Hey everyone, I've got a 410-65, 2275-65 chassis on the bench that I can't get stable. Before I saw the amp someone tried to run it at "half-power" by pulling one of the power tubes. Apparently the whole output section blew up including the output transformer. The transistor driver board was rebuilt by another tech and the output transformer was replaced (incorrectly)with a P-TM050 Marshall-style 50 Watt transformer.
The rebuilt amp came in with a blown output tube and one fried driver transistor. I replaced both driver transistors with 2N4488's and installed new power tubes. I rearranged the secondaries on the output transformer so rather than the 3.2K load of the Marshall-style OT the tubes are seeing a reflected impedance of 6.4K (closer to the original 6K impedance). The negative feedback is coming off the 16 ohm tap.
Now I'm getting 117KhZ oscillation on the cathodes of the power tubes. It's enough to make the tubes over-dissipate. The source of the oscillation is somewhere in the driver stage after the master volume and before the cathodes. I've been able to get it a little more stable by adding small amounts of resistance and capacitance in parallel with the negative feedback resistor but it seem pretty finicky. I've replaced the flyback diodes and 3KV caps on the tube sockets. Voltages at the grid and screens are stable. The oscillation is visible on the scope at the cathodes.
The obvious thing here is the OT. I'm trying to save the owner the cost of a proper replacement from Mercury. Does anyone else have any idea how I can get the amp stable otherwise?