Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dunner84 on January 31, 2017, 10:30:57 pm
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My most recent build is an ab763 type amp with a NFB lift switch and a multi-tap OT. The NFB resistor value is based on an 8ohm load, and this is also where the NFB is attached to the impedance switch; however, I will be primarily running the amp off the 4ohm tap in a 2x10 configuration. Should I adjust the NFB resistor, and attach it to the 4ohm tap? or will the amp work properly with it as is? Will the NFB switch work properly when I am on the 4ohm setting even though the NFB circuit is connected to the 8ohm tap?
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I ran into the same issue when I did my Schedule 40 build with a multi-tap OT. If the circuit was designed for 8ohms, just connect the NFB to the 8ohm tap even if you are using the 4ohm out to the speaker. It will work fine regardless of which tap you have connected out to your speaker. I don't believe there is any benefit to lowering your resistor value to match the 4ohm tap and getting it from the 4ohm tap (although that will work).
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People typically do 1 of 3 things: leave NFB as-is when you switch taps; SW the NFB circuit to the tap you're using; or, use a pot to adjust NFB. There is no right or wrong answer. Also, the effect of NFB varies with the relationship between the amp & the specific speakers being used. If you like the as-is circuit with your 4 ohm load, there's no need to change it. Or, you can experiment to see what you prefer.
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... Will the NFB switch work properly when I am on the 4ohm setting even though the NFB circuit is connected to the 8ohm tap?
Yes.
Say you have 4-8-16Ω taps. When you play through the amp, all taps are energized even if you're only hooked to the 4Ω tap. Or only the 8Ω tap. Or...
Another way to think about this to remember is to ask yourself, "Am I willing to grab the disconnected high voltage tap of my power transformer, even though I've only hooked up the bias tap?" I sincerely hope your answer would be no (that you know voltage is present at the unused high voltage tap).
Same goes for the output transformer. The negative feedback components are sized based on expected voltage on a given tap when the rated power output is passed through the OT, and that expected voltage is at that given tap regardless of which tap you hang a speaker.
I've seen other folks speculate that you oughta move the feedback to the actual tap with the speaker (with appropriate resizing of values) because "it'll damp the speaker better." The reasoning goes that the bit of secondary winding between the speaker tap & NFB tap will somehow isolate the speaker a little from the NFB. I don't think this argument is valid, as all parts of the system are part of the negative feedback loop all the time.
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> "it'll damp the speaker better."
When you get to VERY high damping factors, you want to measure at the tap to reflect the small parasitic resistance differences.
I can't think of ANY commercial amplifier which did this, except verbatim copies of Williamson with the 16 secondaries and the formula for the resistor. Certainly NOT an issue in guitar amps.
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Wow, thank you for all of the input. I think I have a decent understanding of it now.