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In Reply to: Re: a thought on speakers; my own limited experience showed..... posted by Naz on July 8, 2006 at 17:51:17:
They are good when you need power: orchestra, big choir, organ.They allow using broad variety of speakers.
Pentodes sound natural and undistorted when used properly. I have been there, I've heard sound of triode and pentode connection, and I like them both.
Saying pentodes are not good is same as saying that classical tube amplifiers are no good. Macintosh, Marantz, Scott, Harman Kardon, all no good. Sorry guys, I disagree.
Follow Ups:
probably anything can be made to sound good if optimised. I personally don't believe in absolutes, especially if I don't understand the forces at play that makes one thing different from another. And I can't say I understand or agree with (yet) the theories I've read for what makes Triode strapping sound different.I can only comment on my personal experience thus far and in every case Triode strapping subtly and sometimes dramatically made the sound more relaxed and three dimensional. I'm also reasonably sure from memory that the distortion characteristics are improved in Triode mode. How much of this is due to the fact that internal rp is reduced (and its consequential effect on the load line) I don't know.
If that has not been your experience it may be interesting to explore how you are using Pentodes in a different way to try and make some sense of it. Can I ask what sort of load criteria do you apply?
Naz
in a heated triode-vs-pentode exchange here a couple of years ago. My points were:1. One of the ways to reduce distortion is to use the amp at a small fraction of its power. Can be easily done with pentode amps, and not so easily with triodes.
2. Pentodes are quite linear with Ug2
3. Triode sound is euphonic, but pentode sounds closer to reality. 3. Due to high plate resistance, some NFB is needed to reduce amplifier's Zout. I know that for many NFB sounds like obsenity, but it is my belief that 10-15 dB of NFB don't do much harm. Pentode amplifier can can work very well without NFB at all if it is used for midrange and highs only, preferably with a fast decay driver, like BG Neo-8.
Its a pity that the overwhelming majority here are so convinced in the superiority of triodes that they never bother to experiment with pentodes.
With Ug2 I meant that it should be considerably higher than Ua and have good regulation.
If you have a class A amp with no feedback, distortion is a function of signal level and since most listening is done at small (average) signal levels the sound will be nice and clean. But with a class AB amp the feedback loop introduces a distortion that is not signal dependent and might hurt low level detail.If my thinking is correct it would imply that the class A amp would measure best at low level but the class AB amp would measure better at higher output (perhaps best right before class B). Maybe someone has the ability to measure and report on this.
Throw in the facts that pentodes are not as tolerant of load variation (and we have a big one when we transistion to class B)and the fact that I for one can't think of a class A pentode amp with global feedback and it makes it even harder to say just what impact global feedback alone has and how much it is a mixture of design compromises that seem to be included with global feedback amps in general.
Russ
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
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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