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Hi all,

I posted before while I was fixing up a couple of Quad II amps. After working fine for a number of weeks, one of them just started oscillating like mad before I even could put on a record. The "starting up a harley davidson"-type of oscillation, ending with a squeal when I shut it off. Now, the circuit is almost original, but a lot of things and wires have been resoldered and I have replaced the OPT with another original one, which I strapped for 9 ohms.
Swapping out all the tubes for known good ones did not help.

Last time I checked, all the original resistors were ok and less than 10% from spec. I have raised the shared cathode R to 220 ohms to be able to run 6L6GC. The C's are all renewed: C1 is a Wima pp, C2/C3 are NOS Good-All glass sealed PIO with metal case, C4/C6 are replaced with modern 105C 33 and 47uf. C5 is also modern, but raised to 100uF/100v.

I know I will soon have to fire up the scope and do my own work, but have any of you had this happen before? Is the most likely cause a bad solder joint, or something like a shorted resistor in the global negative feedback loop?

Thanks!
Martin