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Old 17th April 2013, 06:17 PM   #81
McGyver is offline McGyver  Poland
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You are right. I'll give a try 1N4148.
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Old 18th April 2013, 11:32 AM   #82
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I found the real cause (source) of the distortion. It is the diode and not so much the R values. No matter what type of diode is used 1N4007 or 1N4148.
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Old 18th April 2013, 11:42 AM   #83
McGyver is offline McGyver  Poland
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If so, the only way to protect the tube from arcing at startup is to delay switching on the anode supply. 40-60 seconds will do.
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Old 18th April 2013, 11:43 AM   #84
DF96 is offline DF96  England
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That means the first stage has too high output impedance because of the cathode degeneration, or the signal level is too high.

This diode trick is commonly used so should work OK given the right circumstances.

A neon is an alternative.

Last edited by DF96; 18th April 2013 at 11:43 AM. Reason: add alternative
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Old 18th April 2013, 01:45 PM   #85
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Quote:
Originally Posted by DF96 View Post
That means the first stage has too high output impedance because of the cathode degeneration, or the signal level is too high.

This diode trick is commonly used so should work OK given the right circumstances.

A neon is an alternative.
You are correct. I played around with the sim and if the Rp and Rc values on the first stage are "tuned" The distortion with and without the diode remains virtually the same. As McG has it they are not.

Last edited by valvesound; 18th April 2013 at 01:49 PM.
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Old 23rd May 2013, 12:14 AM   #86
Kiwi69 is offline Kiwi69  Australia
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Originally Posted by ThorstenL View Post
Konnichiwa,



Surprisingly this circuit is practically exactly that of the linestage of the full function Kondo M7, including the paralelling of unequal value cathode resistors, except for the most important part, the valve. This should be a 6072A, NOT 12AU7.

Also, Kondo builds the circuit hardwired, on copper chassis and using his own Silverfoil & Mylar capacitors plus tantalum resistors....

Sayonara
Hi where can I find the full schematic for amp and power supply plus spec's on it
Cheers.
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Old 16th July 2014, 04:57 PM   #87
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Originally Posted by john65b View Post
Yes, I agree.

Is there one operating point that is optimum for a 5687? I like the one I have a lot. Seems more bias current yields better sound, just like class A SS amps, with guidlines and limits not exceeded that is. I have read Low mu tube, high current, high mu tube low current seems best.


And one more Q for you all. Paralleled tubes - the output impedance is halved? Gain doubled? I can see the smear issue, but both sides in a 5687 tube should be matched?
the operating point aspect may be easier to grasp by starting at the output and working backwards. the output stage has a lot of current gain and slightly negative voltage gain. from the large k series resistance we can infer that the designer has assumed that the load will be hi-z. the output stage will present an even higher-z to the previous one. thus the previous stage will not need to source much current--nearly none. the allows for the use of both a reasonable plate supply voltage and a large anode resistor which behaves more nearly like a current source than a lower value would thus improving linearity. the high bias point allows for a reasonable anode supply voltage despite the large anode resistor. of course if the operating point is pushed too low (ie: even higher negative bias) linearity will degrade. so what we have here is the designer finding the sweet spot where he can exploit the cathode follower and its assumed high-z load to present an extremely light load to the previous stage and that allows him to pick an operating point that is very favorable in terms of linearity while keeping the plate supply voltage within reason. paralleling lowers the characteristic internal plate resistance of each stage which furthers the designer goal of making the anode feed closer to current source behavior. IMHO it is all very clever. the more traditional approach isn't all bad, however. all this hi-z, low current stuff has its price in terms of not supporting ill-mannered loads too well and increased sensitivity to external noise sources. both because of that aspect and the low PSRR of a pure single-ended design, this circuit will be quite sensitive to weaknesses in the PSU.
Now armed with all this it is easy to see that the 6922 design a few posts back is not really an AN7 clone. It's actually a good and relatively standard-design 6922 setup (except for the diode which would be replaced with a shunt neon bulb or removed as made needless by dint of a PSU soft start). see, e.g.: http://www.augustica.com/designing-6...ollower-ezp-17

Last edited by sfthurber; 16th July 2014 at 05:20 PM.
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Old 16th July 2014, 05:03 PM   #88
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Originally Posted by McGyver View Post
If so, the only way to protect the tube from arcing at startup is to delay switching on the anode supply. 40-60 seconds will do.
the neon shunt will work and is very quiet
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Old 11th September 2014, 09:54 PM   #89
McGyver is offline McGyver  Poland
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Originally Posted by sfthurber View Post
(...)
Now armed with all this it is easy to see that the 6922 design a few posts back is not really an AN7 clone. It's actually a good and relatively standard-design 6922 setup (...)
I never claimed that my approach is really a "clone", I only stated that is inspired by Kondo M7 and Klimo Merlin. And my setup isn't accidental, it allows me to drive almost any power amp, be it tube or solid state.
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Old 11th September 2014, 10:29 PM   #90
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and i never said that you made such a claim. i merely opined that the design was not a clone (which is a true statement IMHO).
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Last edited by sfthurber; 11th September 2014 at 10:45 PM.
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