SVT 3 Pro Tube drive

[MstrB]

Hello, I have a SVT 3 Pro on the bench and I am seeing a reasonable swing in the tube driver plate voltage (100v - 250V) but not experiencing any major DRIVE change.

[Enzo]

Be aware the two tubes in this are not the same. The input buffer to the power amp is a 12AX7, but the driver tube is a 12AU7. I often find them in their wrong sockets respectively, or someone has replaced the 12AU7 with another 12AX7.

Even when the tubes are correct, the single most common repair I do to these amps is replace bad 12AU7 driver tubes. For the 10 seconds it will take, just try a different 12AU7 in the socket and see if it corrects whatever the symptom is.