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Old 8th March 2008, 03:33 AM   #1
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Default The Valve Wizard

This site (still growing) covers vacuum tube (valve) theory from a guitar amp perspective. I have been watching this site grow over the past year and it now contains plenty of information useful to amplifier builders both MI and HiFi. A newly posted paper called "the triode gain stage" goes into detail on how to design one, and how to get several different flavors of distortion out of it (or how not to). Several circuit variations are presented.

http://www.freewebs.com/valvewizard/index.html
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Old 8th March 2008, 03:49 AM   #2
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Thanks for the link. I've been looking for something like that...
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Old 8th March 2008, 05:57 AM   #3
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Yes, good site, I tripped over it while searching a few weeks back and had a trawl through it, worth a look.
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Old 8th March 2008, 06:12 AM   #4
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Hi,

I haven't checked everything on this site but it seems that it contains some faulty information.

Look at the chapter of Cathodyne splitter where they convey the old false statement that the output impedance is different from cathode and anode and they even advice the use of a series resistance on the cathode side to "equalise" the driving impedance to the next stage.

In reality there is nothing to equalise as the ou2put impedances are equal.

It would be nice if a site that claim to be the "Valve Wizard "did teach what is correct and not the opposite.

Regards Hans
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Old 8th March 2008, 07:21 AM   #5
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The output impedances are not equal , but if the loads are equal , the balance of the stage is perfect . Look here : www.aikenamps.com/cathodyne.pdf And they say to add a resistor in a guitar amplifier for imbalance ( more distortions ) .
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Old 8th March 2008, 07:54 AM   #6
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Sorry but you have interpreted the text wrong, that text confirms that the the output impedances ARE equal, Quote "In the case of the phase splitter circuit Zk=Zl=Z" (page 2 in the middle).

The ouput impedance can be calculated as Rout = RL*ra/(RL*(µ+2)+ra) Where RL is the anode or cathode resistor, ra is the anode resistance of the tube.

This subject has been discussed many times on this forum and the correct calculation of the ouput impedances is described in many books e.g Morgan Jones Valve amplifiers 3rd edition.

That a faulty description is circulated in many web forums and even in some books doesn't make it more less faulty.

The reason for the mistake in calculation is that the output impedance of the anode and cathode is calculated separately which of course is wrong as both sides are loaded, calculating the output impedances with both sides loaded give the result above. It is very easy to confirm this by either a spice simulation or measurements in a real amplifier, I have both simulated and measured the output impedances and can confirm that it is correct, it is interesting to notre that the output impedance is so low, in my case with a 12BH7 and 15k anode resistor you get ~300ohm.
Adding a resistor from the cathode will introduce imbalance and increase distortion.

Regards Hans
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Old 8th March 2008, 09:02 AM   #7
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I didn't interpreted the text wrong , Zk , Zl are the load impedances . The general formulas are those from my text , Zop and Zok , and only if you put the condition that loads are equal , the output impedances ( anod and cathode ) are equal.
It's wrong to say that output impedances are equal ( there are not ) without specify the load condition.
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Old 8th March 2008, 09:29 AM   #8
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This is a phase splitter is it not? In that case the loads are equal, and then the output impedances ARE equal. The only valid definition of what output impedance you have in a circuit is how it behaves e.g how the frequency response is affected and how it reacts on differences in load conditions e.g if you change the following stage grid resistors, and the only to measure it is during the same conditions, any other measurents are not valid as they don't reflect the real operating conditions. Therefore we can conclude that the output impedances of a cathodyne phase splitter are equal and it is therefore completely wrong to add any series resistor from the cathode.

Regards Hans
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Old 8th March 2008, 09:35 AM   #9
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I came across this website through a link that was posted some time ago here at diyaudio. It will be the website I will refer to when somebody asks for "an introduction to valve amplifiers".

About the cathodyne. I do not have Valve Amplifiers handy, but Broskie also seems to disagree on the output impedance of the cathodyne. Here is his explanation (the most recent blog, actually).
http://www.tubecad.com/2008/02/blog0135.htm

Erik
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Old 8th March 2008, 09:42 AM   #10
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Quote:
Originally posted by tubetvr
This is a phase splitter is it not? In that case the loads are equal, and then the output impedances ARE equal. The only valid definition of what output impedance you have in a circuit is how it behaves e.g how the frequency response is affected and how it reacts on differences in load conditions e.g if you change the following stage grid resistors, and the only to measure it is during the same conditions, any other measurents are not valid as they don't reflect the real operating conditions. Therefore we can conclude that the output impedances of a cathodyne phase splitter are equal and it is therefore completely wrong to add any series resistor from the cathode.

Regards Hans

I totally agree with you . No extra resistor needed. I said that from the beginning .
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