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Check the bias resistors. I've got a Delmonico that does something similar. After it's been on for half an hour the volume drops, eventually away to nothingness. Typically these sorts of problems are caused by old, drifting carbon composite resistors. Go in with a meter and start checking anything over one meg or so. Replace any that are out of spec. Those old carbon composite resistors like to drift upwards in value with age, and tend to change value when they get hot. Modern carbon film resistors are available cheaply at any good electronics store, and even most of the bad ones (Rat Shack). More than likely you'll need 1/2 watt ones. Rat Shack has a 100-piece assortment pack for a few bucks that works pretty well, remembering of course that you can combine resistors in series to add values together to substitute for unavailable values. Connected in parallel, the sum of the reciprocals of the values of the resistors is equivilant to the reciprocal of the total resistance.
Good luck.
-Ian
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