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Old 2nd July 2012, 10:55 PM   #21
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Old 2nd July 2012, 11:02 PM   #22
7N7 is offline 7N7  United Kingdom
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Originally Posted by Mr. dB View Post
My question, though, is why bypass the cathode then throw away the gain with a divider, why not instead leave the cathode unbypassed and then connect the next stage directly to the first stage's plate?

Anyway, I hope formula 22 won't be discouraged by the initial reception this thread has received, and will continue with this project.
I should think that an unbypassed cathode resistor would provide even more distortion surely? Especially with such a small anode load. Characteristics all over the place I should think.

I agree with your second comment of course, valves are great and for using!

Paul
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Old 2nd July 2012, 11:48 PM   #23
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I should think that an unbypassed cathode resistor would provide even more distortion surely? Especially with such a small anode load. Characteristics all over the place I should think.
??? Howso?

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Old 3rd July 2012, 12:02 AM   #24
7N7 is offline 7N7  United Kingdom
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??? Howso?

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Chris
Variation of ra and its consequences (Rout etc.)

Much modern thought , using usually solid state devices, aims to nail down valve characteristics; I am doing it in a small way with the pre-amplifier I mentioned. For a most interesting approach see the thread concerning type 3A5.

Paul
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Old 3rd July 2012, 12:07 AM   #25
kevinkr is offline kevinkr  United States
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Guys be nice to the OP.

A few well directed comments about potential improvements (with examples) would be nice - even if the OP chooses to ignore all of the excellent knowledge on tap here.

Led bias, CCS loads, white followers, etc. You know the score. Referencing Allan Wright's (RIP) excellent writings is a prod in the right direction.
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Old 3rd July 2012, 12:19 AM   #26
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Originally Posted by 7N7 View Post
Variation of ra and its consequences (Rout etc.)

Much modern thought , using usually solid state devices, aims to nail down valve characteristics; I am doing it in a small way with the pre-amplifier I mentioned. For a most interesting approach see the thread concerning type 3A5.
Under what possible circumstance would degeneration not improve linearity?

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Chris
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Old 3rd July 2012, 12:41 AM   #27
kevinkr is offline kevinkr  United States
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Under what possible circumstance would degeneration not improve linearity?

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Chris
In theory when RL is a lot less than the degenerated rp of the triode in question.. The RL in this design is already far too low for good linearity even without cathode degeneration. (The minimum RL for an ECC88 for acceptable linearity IMHO is in the range of 10 - 12K)

I've found there was an improvement in linearity in most cases with the addition of local cathode degeneration, but I don't deliberately load them below rp so I am not certain this would apply in a case where RL was significantly less than degenerated rp - probably a quick sim would indicate what to expect in this case. I've also found no reason to use degeneration for any reason other than to reduce stage gain..
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Old 3rd July 2012, 01:11 AM   #28
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I think one thing we're saying in different ways is that there are better ways of loading the anode than a resistor, whatever the value. 7N7 together with many others is advocating anode loads and solid state devices and the #26 preamp thread is advocating either a good plate choke or an output transformer. When it comes to the cathode, the #26 pre amp thread has been promoting filament bias with cathode resistors as small as 5 ohms, so bypassing such a resistor isn't even an issue. Kevin and many others have been using battery grid bias for ages.

You can't just slap the term "high end" on generic anode resistor+bypassed cathode resistor stages without having done some intelligent exploration of all the more advanced topologies modern preamp designers have been developing and using. Look at all the work that's gone into the Aikido project, for instance. By all means use generic gain stages that date back to the Bronze Age if you want to and like the sound. Drive them with water mills and mice going round in wheels, but don't trivialise the huge amount of work that so many designers have put into optimising the simple tube gain stage. And as 7N7 says, we haven't even started on the power supply.

Andy

Last edited by andyjevans; 3rd July 2012 at 01:38 AM.
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Old 3rd July 2012, 04:55 AM   #29
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Guys, any changes to this schematic preamplifier that lead to sound better, I accept.
So give suggestions to go further, ...
thank you!
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Old 3rd July 2012, 06:18 AM   #30
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The problem, unfortunately, is that if you use anode and cathode loads more appropriate for linear operation, as recommended above, is that you will end up withtoo much gain.

Please note how this design already has "tapped" the anode resistor (taken the output from the mid point of the 2 anode resistors) to throw away about half of the gain of the 1st stage. ECC88 in Grounded Gathode has too much gain for a line preamp.

An anode follower as 1st stage will do the job i think. It has adjustable local feedback.

Last edited by costis_n; 3rd July 2012 at 06:43 AM. Reason: clarification
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