when it first turns on, it sounds impressive / works perfectly. however, once it has been on for 5-10 minutes the sound gradually and evenly fades from nice and clean to distorted (almost as if you are overdriving the tubes) but the signal also gradually fades in amplitude as well. this is consistent across both the right and left channel.
when i was doing some initial readings with a dmm, i noticed that the b+ supply voltage was slowly dropping. taking a reading after the last cap in the power supply, it hovers around 285 v dc when first turned on (as per spec, when cool) and slowly drops to below 240 v dc over the course of 5-10 minutes -- and by this time the audio is very distorted and i just turn it off. if the unit is then quickly powered back up before it has had some time to cool down -- after the initial charging of the system, the voltage drops much quicker this time into low voltages and distorted audio. however, if the unit is allowed to cool for sometime, it will return to working great for the initial couple of minutes before again dropping the b+ voltage and distorting the audio.
the voltage readings slowly and consistently drop when the amp is turned on, all the way back to the ac transformer leads (before the rectifier) while everything is in circuit.
any ideas as to which component(s) might be at fault? i have yet to experience these symptoms with a tube amplifier...
attached is an image of the schematic - any help would be appreciated, thanks!