Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on June 09, 2022, 03:35:38 pm
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2204 with #36 mod. also Jose Pre-TS master. The amp is sounding killer but the NFB i added to it i have to have it on zero because the amp has too much low end. I already have the usual coupling cap mods .022 to .0022 done. Too much bass on this amp. The nfb resistor is 100k and the nfb pot is connected to the 8 ohm tap (if i recall, possibly on the 4 ohm not sure). What do i need to change to shift the low end freq up (tighter) or reduce the very low end?
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maybe take the "bass" out before it needs NFB's help
start post TS and drop the bypass caps in half, adjust til NFB isn't needed under "normal play". make that your new START point.
NFB can only do sooo much
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What would be the result of any of the following:
Change presence cap value
Change NFB resistor value
Change NFB pot cap value
Change NFB tap to a different OT tap
Change slope resistor
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here's where a decade box would help, you could hear what changing cap and resistor values would do
changing the ot tap that the negative feedback is taken from will provide more negative feedback voltage (if moving from 8ohm to 16ohm tap) or less negative feedback (moving from 8ohm to 4ohm tap). power formula and ohms law give us P=V^2/R (since we'll hold P constant, if R goes up V also goes up)
the suggestion to get the bass sounding good with just a negative feedback resistor in circuit and no fancy resonance pot is a very good one. once it's up to snuff you can add more stuff in
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If your coupling caps are changed down to 0.002 and you're still getting boomy low end, I'd look for an error somewhere in the build.