yngzaklynch
New member
From the JVM410.COM forum the JVM's designer tells you how. I might do this later if I can find my multimeter. I'm curious to see where's mines at.
The biasing a JVM is similar to a DSL/TSL. There are independent bias controls for each side of the push pull and shunt 1 ohm resistors to measure the current.
Again, I cannot stress enough that biasing an amplifier is not a user adjustable feature and that working inside an amplifier can be very dangerous. An incorrectly biased amplifier can lead to valve and/or output transformer failure. Leave it to qualified people.
Procedure is as follows:
With the amp open and upside down, place both master volumes fully counterclockwise (OFF), highly recommended a speaker or resistive load into its relative socket and let the amplifier warm up for few minutes. Switch the standby to ON (playing position). Underneath the power valves, there are two small trimmers and a 3 pin connector (I think they are VR2, VR3 and CON1 but just talking off of my memory). VR2 controls the bias of V1 and V2, and VR3 does the same for V3 and V4.
To measure the current of each side place a multimeter set to millivolts, mV, between the center pin of CON1 and each side. Pin on the left indicates the current flowing through the left pair of valves, pin on the right indicates the same for the other pair.
the conversion is as follows: Voltage in mV=current in mA per push/pull side (if valves are matched current per valve will be half of that). So, if the reading is 60mV that means 30mA per valve (factory setting). I wouldn't go any higher than 45mA per valve (90mV), 30-35mA is a good compromise tone/valve life.
S.
The biasing a JVM is similar to a DSL/TSL. There are independent bias controls for each side of the push pull and shunt 1 ohm resistors to measure the current.
Again, I cannot stress enough that biasing an amplifier is not a user adjustable feature and that working inside an amplifier can be very dangerous. An incorrectly biased amplifier can lead to valve and/or output transformer failure. Leave it to qualified people.
Procedure is as follows:
With the amp open and upside down, place both master volumes fully counterclockwise (OFF), highly recommended a speaker or resistive load into its relative socket and let the amplifier warm up for few minutes. Switch the standby to ON (playing position). Underneath the power valves, there are two small trimmers and a 3 pin connector (I think they are VR2, VR3 and CON1 but just talking off of my memory). VR2 controls the bias of V1 and V2, and VR3 does the same for V3 and V4.
To measure the current of each side place a multimeter set to millivolts, mV, between the center pin of CON1 and each side. Pin on the left indicates the current flowing through the left pair of valves, pin on the right indicates the same for the other pair.
the conversion is as follows: Voltage in mV=current in mA per push/pull side (if valves are matched current per valve will be half of that). So, if the reading is 60mV that means 30mA per valve (factory setting). I wouldn't go any higher than 45mA per valve (90mV), 30-35mA is a good compromise tone/valve life.
S.
