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Old 15th September 2019, 11:17 PM   #1
Hearinspace is offline Hearinspace  Canada
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Searching math for circuit design, it's clear there are more than several ideas about the meanings of various tube specific terms. Even some of the more authoritative (looking) texts seem not to agree.

eg. Today I've been searching for formulae to understand and work out requirements for a power amp circuit I want to do and find Ra, ra, Ri, fi, Rp and rp used willy-nilly for the same thing.

I know I've been guilty of ignoring the differences and have seen them all used so many ways now I can hardly remember what the original meanings are.

European data sheets often use Ri for Rp and I've seen some use ri or rp for the same thing.

Looking for clear definitions I find most Googled online glossaries don't contain any, or more than one of them. Even the Radio Designer's Handbook (RDH3) only has Rp.

May I ask for some clarification?

Thanks
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Old 16th September 2019, 12:08 AM   #2
rayma is online now rayma  United States
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Standard practice is for lower case letters to be for AC parameters. A subscript is used to qualify them. Until word processors, the use of subscripts was a problem, and sometimes a capital letter with a lower case subscript was used instead.

Capital letters are used for DC (static) parameters. I believe Ri was used in Europe instead of Rp as used here. Notice I had to type the capital letters because of the text editor used here.

Some examples: Tube Parameters | Electron Tubes | Electronics Textbook Tube parameters - vacuum tubes

Last edited by rayma; 16th September 2019 at 12:14 AM.
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Old 16th September 2019, 02:02 AM   #3
Hearinspace is offline Hearinspace  Canada
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So if looking at a standard data-sheet page with Plate Characteristic Curves, Rp is determined by working along a grid bias line and rp can be determined working along the load line?

Quote:
Originally Posted by rayma View Post
Notice I had to type
the capital letters because of the text editor used here.
Not sure what you were referring to.
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Old 16th September 2019, 02:06 AM   #4
rayma is online now rayma  United States
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You have to type Rp here instead of the correct r with a subscript p.
There's no static Rp, just the dynamic rp.
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Old 16th September 2019, 02:41 AM   #5
Hearinspace is offline Hearinspace  Canada
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So are you saying that wherever Rp appears it means they want to say r sub p but they can't?
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Old 16th September 2019, 02:51 AM   #6
rayma is online now rayma  United States
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Exactly, there is no such thing as static Rp, just the dynamic rp. There is static Ip and Vp though.

I believe there was at some time in the past an archaic term (perhaps something like "beam resistance") which was the value of resistance which, when substituted for the tube, gave the same static DC voltage where the plate was (that is, beam resistance = Vp/Ip). Or maybe not. It would not be useful for design. The static power dissipation Vp x Ip is useful.

Last edited by rayma; 16th September 2019 at 02:58 AM.
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Old 16th September 2019, 03:18 AM   #7
rayma is online now rayma  United States
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Originally Posted by Hearinspace View Post
I was just about to call it settled in my mind until I took another look at this piece from Crowhurst. It's clear his typesetter had the option.
Nope, that Rp is actually rp. This equation is for AC signal gain, so only dynamic values appear.
Just chill on that.

Last edited by rayma; 16th September 2019 at 03:20 AM.
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Old 16th September 2019, 03:20 AM   #8
Hearinspace is offline Hearinspace  Canada
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I was just about to call it settled in my mind until I took another look at this piece from Crowhurst. It's clear his typesetter had the option. Hence, my difficulty in figuring out who means what.

OK, I was editing while you posted above. I get it. Thanks for the confirmation.
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Old 16th September 2019, 03:28 AM   #9
rayma is online now rayma  United States
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See also "Variable notation" in Wiki.
Small-signal model - Wikipedia
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Old 16th September 2019, 03:52 AM   #10
Hearinspace is offline Hearinspace  Canada
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Thanks for that. It makes it clear and easy to remember ,
. . . . . though it still bugs me a little that we can't use the notation to learn the principle, we have to already know the principle in order to interpret the notation. I don't like the idea much.
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