Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 18, 2005, 06:04:41 am
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Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> NFB Questions
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jbrew73
Junior tube assistant
Posts: 107
(4/1/04 9:40 pm)
NFB Questions
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i understand that the value of the nfb resistor would need to be changed if the output tap used was changed(27k on 2 ohm, 47k on 8 ohm , 100k on 8 ohm etc.) but what happens if you are running a 4 ohm cab on the 8 ohm tap or you pull 2 tubes on a 100watt amp or using a 6.6k transformer where the amp in question came with a 4k. would this change the level of nfb? what i'm after is whether i would need to change the value of the nfb resistor depending on the impedence ratio of the output transformer? don't tell me to try it and see what sounds the best because i'm tone deaf as the rejects on american idol. i want to make sure it sounds good to you guys with golden ears
Tiny Daddy
I will work on all amps
Posts: 587
(4/1/04 10:56 pm)
Re: NFB Questions
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The NFB should reduce the volume 3 or 4dB, a barely audible change. Or you could measure the gain change with load attached and adjust the resistor.
jbrew73
Junior tube assistant
Posts: 108
(4/3/04 12:10 am)
NFB Questions
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so you are saying to adjust the resistor so the nfb drops the output by 3-4db. and this would compensate for the changes made to the output transformer impedence ratio.
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