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

Posted by Anthony Dockrill (A) on January 19, 2007 at 01:24:09

In Reply to: 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:

Kurt,

I don't post around here much anymore but what you just posted matches my findings and Kurt you’re a braver man then myself for posting those findings. They fly in the face of dogma and fashion. “Feedback is good when used wisely on single ended amps” Dangerous stuff.

Putting negative feedback on a SE triode in my case of 845 running at sub 500 volts was like putting a wet blanket on a sickly kitten. While distortion was lower with the amp playing near its limit it just sucked all the life out of the music. I reduced the feedback to very low levels but it always sounded better with zero feedback.

I then experimented with the same amp less the 845 running a EL34 running in triode mode. Without feedback the EL34 sounds not bad at all – I was actually surprised how good it sounded - not as open as a DHT but perfectly OK. Feedback had the same effect as the 845 - The sound of death!

BUT hooking the EL34 up as pentode and adding feedback was an eye opening experience. Not only did the amp not suck but the huge surprise was that the amp was full of life and musical. Very good tone and timbre and it could play loud. After changing the driver so it was more suited to driving a pentode then a triode (ie. going for a single 6SL7 triode input/driver rather then a 6H30 cascode) the final amp was really very nice on any measure.

6L6GC and KT88 also kicked arse as pentodes but the KT88 and EL34 were the best overall with the KT88 being a bit more weighty and darker sounding.
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My feeling with feedback is that if you use it to lower distortion on triodes it can only suck.

If you use it only to lower the impedance of the amp then it has a fighting chance of being very beneficial.

BTW - I tried local and general feedback on pentodes and I personally preferred general feedback. The amp in question was a 6SL7/KT88 single ended (with MQ parafeed iron) - the amp only has two stages may that is the key.

We live and learn - well lets hope.

Anthony

BTW – Try some form of regaulation on the screens of the output.

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