AMPEG SVT-2 pro
This beast came into the shop, followed by the local structural engineer to determine if the shop foundations were strong enough to take its weight!
Report was a general service required and it was making a terrible noise.
So, put on my industrial surgical support and heave the thing onto the bench. The amp is a hefty 300 Watts rated, 6 x 6L6Gs, 6550As in parallel and a mix of 12AX7s, AU7s. All connected up, there was indeed a nasty noise, sounding like a fish and chip shop on a Friday night closing time.
These are not the easiest to work on. 2 main boards, one containing the preamp and control sections, the other with all the big grown-up stuff. The preamp board can be withdrawn and worked on, after screws and control nuts and bolts have been removed. There were various dry joints found and some previous repair work around the MUTE circuits and other areas. All these bad areas were dealt with and an injection of cleaning fluid into the user controls. Nice big proper size potentiometers, not these silly little things you find today. With everything cleaned up and the output valves tested on the AVO, we powered her up. All good, plenty of power and slight adjustment of the bias conditions on the output stage. Departure from the normal, the drive to the final stage being cathode follower, set to include the negative bias. Added to this is a circuit providing LED indication as to the status of the bias.
Have to say the sound was very impressive and very little background noise, give the gain the big population of valves.
