[Jukebox-list] Any ideas on AMI 'K' amplifier fault?

[Paul Taylor]

The amp set up is mono - just using 1 channel of the stereo pre-amp  
and the basic power amp (does not use the add-on power amp).
Several weeks ago I rebuilt the amplifier with new caps and valves  
and it sounded really great. Recently it has developed the following  
fault: When a record is played it sounds fine for the first 40-60  
seconds then gradually the sound becomes very distorted. The amp also  
seems to have developed an uncontrollable 'AVC' effect whereby the  
volume just keeps rising.
Despite the fact that everything is then 'warm' (valve heaters on /  
caps charged etc.) it will behave the same way with any subsequent  
records played - i.e. good sound initially, then becoming distorted.
I've changed all the valves for known good ones  and substituted  
another tonearm/cartridge/lead with no effect. Any ideas where to  
investigate? Many thanks for any responses.
Paul in the U.K.