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Old 27th August 2004, 05:42 PM   #11
SY is offline SY  United States
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Default "You CAN, guru, you CAN!"

Whether you get a bump or not ought to (I think) only depend on the relationship between the time constants of the amp itself and the feedback loop, not their topology. At low frequencies, the circuit shown tends toward unity gain. With a conventional (by valve standards!) feedback connection, the gain tends toward... well, the "normal" closed loop gain. When you get peaking, it's because the time constants in the open loop (coupling caps and transformer, mainly) aren't sufficiently separated. That would be the case even more so if I were taking the closed loop gain to unity. In either case, staggering the time constants properly will eliminate any tendency to peak for either connection.

What am I missing?

(note in passing: if you could read those values, you were a much better behaved teen than I was)
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Old 27th August 2004, 05:56 PM   #12
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Well, the output transformer primary inductance changes rather a lot with level. Juggling the time constants to achieve stability with the traditional feedback scheme generally means making a coupling capacitor rather small. And that means that feedback that should have been mopping up distortion in the output transformer is correcting LF loss in the open-loop amplifier.

Of course, you could juggle the time constants by making that cathode bypass electrolytic really large. But that ensures trouble when you overload the amplifier.

I couldn't read those values either. (The hair on the palms of my hands gets in the way.)
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Old 27th August 2004, 06:15 PM   #13
SY is offline SY  United States
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Default I'm blind, I tell ya!

Well, let's take an example where the feedback resistor is tied to the top of the cathode bias resistor and hence directly to the cathode. In fact, let's do that with the example circuit. If we go to a 12AX7, we will need roughly a 1K cathode resistor, with a proportionately larger feedback resistor. With 100K on the plate, the first hole should give a gain of roughly 35 (open loop), which is adequate given the gain of the following diff amp. For a plate resistor of 200K, the gain rises to roughly 55. The feedback resistor will be something like 20-40K, very workable values.

So, where would the advantage be in taking the closed loop gain down to unity? Doesn't that challenge the stability of the circuit by increasing the feedback factor if it happens before the open-loop rolloffs? And if it happens at a frequency well below the open loop rolloffs, what good is it doing?

Sorry to be a pest, I just want to wrap my head around this a bit better.
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Old 27th August 2004, 10:22 PM   #14
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Agreed, taking the closed loop gain to unity means that the feedback factor is greater, and that risks LF instability. But look at the values that were typically used for that cathode capacitor. At smallish signal levels, the primary inductance of the output transformer is high, so the amplifier has gain down to a very low frequency. But the cathode capacitor prevents negative feedback, causing a bump in the LF response, typically at about 1Hz, putting it right into motorboating territory. The result is a frequency response and stability that's dependent on level (output transformer primary inductance).

I think it's a really good idea to try to make everything stay as stable as possible with level. Mind you, I would rather not have that cathode capacitor in the first place. I'd rather use a differential pair for the feedback or just ditch that capacitor. Another interesting possibility is to say "Who cares if the amplifier inverts?" and use shunt feedback.
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Old 27th August 2004, 10:49 PM   #15
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Quote:
This his a WAD schema. On this net URL you can see a recent build version based on with an optimised PSU. Sorry but it's in french.
Yeah, that is same thing, and values are easier to read . Sorry for that but I have to resize it to put it in here, I think I 've done hell of a job. Still, there is a matter of 6au6, I'm bit confused what I've got to do to replace it

Lurking around, bump into this http://www.ptsoundlab.com/sectubes/s...ssa40w6550.htm is it possible to convert this one from ultralinaer into triode mode ( again, what to do?).

Sorry to be 'pain in ...' ,Thanks in advance
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Old 27th August 2004, 11:56 PM   #16
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To convert to triode, remove the 2k2 screen resistors. Tie the screen of each power tube to its plate using about a 100 ohm 2W resistor.

You may have to diddle the compensation a bit to get the cleanest square waves.

So, what's the theme? Convert all the triodes to UL and all the ULs to triode?
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Old 28th August 2004, 12:05 AM   #17
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Hi,

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Still, there is a matter of 6au6, I'm bit confused what I've got to do to replace it
You could replace it with half of a 12AX7 for instance after adjusting plate load resistor and bias resistor or configure it as a diffamp with the second triode's grid either grounded or as input node for the NFB loop.

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So, what's the theme? Convert all the triodes to UL and all the ULs to triode?
ROTFLMAO. I thought school hadn't started yet...

Cheers,
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