turn the bias pots much more counterclockwise to get the 6L6s down to 40mA than with EL34s. I measured the bias at -37V. The 6L6 data says that at 300V on the screen and 400V on the plate, the bias should be around -25V. I think the tube simply isn't designed to have such a high voltage on the screen.
Bottom line is that only 6L6GC tubes are designed to have 400+ volts on the screen.
Tim
Last edited by Tubes4ever on Wed Jul 15, 2015 9:58 pm; edited 1 time in total (Reason for editing : additional information)