feed back etc?

[steve s]

Originally Posted by acorn

Hi all

I believe mullard had a problem concerning phase shift with there 5/20 valve amp, apparently the more stages you have the more phase shift distortion you get so the general rule is to keep it as simple as possible, so the sweet sound you get from smaller power amps may account for fewer stages used compared to there larger counterpart which use more stages to obtain the power.

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Regards Acorn

thats right they manged to get within 10 degrees on the 5-20, but the 5-10 el84 amp was alot worse.. nearly double..
they are both 3 stage amps, voltage amp,phase splitter and p/p output
there are not many amps with less

I'm not sure about the number of stages affecting phase shifts, my old wireless world mags say phase shifts are not present in any amount until global feed back is introduced,
they also stated that the feed back is required in pentode/tetrode amps to right the uneven amplifaction acros the frequency range... they had name for it ending with distortion.. I'll have to have another read..

I'm sure when i get near 5% distortion i can tell through the lowthers..
the bass can sound like it has more body and depth..
but the clean sound i was getting through the pt15 amp was very low distortion.. but was only giving me 2-3watts but it sounded so good
I've been going through it the last week or so trying to get the det25 working in place of it.. now I'm trying to find a suitable mains transformer..
I'll probably end up having to get one made
in the meantime have quite got used to the sound of distortion...

cheers steve



cheers steve

[petercom]

Feedback is necessary in pentode/beam tetrode amplifiers because the anode impedance is incredibly high, basically the valve just conducts a current which is a function of the grid voltage. If you didn't apply feedback the output impedance would be high and response would be all over the place dependant on transformer and loudspeaker impedance.

Applying feedback flattens the response and provides a level of speaker damping due to the reduction in output impedance. If you like to look at it another way the feedback tells the valve how much current to deliver to provide a linear frequency response and, if the output impedance changes due to the loudspeaker load, how much current to provide to maintain the output level.

You don't need much feedback to do this, 12-14dB will do it. Some designers like to increase feedback level to lower distortion - you can dial it in to personal taste.

There is poor evidence to indicate whether harmonic distortion is damaging to our appreciation of a musical performance. The ear seems quite tolerant of levels of harmonic distortion up to 2 - 3 % providing that the harmonics are presented in proportion to the music signal. In other words a graded series of harmonics will not unduly 'spoil' the sound quality.

If the distortion is not harmonically related, however, like IM distortion, it stands out like a sore thumb. Similarly if harmonic distortion has higher levels of, say, odd harmonics compared to even harmonics the ear hears them as a coloration.

With low powered amps the distortion may well be high on musical peaks but, as the ear is more sensitive to distortion at low sound intensity levels, it is the sound quality in the 0.1 - 1 W region that really counts.

[NealG]

Re the point about the 5-20, are you not mixing this up with the Williamson 4 stage amp?

The 5-20 is a very stable 3 stage amp with just one RC coupling. I don't have the Mullard book to hand but I recall 10db of headroom. Phase shift is present in all amps it is only of concern in a loop feedback amp if you have enough RC time constants to shift the phase 180deg and the loop gain at the 180deg shift frequency is not less than 1 IE if the loop gain at the 180deg F is greater than 1 positive feedback turns the amp into an oscillator.

As Peter has stated FB lowers output impedance, extends bandwidth and flattens response, however, there is a whole raft of issues with it that cause more issues than FB generally solves.

[petercom]

Originally Posted by NealG

As Peter has stated FB lowers output impedance, extends bandwidth and flattens response, however, there is a whole raft of issues with it that cause more issues than FB generally solves.

Entirely agree, however the message was in response to the question as to whether you have to have feedback with pentode/tetrodes. If you want to dispense with global feedback on triode amps then you can.

[James D]

In that case, then I totally disagree with you and since I have two working power amps with pentode SE outputs without global or local feedback and about seven without global feedback I can prove you are wrong...

It just the conventional and 'established wisdom' that it is necessary to employ negative feedback in pentode power amplifiers to get them to work... And as usual with such assumptions there is a lot of truth in them but it isn't a necessary condition - it is the easiest way to get them to work but it isn't necessarily the best sounding...

One shouldn't let 'conventional thinking' restrict your consideration of solutions when looking for the best solution... conventional wisdom is usually hidebound by commercial and expedient considerations - if you can't think your way around them then what is the point in being an engineer? I wish I had a pound for everytime I have had to 'push' engineers to think there way through problems from first principles so they actually understand what is happening and can form a judgement on what is the best way forward rather then just copying what has worked before... Its amazing how good there solutions can be...

Sorry this sound like me critising Peter, I not, he has proven many times that he thinks things through and finds interesting solutions, as the WD25 shows. I just can't let the pentode and feedback remark pass

James

[petercom]

Not at all, James, I am pleased you feel you are getting good results from pentode SE without global feedback.

The problem with a forum such as this is that we often have to deal with generalisations when discussing the rights and wrongs of certain topologies. Of course there are the exceptions to every 'rule' as the rules are only there to guide us and, you are quite right, we shouldn't take them as 'gospel'.

It isn't absolutely necessary to employ global feedback with pentodes to get them to work (sorry that the 'necessary' in my previous post inflamed you). However, as I pointed out, without feedback they are very sensitive to load impedance. I can imagine that a full range driver with only a small vc inductance would be best for the 'no feedback' arrangement. I would be interested to see how you have coped with this.

What I didn't want to happen was for people to rip the global feedback out of their Kat6550s and expect good results!

[steve s]

can't the right cap across the transformer secondary do nearly the same job as feed back with a pentode?...
but would that only work with a single driver..or would it have to be calculated to include the xover..

go back 50 years when they knew much more than we do now.. the feed back was designed to suit the type/ size/performance of the speaker used..so speakers and amps where 'matched'...


steve

[NealG]

However, as I pointed out, without feedback they are very sensitive to load impedance. I can imagine that a full range driver with only a small vc inductance would be best for the 'no feedback' arrangement. I would be interested to see how you have coped with this.

Exactly Peter that was my understanding as well so James post is very curious and interesting, I would like to know more about your two non FB Pentode amps James.

Steve, no, not that I know of.

[steve s]

just found the info about a cap replacing the feedback


in 1944... it was customary to use negative feedback or a condensor across the output transformer primary to overcome the 'excessive high note response'... this is because pentodes are approximately a source of constant current, and triodes are not....

so you can run a pentode without feedback..

[NealG]

Sure you may be able to operate one without feedback but I can't see how that cap compensates for the dynamic load nature of a 'speaker which causes a Pentode output stage to produce large amounts of nasty third, seventh etc harmonic distortion when operating into a none optimal load. I suspect that cap bandwidth limits the output stage or I could be talking the normal amount of spherical objects...