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In Reply to: Tell me what you think posted by Russ57 on July 11, 2006 at 08:57:34:
We are mixing terms a bit here. In theory, NFB is applied as a ratio and therefore should affect all levels by the same percentage. Whether it does in reality is another thing. Obviously NFB can be applied to any type of amp using any tube but you are correct that zero feedback Class A will indeed produce lower distortion at low levels.Regardless of tube type, Class A is inherent in SE but could be used in PP whereas Class AB refers only to PP. AB suffers most on the transitions from A to B and that translates into higher % distortion at low levels due to the fact that all tubes are very non linear near the cutoff point. This could be improved by reducing the applied load but it would have to be much less than would normally be used in the OP stage of a power amp. The problem with this type of distortion, as you rightly put it, is that it occurs at levels where we typically do a lot of our listening.
If there is a consistency in my book it's that those who like PP tend to prefer them for their high power or their sound at higher OP levels. Few seem to choose them for their sound at low levels whereas for SE it's the opposite. This does seem to coincide somewhat to the distortion characteristics of the two topologies ... at least as far as ratios go.
The high output impedance of pentodes compromises the OP stage too much and considerable NFB would have to be used to improve the situation. In this regard my comment was borne from the observation that every time I've triode strapped any tube it sounded better and since no other changes were made I wondered how much of this was due to the fact that triode mode offers lower OP impedance and therefore lower distortion by default. Personally, I think it's not the primary reason and would like to hear other theories.
I also think that NFB is the least damaging to amplifiers that are inherently wide-band before NFB is applied. In other words, I think it can somewhat successfully be used to correct for non-linearities but probably causes too many phase related issues where its primary use is for extending bandwidth.
Naz
Follow Ups:
When you triode strapped a pentode it was a Pentode amp with a typical ( read CRUMMY ) G2 supply. sser2 is talking about a Pentode amp with an optimized G2 supply.I built such optimized G2 supplies in my Paoli 60M projects ( posted on this forum ) and I must say, these amps were nothing like the amps we typically experience. A whole new ballgame, with detail and lotsa good old fashioned and TRUE authority.
Jeff Medwin
You are correct in that the mod was the standard typical mod. I only comment on what I've experienced so I can't argue that what you say is wrong. Where a new experience supersedes a previously held belief then I have no problems reporting that. A good example is the difference of opinion we once had regarding bias. I have not changed my mind per sae but have since experienced that under SOME conditions lower idle current can be beneficial eg, in the driver stage of an SE amp.This is something that I observed to sound better and because it was at odds with almost all of my other experiences I investigated in pursuit of a plausible reason ... hence the primary reason for this whole thread. BTW, I still believe that higher idle current sounds and measures better in most situations.
sser2 did make the comment that triodes are euphonic, and that, I most definitely don't agree with. Done right they are very natural and provide a real sense of space that I have not heard from other tubes ... not that I'm saying it can't happen. I'll try optimising the G2 supply one day and report back with my impressions and with measurements. I wonder if it might happen to work well as a driver, ala lower bias on a triode?
Naz
Hi,Well, someone like Dennis suggests on this forum that tubes, transformers, lawnmower and car engines, etc., all work best at about 5/8ths to 2/3rds of their maximum ratings.
The rating on a tube is derived of two components which we can adjust........current and voltage.
Maybe we need to routinely use lower voltages and "decent" current, ( below the published A1, etc. " by the book" operating specs ), to gain a more relaxed sound. Also gained, MUCH higher component reliability and tube life.
I've heard this effect, a "sweet spot" on Finals listening to music, and Wm. Lee has made similar reports up here. Listening to the "by the book" ratings sounded strained in comparison. This listening was done by me on amps whose supplies were quite optimized, and dare I mention, low in DCR.
Jeff Medwin
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