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Old 12th February 2013, 12:39 PM   #61
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So drop the cap, replace 1K with 1M, and be happy??? (And tap the valve at the upper cathode? Or keep the tapping point???)
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Old 12th February 2013, 12:44 PM   #62
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Just an idea: They use the lower tapping point, because they have a follower anyway... It's because of better DC conditions, and with the 1M we would correctly "ghost" the lower tube...
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Old 12th February 2013, 01:16 PM   #63
45 is offline 45  Italy
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Quote:
Originally Posted by HVfanatic View Post
So drop the cap, replace 1K with 1M, and be happy??? (And tap the valve at the upper cathode? Or keep the tapping point???)
I would consider to drop that circuit tout-court. Maybe you will get a much better preamp making a cascode + DC coupled cathode follower where you take the bias for upper valve of the cascode from a partition of the cathode resistor of the cathode follower and that will work as feedback network as well. You need 3 ECC88's in total for both channels. Such circuit is described in the Morgan Jones book.
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Old 12th February 2013, 07:24 PM   #64
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Hi,

that's not the point. I have a cat 1 Ebay set, I got it as a birthday present, now I would like to listen to the Clone sound magic... ;-)

So my technical question remains: Change 1K to 1M, and keep the tapping point? Or do I need to change the tapping point? I guess, the lower tapping point is due to DC conditions...

And yes: I think the is a superior topology: Maybe use the Impasse input stage without transformer and a SLCF as second stage...

But this is my idea, not CAT 1 topology. And I would like to understand the topology behind...
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Old 15th February 2013, 08:55 AM   #65
Asen is offline Asen  Bulgaria
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My two cents:
I think things with this circuit are much more complex than they appear at first glance.
My simulations show that if you put 1m instead of 1k the distortion doubles. For example the second harmonic goes up from 0,440 mV to 0,750 mV in the same measuring conditions. In addition I can easily observe some nonlinearity in the lower register.
The most specific and important thing about the CAT pre is that it’s a current feedback circuit. I think that makes all the active components and stages interdependent. I think one cannot easily change only one resistor value. Changes should be applied everywhere to work properly.
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Old 15th February 2013, 10:22 AM   #66
DF96 is offline DF96  England
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Quote:
Originally Posted by Asen
The most specific and important thing about the CAT pre is that it’s a current feedback circuit.
No it isn't. The feedback samples the output voltage, and subtracts a fraction of it from the input voltage. Normal global negative feedback.

The increased distortion you see with the 'correct' circuit could be due to the 12AX7 being badly biased. If it is on a curvy part of its characteristic then running a similar identical circuit as an active load, as this circuit does, will give some cancellation of even-order distortion. This is a useful trick for non-linear valves but not necessary for the 12AX7, which is very linear when used properly.

Given that nobody would design a circuit like that, I can only assume that the 'designer' fiddled with it until he got acceptable results, being unaware of how the circuit actually functions.
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Old 15th February 2013, 11:00 AM   #67
Asen is offline Asen  Bulgaria
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Well, I am far from saying I understand the circuit completely, but I was referring to R14. I thought it makes the input stage sensitive to current as far as all the idle current of the end-stage tube flows through it, and after that through R8 together with the current of the input stage. Maybe I’m not reading the topology correctly.
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Old 15th February 2013, 11:12 AM   #68
DF96 is offline DF96  England
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It is slightly unusual to put the output CF quiescent current through the input stage cathode resistor, but that does not make it current feedback.

Two snags with doing it this way:
- the input stage bias could be disturbed by the output stage
- the feedback loop will try to maintain frequency response down to DC but it can't do this because of the interstage coupling cap so any large subsonic input will create distortion in the first stage (there is no input coupling cap to limit this problem).

This is one of those circuits which shows more and more mistakes the longer you look at it.
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Old 15th February 2013, 12:15 PM   #69
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Hi.

I believe that the way to design such a circuit is to first design it on paper, using published tube characteristics and a ruler. I know that 12AX7 has the reputation of being very linear, but that is on paper and not in an improperly biased circuit. I would then breadboard the circuit and verify how well it works. I would start without negative feedback, which is a little hard in a circuit with DC coupled feedback. When I have verified that the circuit operates properly and "clips" symmetrically for a slightly overloading input, I connect the feedback. The reason I do it like that, is that negative feedback hides so many design errors that are difficult to find with the negative feedback active. My design methodology does require the use of an oscilloscope and a sine-wave generator, but that should be minimum requirements for a design-effort.
Circuit simulation is in my opinion not replacements for reality. If simulation says that there is distortion in a circuit, that may mean nothing more than the simulator has distortion, and that is it. The models used for characterizing a device is nothing more than a model.

Hans J Weedon.
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Old 15th February 2013, 12:27 PM   #70
DF96 is offline DF96  England
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Yes.
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