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Old 25th August 2012, 11:45 PM   #31
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Quote:
Originally Posted by jonusgrumby View Post
I bought one of the blank circuit boards on ebay a few years ago. This is the circuit that I used on it. It worked well and sounds fine, but I agree it is very complicated.

I really didn't think the original circuit was biased optimally, so I made several changes. I used the 6DT8 since they are similar to the 12AT7 but very avaialble and very inexpensive. I didn't need all the gain of the 12AX7.

The board required a trace to be cut or you'll but too much voltage on the heater-cathode. I used two heater supplies and boosted the top tube heaters.

I'd really only recommend this board if you want to experiment with the mu follower in a feedback loop.
Have you ever tried to remove C4 820pF and listen to the preamp or measure it's frequency response to see the differences with and without the C4 capacitor .
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Old 26th August 2012, 12:24 AM   #32
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Yes, if you remove the 820pF capacitor their is significant overshoot and ringing. Their are other ways to compensate the circuit but adding that zero in the response takes care of instability and the capacitor is already available on the circuit board.
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Old 26th August 2012, 05:23 AM   #33
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Quote:
Originally Posted by jonusgrumby View Post
You can use a 12AT7 directly, just make sure you have the heater wired correctly.

I don't understand what you mean by "Have you ever try to maintain the ecc82 input drive?"

I originally tried 1M at R6, I don't remember why I went to the lower value. The board showed 1K there but I made lots of changes to it. I'll see if I can find my notes.
HI,

the original circuit board design have the 12AU7 (ECC82) as the input stage while I saw it has been changed to 6CG7 on your schematic.
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Old 26th August 2012, 05:41 AM   #34
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Anytime you do a direct coupled feedback the bias point of the input tube is critical. I chose the 6CG7 since it had a greater grid-cathode voltage at my operating point than the 12AU7 did and that allowed me to set the closed loop gain where I wanted it to operate. The voltage swing from the 6CG7 is minimal, but I have found it ot be a very linear tube if biased properly.
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Old 26th August 2012, 02:33 PM   #35
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Quote:
Originally Posted by jonusgrumby View Post
Yes, if you remove the 820pF capacitor their is significant overshoot and ringing. Their are other ways to compensate the circuit but adding that zero in the response takes care of instability and the capacitor is already available on the circuit board.
I never had any problem of instability or overshoot and ringing in any stage with completely unbypassed cathode resistor , IMΗO the problem is somewhere else and you can fix it with other ways , it depends on the experience of each one ( this is not a hint to anyone ) Thanks .
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Old 26th August 2012, 02:59 PM   #36
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As I stated there are many ways to compensate this amplifier, the capacitor is one of them and there was already a place for it on the board. I altered the value to compensate the amplifier as I designed it.

When you put a mu-follower in a feedback loop you have to account for the open loop frequency response. The problem is you have three poles in the circuit and and they are too close together. Other possibilities are to alter C1 and R29 or C2 and R6 and their associated components. Be careful not to get too close to the roll off from R4 either. You could rebias the input stage as well.

You are correct, there are lots of possibilities and other ways to handle the required compensation.
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Old 26th August 2012, 05:27 PM   #37
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Quote:
Originally Posted by jonusgrumby View Post
Anytime you do a direct coupled feedback the bias point of the input tube is critical. I chose the 6CG7 since it had a greater grid-cathode voltage at my operating point than the 12AU7 did and that allowed me to set the closed loop gain where I wanted it to operate. The voltage swing from the 6CG7 is minimal, but I have found it ot be a very linear tube if biased properly.
So what is the bais voltage of the 6CG7 tube?
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Old 26th August 2012, 05:41 PM   #38
DF96 is offline DF96  England
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Quote:
Originally Posted by Dimitris AR
I never had any problem of instability or overshoot and ringing in any stage with completely unbypassed cathode resistor
The problem is not in the stage, it is in the entire loop. The capacitor gives some phase advance so the loop is stabilised.
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Old 26th August 2012, 07:23 PM   #39
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Originally Posted by DF96 View Post
The problem is not in the stage, it is in the entire loop. The capacitor gives some phase advance so the loop is stabilised.
Yes , it is true , yet I haven't any problem with such stage even if it is in an preamp consisting of two or three stages and with the use of NFB , I suspect the way that the NFB is placed in this preamp , and other cases like the layout of the PCB .
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Old 26th August 2012, 09:44 PM   #40
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I don't think it is the PC layout. I know it is a result of the multiple poles at close to the same frequency.
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