Bias on Marshall JCM 900 SL-X 100 W

[Mats A]

My brother has converted my Marshall JCM 900 SL-X 100 Watt who originally had 5881 valves to EL 34´s using a Marshall conversion manual i got plus an SL_X schematic with EL 34´s i got at the Marshall factory in 1993. Question is how should the bias be set? According to a schematic on Dr Tube it should be set at -45. We can only get it to 38.

[surfnorthwest]

You can measure the plate voltage and use this tool to determine the bias.

You want about 70%

http://www.ax84.com/biascalc.html

[Anitoli]

EL34's will be in the -35 to -50 volt range.
5881/6l6 will be in the -45 to -60 volt range.
Remove all the power tubes and measure the negative bias voltage at both ends of the potentiometers range/ high and low. what are you getting?