Just got this thing fired up the other day and one issue is really screwing with me, and that is I'm getting a serious buzz from the nfb.
This amp is basically a plexi style preamp/pi into a pair of cathode biased EL84's. The PI and Presence control are straight JTM45 (or 5F6A) , the nfb is taken off the 16 ohm wire at the impedance switch, thru a 100k resistor to the junction of the .1uf cap (to the PI grid opposite of the input grid), the ground side of the PI tail resistor and the input of the Presence pot (5K). Nothing unusual really.
If I ground the PI tail resistor the buzz goes away. But of course, I also loose the Presence if I do that. Lifting the nfb cures the problem , and the amp sounds gorgous like that, I'm tempted to just leave the nfb off. But I know that there's got to be a reason it's causing the buzz,
I tried reversing the OT leads at the tube plates, thinking I might have positive feedback, but that just made things much worse, the added a loud squeal to the problem, so I re-reversed the plate leads, putting them back to where they were originally. I've replaced the .1uf cap, thinking perhaps it was causing the problem, that did not help. I replaced the Presence pot, that didn't help. I even took the Presence pot out of the circuit and just used a 6k8 resistor (closest I had to 5k on hand) that didn't help. It's definitely a buzz caused by the nfb into the PI, but why?
Note, in my grounding scheme I grounded the Presence pot to the smae point as the filter cap that supplies the PI, and I also grounded the isolated speaker jacks there. In troubleshotting I've moved both of those grounds around to different locations, none of which helped the buzz at all.
Any thoughts?
This amp is basically a plexi style preamp/pi into a pair of cathode biased EL84's. The PI and Presence control are straight JTM45 (or 5F6A) , the nfb is taken off the 16 ohm wire at the impedance switch, thru a 100k resistor to the junction of the .1uf cap (to the PI grid opposite of the input grid), the ground side of the PI tail resistor and the input of the Presence pot (5K). Nothing unusual really.
If I ground the PI tail resistor the buzz goes away. But of course, I also loose the Presence if I do that. Lifting the nfb cures the problem , and the amp sounds gorgous like that, I'm tempted to just leave the nfb off. But I know that there's got to be a reason it's causing the buzz,
I tried reversing the OT leads at the tube plates, thinking I might have positive feedback, but that just made things much worse, the added a loud squeal to the problem, so I re-reversed the plate leads, putting them back to where they were originally. I've replaced the .1uf cap, thinking perhaps it was causing the problem, that did not help. I replaced the Presence pot, that didn't help. I even took the Presence pot out of the circuit and just used a 6k8 resistor (closest I had to 5k on hand) that didn't help. It's definitely a buzz caused by the nfb into the PI, but why?
Note, in my grounding scheme I grounded the Presence pot to the smae point as the filter cap that supplies the PI, and I also grounded the isolated speaker jacks there. In troubleshotting I've moved both of those grounds around to different locations, none of which helped the buzz at all.
Any thoughts?
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