Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on April 24, 2026, 07:57:49 am
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Another question on the Super Twin. I see that the stock NFB resistor is 820 ohms but that was for a guitar amp. I use it as a bass head. Plus, I see for the SVT they used a 47k and Traynor used a 100k.
What would al think would be the right values to test out?
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Fender used the 820Ω/100Ω NFB resistors in a lot of the blackface amps, including many blackface Bassman amps. Don't get confused by the feedback resistor values in other amps that use a different power amp circuit. Maybe replace the 820Ω with a 470Ω in series with a 5K pot. Then experiment. I suspect you will spend a lot of wasted time and eventually go back to the 820Ω.
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You're probably right, someone suggested a 1.5k. Any thoughts on that? Thanks!
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Try it and let your ears decide. What is your goal? The stock values work very well, especially for a clean bass power amp. Nice balance between fidelity, stability, and reliability. Most of the people you see tinkering with NFB are guitar players that are trying to get a bit of dirt in the power amp. That's OK for guitar, but not so for bass. IMO, a bass power amp should be powerful and clean. The original circuit already gives you that.
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Based on your experience and help from the past, I'll leave it
Thanks!