I've recently gutted my "Champion 600" reissue amp & built myself a 5F1 reusing the chassis, cabinet, transformers & speaker with plans to replace the speaker in the near future. My build follows the 5F1 circuit closely however I could see no great benefit using a 5Y3 rectifier, using 4 1N4007 diodes instead & the Chinese iron has no center tap. I also have deviated from the original by using the original 18/3 power cord (properly grounded of course), deleting the .047mfd "death cap" & twisted my heater supply lines & added 2 100 ohm resistors running from each leg of my replacement pilot lamp to ground. Upon completion of my project turning the volume control above 3 yielded a "farting" kind of distortion. Researching the problem I found several suggestions so I opened it back up & added a 25mfd/50vdc cathode bypass cap across the 1.5K resistor from pin 3 of the 12AX7 tube & eliminated the 22K negative feedback resistor, both of which seemed to help a great deal however now when I turn the volume above 9-10 I get some sort of oscillation that kind of sounds like a motorboat & hope somebody can assist me in eliminating the problem. So far my thoughts on this have been:
1.) try adding another 25mfd/50vdc cap across the 1.5K resistor from 12AX7 pin 8
2.) reverse the red & blue wires going to the OT (red now runs to A+ supply node - blue to pin 3 of the 6V6)
3.) change the values of the supply resistors from 10K/2W & 22K/2W to 1K/2W & 10K/2W respectively to increase the plate voltages a little
I kind of also remember reading somewhere about using shielded wire at 2 key spots in the circuit but I've been unable to find that article again & I didn't take notes when I 1st came across this. Can anybody help shed some light on what I should do or perhaps offer another suggestion & please if possible don't just provide what to do but a bit of an explanation as to why so I can learn & not have to be a bother in the future?
1.) try adding another 25mfd/50vdc cap across the 1.5K resistor from 12AX7 pin 8
2.) reverse the red & blue wires going to the OT (red now runs to A+ supply node - blue to pin 3 of the 6V6)
3.) change the values of the supply resistors from 10K/2W & 22K/2W to 1K/2W & 10K/2W respectively to increase the plate voltages a little
I kind of also remember reading somewhere about using shielded wire at 2 key spots in the circuit but I've been unable to find that article again & I didn't take notes when I 1st came across this. Can anybody help shed some light on what I should do or perhaps offer another suggestion & please if possible don't just provide what to do but a bit of an explanation as to why so I can learn & not have to be a bother in the future?
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