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 cascode vs asymmetrical cathode coupled
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 16th April 2008, 08:22 AM #31 revintage diyAudio Member   Join Date: May 2007 Hello Darius, My guess from yesterday about feedback was right. This is also confirmed in tubetvrs post and some simming this morning. Simplified there is local feedback formed by the output impedance of the CF and the anoderesistor of the preceding stage. So my talk about 3dB or 6dB was wrong, it is componentvalue depending, sorry! It will allways be a loss in the CF added to the feedback-loss(lower gain, better linearity and higher output impedance) from the next stage. So to minimize this one should use a high gm CF followed by, for example, a cascode. Brgds Lars
 16th April 2008, 09:03 AM #32 oldeurope   diyAudio Member   Join Date: Mar 2008 gain calculation explained http://coupling-triode.blogspot.com/ Voltage gain calculation of the asymmetrical cathode follower in three steps: 1. gain of the cathode follower: Input resistance of the grounded grid stage is 8KΩ. This resistance is parallel to R4 =22KΩ. R load 8KΩ ll 22KΩ = 5900Ω The output resistance of the follower is 1/mutal cond. = 400Ω This value comes from the data book ECC83 2mA. The output resistance of the follower and the load resistance are dividing the no load output voltage of the cathode follower: 5900Ω / (5900Ω+400Ω ) = 0,94 [gain reduce caused by load] The no load output voltage of the cathode follower is reduced by the amount of signal voltage between grid and anode divided by µ. See triode transformer mode. This is calculated by 1-1/µ [ECC83 µ=100] 1-1/100 = 0,99 [open output gain] The no load gain multiplied by the gain reduce caused by the load gives the gain of the cathode follower stage: 0,99 x 0,94 = 0,93 [this is the voltage gain of the cathode follower] 0,93 2.gain of the grounded grid stage: The mutal conductance s is 0,5mA/V. This value comes from the data book. The anode load resistance is 800KΩ it comes from the resistor multiplier triode V3 of the SRPP arrangement. The no load resistance Ri at the anode is: Ri = µ/s = µ x 2KΩ = 200KΩ . Both in parallel: Ri ll Ra = 200KΩ ll 800KΩ = 160KΩ The voltage gain is (Ri ll Ra) x s = 160KΩ x 0,5mA/V = 80 3.To get the total gain of the asymmetrical cathode follower the gain of the cathode follower and the gain of the grounded grid must be multiplied: vu = 0,93 x 80 = 74 [this is the voltage gain of the asymmetrical cathode follower amplifier] 74 = 37dB Kind regards, Darius
tubetvr
diyAudio Member

Join Date: Jan 2003
Location: Sweden
"Oldeurope" in answer to your post 30.

You still don't get it, the gain of a cathode coupled stage and a grounded grid stage is not the same as I explained in my post, this is well known and if you don't understand this you can look it up in any professional book about tube amplifiers. Please find out what the input impedance is for a grounded cathode and a grounded grid stage is and then you can start to understand how this affects the stage gain.

The book I refered to that was written in 1936 describe exactly the same circuit as you describe and give values for gain either for each part or for both tubes, (here they assume that both tubes are equal but the formula can easily be modified for a case with different tubes).

It is difficult to understand your comment
Quote:
 As posted in #24 these books will definitely not help you understanding.
as the book I refer to describe exactly your circuit.

If you believe that you have invented a new circuit I must tell you that you are wrong, this circuit is very old and has been described in several classic tube books, do you just ignore all knowledge that is available from these classic works?

BTW your equation for gain of a cathode follower is also wrong, (hint, yours is too simplified).

Regards Hans

 16th April 2008, 12:09 PM #34 tubetvr diyAudio Member   Join Date: Jan 2003 Location: Sweden More about the grounded cathode - grounded grid amplifier. See here http://www.livinginthepast.demon.co....y/vappccts.htm down on the page "cathode coupled circuit" here a simplified formula for the gain is given and a statement which is of interest: "The cathode-coupled amplifier behaves like a grounded-cathode stage having a valve of amplification factor m and an anode resistance greater than twice rp of one of the triodes" Meaning that the gain with any given load is lower than for the ordinary grounded cathode stage. Regards Hans
oldeurope
diyAudio Member

Join Date: Mar 2008
#34

Quote:
 Originally posted by tubetvr More about the grounded cathode - grounded grid amplifier. See here http://www.livinginthepast.demon.co....y/vappccts.htm down on the page "cathode coupled circuit" here a simplified formula for the gain is given and a statement which is of interest: "The cathode-coupled amplifier behaves like a grounded-cathode stage having a valve of amplification factor m and an anode resistance greater than twice rp of one of the triodes" Meaning that the gain with any given load is lower than for the ordinary grounded cathode stage. Regards Hans
Hello Hans,
oh oh, your link does not look good
And I think it is the same with your books ...

Darius
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 16th April 2008, 02:16 PM #36 revintage diyAudio Member   Join Date: May 2007 Hans: I see no wrong in simplifying some of the tube equations. IRL the spread af tube data have to much influence. Darius: In simple terms, you forgot point 4 where you should include the current feedback formed by the source impedance of the CF and the load of the second stage load !
oldeurope
diyAudio Member

Join Date: Mar 2008
Quote:
 Originally #36 posted by revintage Hans: I see no wrong in simplifying some of the tube equations. IRL the spread af tube data have to much influence. Darius: In simple terms, you forgot point 4 where you should include the current feedback formed by the source impedance of the CF and the load of the second stage load !
Hello Lars,
this is done by calculating the cathode follower, see [gain reduce caused by load].

BTW: source impedance cathode follower is <400&Omega;
grounded grid input impedance is >8K&Omega;
Please note: If you take a pentode, Transistor, Cascode ...
the input impedance is only the mutal conductance of the
element. (Here 2K&Omega; ). In this case the loss caused by the current feedback formed by
the source impedance of the CF and the load of the second stage, means the loss
of the cathode follower caused by output voltage dividing, would be unacceptable.
This is why I wrote in post #24 that this is a special triode topology.
I need no cathode decoupling cap (LED etc.) and don't have its influence in sound.

thanks and
kind regards,
Darius

tubetvr
diyAudio Member

Join Date: Jan 2003
Location: Sweden
Oldeurope I must say that your way of argumentation is odd to say the least. I think the least you can do is to read what other people write, I wrote:

Quote:
 See here http://www.livinginthepast.demon.co...ry/vappccts.htm down on the page "cathode coupled circuit"
Which you chose to ignore and copy what is written on the same page about "Modified cathode follower". If you scroll down a few lines on the same page you find Composite amplifiers which describe "cathode coupled circuit" and the cascode. You must be the only one who didn't found this on the page.

Regards Hans

oldeurope
diyAudio Member

Join Date: Mar 2008
#38 I am sorry but...

Quote:
 Originally posted by tubetvr Oldeurope I must say that your way of argumentation is odd to say the least. I think the least you can do is to read what other people write, I wrote: Which you chose to ignore and copy what is written on the same page about "Modified cathode follower". If you scroll down a few lines on the same page you find Composite amplifiers which describe "cathode coupled circuit" and the cascode. You must be the only one who didn't found this on the page. Regards Hans
Good evening Hans,
at first you have to learn how to calculate a cathode follower.
I explained it several times in this thread and in the blog but
you ignore it. Should I ignore your posts ?

D.

tubetvr
diyAudio Member

Join Date: Jan 2003
Location: Sweden
ReVintage,
Quote:
 Hans: I see no wrong in simplifying some of the tube equations. IRL the spread af tube data have to much influence.
I don't agree, one uncertainty is then added to a simplification which gives an even higher uncertainty about the end result, design is about predicting results by calculations and after measuring the end result also explaining any possible differences between theory and practice.

We can take a cathode follower as an example to show my point, if you calculate the output impedance of a high µ triode with the simplified formula that oldeurope gave us in this thread you get a result that is reasonably accurate, it is OK but still incorrect. However if you use the same formula for a low µ power triode as a 6C33C the result is very inaccurate. A 6C33C has a µ of typically 2.7 and a Rp of 80 ohm which gives a gm of 33.75mA/V, with the faulty formula this would give an output impedance of 1/gm which in this case is 29.6 Ω. If we instead use the correct formula Zo = 1/[(1/Rp)+gm] we get 21.6 Ω a 37% difference which can't be seen as insignificant.

The reason why I contributed in this thread is due to the obviously faulty statement that the gain in a cathode grounded stage and a grounded grid stage is the same which it is obviously not. even though the grounded grid stage is not use much in audio amplifiers it is for instance important to know the gain as the typical input for the feedback loop at the first stage cathode is a grounded grid stage from a gain point of view.

How should beginners in the tube hobby learn to do things if we spread inaccurate statements in a forum like this? All tube stages that have any practical use has already been analysed in detail many, many years ago and the literature is not so hard to find. Simplifications are useful but they don't give the exact knowledge that is possible to find and it is a risk that common simplifications is taken as the exact truth which could introduce problems in many cases.

Regards Hans

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