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Old 9th August 2015, 11:42 PM   #1
zlab is offline zlab  Hong Kong
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Default Looking for a SRPP Calculator

Hi
I am looking for a SRPP calculator for 12AX7 & 420a, I am not so good to just getting ibea from raw tube information, thanks. I have a project to rebuild first stage of a tube amp and plan to use 420a.
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Old 10th August 2015, 12:45 PM   #2
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here
Attached Files
File Type: zip tube calc.zip (95.3 KB, 47 views)
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Old 10th August 2015, 03:22 PM   #3
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Hi

I am sorry to say that this SRPP calculator is wrong.

At least there are 2 mistakes.

1 - Rg2 should be a short

2- the output résistance (at top of Rk2) is much more smaller than the value given by the calculator

Regards
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Old 10th August 2015, 03:43 PM   #4
zlab is offline zlab  Hong Kong
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Thanks, it’s still a great step forward.
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Old 10th August 2015, 04:32 PM   #5
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Hi

I would like to say that in order to get the benefit of a low distortion level produced by the SRPP topology, the 2 tubes shall work in the same conditions and for this, Rk2 shall equal Rk1=Rk.

In this situation, the output resistance ( at top of Rk2) is given by :

rs=(Ri/2) * (Ri+(2+µ)Rk) / (Ri+(1+mu) Rk) soit proche de Ri/2

Regards
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Old 10th August 2015, 05:22 PM   #6
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Hi

I have to add that the formula I gave last post is with no capacitor in parallel with Rk1

With capacitor Ck in parallel with Rk1 ( and no capacitor in // with Rk2 ), the output resistance is given by :

rs=Ri*(Rk2+Ri) / (2 Ri+(1+µ) Rk2)

And with capacitor Ck on both Rk=Rk1=Rk2

rs=Ri/2

Personaly, I avoid capacitors and I use a 6SN7 to do the job, less voltage gain but lower output resistance than with a 12AX7.

Regards
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Old 10th August 2015, 05:29 PM   #7
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[QUOTEI am sorry to say that this SRPP calculator is wrong. [/QUOTE]

That is not the only fault in the calculator, as an example Cmiller is wrongly calculated for cathode followers, there can be other faults so be careful when using this.
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Old 12th August 2015, 07:20 AM   #8
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Default SRPP calculator

The attached Excel table proved to be accurate. Conditions: Rk1 = Rk2, there is no Ck1 parallel to Rk1.
Attached Files
File Type: zip SRPP.zip (12.9 KB, 24 views)
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Old 12th August 2015, 09:38 AM   #9
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Hi

@Icsaszar
According to the definition of output impedance, it shall not include the load RL as I can read in the file (column L).

Is my reading correct ?
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Old 12th August 2015, 12:44 PM   #10
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Quote:
Originally Posted by Hyperman75 View Post
Hi

@Icsaszar
According to the definition of output impedance, it shall not include the load RL as I can read in the file (column L).

Is my reading correct ?
That is correct. The output impedance is the unloaded source impedance. The equations came from the Tube CAD Journal, May 2000 issue, available online. It seems I made a mistake when I put them in the Excel. Uploaded is the fixed version.
Attached Files
File Type: zip SRPP.zip (16.5 KB, 15 views)

Last edited by lcsaszar; 12th August 2015 at 01:02 PM. Reason: error in the calculation
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