Weapons Man
Active Member
I have a question for the wise Marshall owners on here: I have a 1959 SLP with 5881 valves. I got a matched quad from Watford vales in the UK and they appear to be rebranded Sovtek valves. There is tons of conflicting info out there regarding the max plate dissipation on these valves. Some claim it is 20.5 watts and others say anywhere from 23 to 30 watts. I found the Marshall conversion manual online (where it talks about how to convert these amps from EL34 to 5881). On the attached schematic, in the upper left hand corner, it says to "adjust bias to -50V." What does that even mean? When using a Bias Master, the info that shows up on the screen is in mA. What I had planned on doing was to turn the amp upside down and take a plate voltage reading off pin # 3. Then take a mA reading from the Biasmaster. Multiply the plate voltage times the mA to the get the plate dissipation in watts. Take the dissipation in watts and divide that number by the plate voltage, then multiply that number by .7 to get the bias in mA. Am I on the right track here? Does anyone know what is meany by "set bias to -50V?" Thanks in advance!
