Damping factor question

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spudwrench2
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Damping factor question

Post by spudwrench2 »

I have a general tube theory question: I'm trying to understand how to calculate the damping factor of an output stage and how it is influenced by output tube type selection, output transformer ratio etc. (in a PP type amp.)
Assume no global negative feedback...

Isn't the anode resistance of a tube dependant on the operating voltage?

Any takers?

Nathan
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Francis_Vaughan
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Post by Francis_Vaughan »

Yes Rp is partly dependent upon operating voltage. So you need to check the spec sheet. Then again just about every parameter of a tube is dependant in some way on almost every aspect of operating conditions, so there is no real surprise. :D

Output impedance is simply Rp multiplied by the impedance ratio of the OT.
Damping factor is a rotten number which is simply the speaker impedance divided by the output impedance.

With no feedback this is all there is to worry about (except that the numbers can be very frequency dependant.) Feedback drops the output impedance roughly with the feedback factor.
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spudwrench2
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Post by spudwrench2 »

Thanks! At least I can make some rough comparisons tube type to tube type. Problem is, it doesn't seem that too many spec sheets list anode resistance at different voltages.

Nathan
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Rawk
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Post by Rawk »

spudwrench2 wrote:Thanks! At least I can make some rough comparisons tube type to tube type. Problem is, it doesn't seem that too many spec sheets list anode resistance at different voltages.

Nathan
Then you have to calculestimate it by drawng on the plate characteristics graphs for your particular valve. Tedious, but if you want to be exact about it, this is the only way... actually, it is still approximate because there are not an infinite number of curves published for every possible grid potential, but then you would have to do your own curves and that is really too keen...
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