09-05-2015, 09:23 PM
OK, looks like I stepped on my own crank! I repopulated the circuit boards on my Magnavox era '76 distortion V4, upgraded the 1/2 watt resistors to 1 watt counterparts. PS cap cans were previously replaced and no hum detected before this project. I replaced the board caps with mostly Mallory 150 caps where possible (due to size), used silver mica caps for the really small values. I'm certainly not a true tube amp tech but also not a complete novice with electronics. I made every effort to be very meticulous with the component swaps, verified the component ID, location and value on the board, in the schematic, and also measured the actual value and color code (or printed value on the caps) of the removed component before replacing with a new component. Moment of truth: I fired the amp up, checked internal voltages and bias OK, but there is a high frequency oscillation (probably >10khz) that is quite loud (drives dogs nuts). It is unaffected by any of the controls. I'm going to isolate this down to the preamp or power section, just hoping that Hangman or another forum guru might have some suggestions as to what might be causing this and where to look. The preamp tubes are original, I did not want to risk damaging replacements until this thing is completely debugged. Any assistance MUCH APPRECIATED! THX, danb435