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NFB - when it's good, it's good, when it's bad, it's bad

Posted by kurt s on January 18, 2007 at 20:13:43

Triodes are the most linear amplification devices known because they possess one important trait - high negative feedback. The electrons hitting the plate do not just go on unabated, they are bounced off the plate by secondary emission and this is fed back into the tube's own "circuit" which is a very quick NFB loop.

Pentodes on the other hand are designed to suppress secondary emission which means the electrons do go on unabated and hence negligible NFB exists inside the tube.

What amplifiers sound good with NFB? Amps with only pentodes do. Which do not? Triodes. The Harmon Kardon Citation II uses a lot of NFB in many forms - local and global and it sounds good. That's because the thing is all pentodes and it needs it. The Futterman used huge global NFB and it sounded good. Why? It had all pentodes.

Why is it that pentodes need NFB and triodes abhor it? Is it really just the output impedance? I think not. Look at the spectrum that they produce in terms of harmonic distortion. Pentodes produce higher amounts of odd order distortion than do triodes because it has no inherent local NFB. When you add NFB to high amounts of odd order distortion spectra, the results are shifted to more even order distortion spectra. But the triode does this for you already.

The triode is already locally NFB'ed where its harmonic content is largely even order. If you add NFB around this you add odd order harmonic distortions at higher orders. The math is there that showed this. Scott Franklin of MFA wrote about it. It's true.

The only amps that sounded good to me with high local NFB were pentode outputs - 47 DHP's, EL-34's, etc. They sounded close to triodes with this NFB in terms of timbre, and the 47 DHP sounded VERY close to a DHT. Add NFB to triodes and it sounds worse, local or global, I don't care which way. Some think local NFB is all super pure and everything, and I disagree. For triodes, local NFB screws it up, it already has enough of that inside the device itself.

You can use pentodes and have them sound superb. You will need NFB to do so. You can use triodes and have them sound superb, also. You will need to avoid all NFB.

My two cents worth for the day.

Kurt

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