Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: catnine on January 24, 2013, 04:13:19 pm
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I disconnected the NFB lead from the output jack just to see if where I placed the switch so close to the inputs was creating some issue , nothing changed . If I switched the switch to add in the 25uf bypass cap then the buzz came back so I guess by having the pos side of the second gain bypass cap so close to the inputs and the cap lug on the switch was the closest to the inputs. I moved the switch to the rear of the chassis and the buzz is now gone. Also I checked all the resisters and the 1.5K that is on the PI cathode per 5E3 through a 56 ohm to ground read 1.889K so I changed it to what all P-P princetons used a 1K .
I installed a 1 meg PI grid stopper between V2 pin 7 and the junction of the .02 coupling cap and the circuit called for 1 meg .
Odd thing was when I powered up the amp the 12ay7 in V1 with nothing plugged in and the volume up all the way at times i would hear this sort of motorboat sound , this would go away on it's own seemed realted to how warm the amp was . I also notinced with the vol up if I tapped the 12ay7 it sounded microphonic even if I tapped the chassis I could hear it. I tried a 12ax7 and it did not do this but then after removing and installing these tubes a few times the 12ay7 was not microphonic anylonger maybe it had a bad contact .
I playing the amp and the buzz was gone and the raspy OD sound was gone .
I really don't understand what putting a 1 meg grid resister on the PI grid does it's on pin 7 of V2 like I said . I can't hear any change in the tone just the raspy OD is now gone. There is already a 1 meg on the eyelet board so this just adds another 1 meg in the same path through the 56K to ground .
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Congrats on getting rid of the buzz. So it had something to do with the layout and minor parts change... Let me get it straight, the 1M that you added to the PI's grid (pin 2) - is it just in parallel with the existing 1M already there? If so, you simply lowered the grid leak (not grid stop) to 500K. Or did you connect from the junction of the 0.02u/1M to pin 2 of the PI? If so, that's a grid stop of 1M, which is way too large imo. Hopefully, it's the first case.
Jaz
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Congrats on getting rid of the buzz. So it had something to do with the layout and minor parts change... Let me get it straight, the 1M that you added to the PI's grid (pin 2) - is it just in parallel with the existing 1M already there? If so, you simply lowered the grid leak (not grid stop) to 500K. Or did you connect from the junction of the 0.02u/1M to pin 2 of the PI? If so, that's a grid stop of 1M, which is way too large imo. Hopefully, it's the first case.
Jaz
Actually the PI grid is pin 7 . I put the 1 meg of the 0.02 and 1 meg . I feel it's to large too . I am going to remove it and just run a wire like I had before. I just don't trust this much . It was from Valve Wizards site . I don't know stock fenders never used them so why suddenly is there a need for one. I just thought I would try it out. I can't say I understand how it works.
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This is why it's best to do things 1 at a time so then you know what and why any change in sound happened.
Brad :icon_biggrin:
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Actually the PI grid is pin 7 . I put the 1 meg of the 0.02 and 1 meg . I feel it's to large too . I am going to remove it and just run a wire like I had before. I just don't trust this much . It was from Valve Wizards site . I don't know stock fenders never used them so why suddenly is there a need for one. I just thought I would try it out. I can't say I understand how it works.
You mean 1M in series with the 0.02u? If so, then yes, it is too large, but it does not hurt to put a smaller grid stop (say 68k) right on the grid pin, which someone already suggested earlier, I think...
(https://dl.dropbox.com/u/1326040/PI%20Mod.gif)
Jaz