Zinnmaster
New member
Earlier this month I changed out my tubes in my VH4 from JJ KT77s to Ruby EL34 BSTR/BW. When I did, the bias went all the way to 100mA and was fluctuating. I immediately turned it down to 65mA and it slowly climbed back up and still was fluctuating. Eventually it blew the fuse for the first power tube.
I posted in this forum asking if further troubleshooting needed to be done. Peter recommended checking the screen resistors. All 4 checked out good.
I replaced the fuse and put the old tubes back in, biased, and enjoyed playing for about 3 hrs with no problems.
Old KT77s bias using shunt method from factory (checked before removing tubes
Yel/red - 70mA
Blu/red - 77mA
I sent the bad tube back and received another tube that was matched with the previous three. I put the new tubes back in and biased. I had no problems with the biasing (no fluctuation) and biased to:
New EL34BSTR/BW bias using shunt method
Yel/red - 65mA
Blu/red - 67mA
I played for about 5 min adjusting my volumes and gain for the channels. Fuse for tube 3 blew.
I know I have been told that Ruby EL34 tubes have been unreliable in comparison to the JJ KT77s, but 2 out 5 seems odd to me. Am I doing something wrong?
EL34 tubes:
Tube 1- plate current 27.8, transconductance: 4760
Tube 2- plate current 28.2, transconductance: 4950
Tube 3- plate current 28.9, transconductance: 5210
Tube 4- plate current 27.9, transconductance: 4900
Turned all vol and gain pots off, speaker 16ohm 4x12, input cable in
All power off, hooked up my leads to transformer (Blu/red), turned bias pot counterclockwise to lower.
Turned on power, waited a minute before turning on standby.
Let tubes warm up for ten minutes, adjusted bias to 65mA and let settle for several min.
Turned standby and amp off. Switched leads to (Yel/red)
Powered on, waited a minute before turning on standby.
Observed bias was around 64mA.
Let tubes warm up 10 min. Adjusted bias to 65mA and let settle for several min.
Turned standby and amp off, switched leads back to (Blu/red).
Powered on, waited a minute before turning standby on.
Observed bias at 67mA and documented on paper my measurements.
Another bad tube?
I posted in this forum asking if further troubleshooting needed to be done. Peter recommended checking the screen resistors. All 4 checked out good.
I replaced the fuse and put the old tubes back in, biased, and enjoyed playing for about 3 hrs with no problems.
Old KT77s bias using shunt method from factory (checked before removing tubes
Yel/red - 70mA
Blu/red - 77mA
I sent the bad tube back and received another tube that was matched with the previous three. I put the new tubes back in and biased. I had no problems with the biasing (no fluctuation) and biased to:
New EL34BSTR/BW bias using shunt method
Yel/red - 65mA
Blu/red - 67mA
I played for about 5 min adjusting my volumes and gain for the channels. Fuse for tube 3 blew.
I know I have been told that Ruby EL34 tubes have been unreliable in comparison to the JJ KT77s, but 2 out 5 seems odd to me. Am I doing something wrong?
EL34 tubes:
Tube 1- plate current 27.8, transconductance: 4760
Tube 2- plate current 28.2, transconductance: 4950
Tube 3- plate current 28.9, transconductance: 5210
Tube 4- plate current 27.9, transconductance: 4900
Turned all vol and gain pots off, speaker 16ohm 4x12, input cable in
All power off, hooked up my leads to transformer (Blu/red), turned bias pot counterclockwise to lower.
Turned on power, waited a minute before turning on standby.
Let tubes warm up for ten minutes, adjusted bias to 65mA and let settle for several min.
Turned standby and amp off. Switched leads to (Yel/red)
Powered on, waited a minute before turning on standby.
Observed bias was around 64mA.
Let tubes warm up 10 min. Adjusted bias to 65mA and let settle for several min.
Turned standby and amp off, switched leads back to (Blu/red).
Powered on, waited a minute before turning standby on.
Observed bias at 67mA and documented on paper my measurements.
Another bad tube?