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Old 12th March 2009, 10:12 AM   #11
SY is offline SY  United States
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You can't tell one class from another from the schematic alone. You need to look at the operating points and load resistances.

The best hundred bucks you'll ever spend is buying Morgan Jones's two books. That will let you get a firmer handle on the theory rather than randomly flailing, and he even has a section on specifically how to tune feedback and compensation in a tube amp.
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Old 12th March 2009, 01:08 PM   #12
john65b is offline john65b  United States
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I have also experimented with my amps feedback. I have a much more simple amp (6SL7 into a pair of 6L6GC). I put a 100k pot in for the 30k feedback resistor. It sounded less anemic as the resistance went up. I accidentally turned on the amp with pot set to low resistance and all sorts of hell broke loose...arcing across some tubes, nasty sound, etc...I shut it down quickly before any major damage....

I now have a switch to turn on 50k FB and turn off FB entirely. The original was 30k (this is a home brew jobbie a friend of a friend built)

I do have some major hum with less FB, but I have plans to add another RC filter to the power supply and maybe put DC on the heaters to reduce the hum.

I cannot detect "flabby bass" that no FB designs often have, so maybe I will leave it with no FB...

I should get MJ's books and tune the FB as Sy says. I just get monster headaches when I read too much.
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Old 12th March 2009, 01:16 PM   #13
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John, Morgan's books are entertaining and astoundingly clear. Not a headache in the bunch.
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Old 12th March 2009, 04:33 PM   #14
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Quote:
Originally posted by SY
John, Morgan's books are entertaining and astoundingly clear. Not a headache in the bunch.
Not quite so.There are some startling omissions.
SY.. My Morgan Jones book 3rd ed Vamps has no mention anything on pole/bode stability. (and I havenít ripped the pages out..) Iím suprised at this omission and no mention with the feedback components. In fact Iím often referring to my switchmode bible. Which book(s) have you got ?

I differ with your intepretation on not being to detect class ABC from schematics. The PSU is often the give-away clue.
.John....One reason why your amp is class AB is that the current through the front end heaters is fairly constant from no to full load condition as the difference in class A even towards AB1(class B on peaks) is a mere 10% more. The long time-constant of the heaters evens this small difference out. If it was true class B or even class C then the ratio between no load and full load would be too large and this heater technique from o/p stage cathode currents cannot be used. The simple concertina phasesplitter cannot cope with class B demands, so again this can only work when no grid current flows, ie o/p stage in class A.

Advocates of the non Global feedback lobby often fail to realise itís the absence of feedback that often causes more troubles!

A sine wave is for setting levels. One can easily determine the amount of global nfb by measuring voltage difference with nfb loop connected/disconnected. Log20 delta (Vout no nfb/Vout with nfb).
A square wave for a tube amp spells truth about its performance. No myth here.

richy
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Old 12th March 2009, 07:00 PM   #15
m6tt is offline m6tt  United States
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I spent months trying to design a indirectly heated pentode power amp with no negative feedback globally (all local). It was noisy, quirky, and did all kinds of strange things when you so much as put your hand near the tubes. It's a boat anchor-style guitar amp/multi-power amp public address, so the usual attempts to make it frequency flat were unnecessary...I just use a ~50-100pF cap in parallel with a 50k pot to find the sweet spot.

Now it's dead quiet, overdrives *better*. Too much NFB is just as oscillation prone as too little, and it all goes crazy when the amp is overdriven. I imagine though that applies to hifi as well, due to transients etc. I find it's better to use the global loop to nail down the hi frequency response and provide a bit of damping for the outputs, and then use local feedback elsewhere in the amp.

Done properly, NFB is important with pentodes, they're vastly different than triodes in that respect. Triodes have inherent NFB, pentodes need it developed externally for similar performance.
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Old 12th March 2009, 07:05 PM   #16
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Since you have a scope you may want to check the frequency response of the amp with and without any nfb. You should see the low and high frequency response roll off earlier with no feedback.

As far as the cap goes.. I believe that it is used for phase correction. As different frequencies go from the input to the output of an amp there is a non equal shift in phase depending on the frequency. As you apply nfb from the output into the input of the amp and especially with a lot of nfb, there is a chance that one end of the frequency (I forget if it's high or low) will will wrap around and come back at positive feedback instead. So you end up having the amp starting to oscillate at certain frequencies with the other end of the scale is dampened. The cap is there to kinda straighten out the phase shift to keep it all negative feeback instead of mixed negative and positive feedback.

You can try different values of pico farad caps and see how they effect the overall frequency response of the amp.

-keith
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