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Re: Motorboating

6/2/2000 12:03 PM
Ray Ivers
Re: Motorboating
Winnie,  
 
I believe what you're seeing is ultrasonic oscillation. The voltage jump on the output tube plates is probably due to the 'flyback effect' from the OT at full power; I bet that if you hooked the amp up to a 'scope, you would see the amp operating full-tilt at some ultrasonic frequency (which the meter may integrate into a DC reading). Turning up the treble and mid controls would make this problem worse, of course. Small animals can be great troubleshooting tools for these type of problems, especially if you can read their facial expressions; got any?  
 
Is it possible you reversed the OT transformer's plate leads? Try disconnecting R35 (feedback resistor) and see what happens. It may also be a lead dress problem, or anything that affects amp stability, really. I would first try disconnecting the feedback loop, and see what happens.  
 
Ray

 
Replies:
Winnie Thomas Ray,Thanks, I'll t... -- 6/2/2000 12:50 PM