JTM45 screen resistor and bias issue
Posted: Wed May 15, 2019 9:20 am
Hello,
I changed the KT66 on my JTM45 clone, but got unexpected results, as with the bias pot set at coldest point, I have the same plate voltage reading on both tubes (460V with a NOS Mullard GZ34, 440V with a chinese GZ34), but about 10ma plate current difference (I.e. 36,5ma on tube 1 and 46,5ma on tube 2).
The KT66 ar Shuguang, supposedly matched.
I tried reversing the tubes, and got the same results on the same sockets, which means the issue is the amp and not the tubes. I checked the and components from the sockets. It appears that one of the screen resistors has drifted in value : around 225 ohms instead of 470 ohms. The socket with the incorrect value is the one on which I monitored the highest plate current.
Does that seem logical or should I check for something else causing the issue?
Many thanks
Hugues
I changed the KT66 on my JTM45 clone, but got unexpected results, as with the bias pot set at coldest point, I have the same plate voltage reading on both tubes (460V with a NOS Mullard GZ34, 440V with a chinese GZ34), but about 10ma plate current difference (I.e. 36,5ma on tube 1 and 46,5ma on tube 2).
The KT66 ar Shuguang, supposedly matched.
I tried reversing the tubes, and got the same results on the same sockets, which means the issue is the amp and not the tubes. I checked the and components from the sockets. It appears that one of the screen resistors has drifted in value : around 225 ohms instead of 470 ohms. The socket with the incorrect value is the one on which I monitored the highest plate current.
Does that seem logical or should I check for something else causing the issue?
Many thanks
Hugues