Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Other Topics => Topic started by: shooter on July 07, 2016, 11:16:04 am

Title: math ?
Post by: shooter on July 07, 2016, 11:16:04 am

I'm slowly building an excel sheet with all the cool math.

I have this formula Ip=(-Vg/Rk).  My question, did I copy the source correctly?
and if so, this must be, *ONLY IF*, you have Rk?  since Divide by zero in excel won't give me imaginary numbers  :icon_biggrin:

Title: Re: math ?
Post by: PRR on July 07, 2016, 12:55:23 pm

> Ip=(-Vg/Rk)

This is so *only* if Vgk is zero. Which is never normal.

Knowing only Vg and Rk, you can't say anything until you know the tube's curve.

And FWIW, if you have an Rk you "almost" always have Vg at zero.

Look at some annotated schematics and see what they tell you.

"Vg" should provably be "V(rk)", the drop across this cathode resistor.

If cathode resistor is zero, you can't say anything.

Grumpy Old Man Mode: I suggest you learn to do these problems on matchbook or a $2 calculator, before you hide them in a spreadsheet program.

Title: Re: math ?
Post by: shooter on July 08, 2016, 08:31:55 am

Thanks PRR, I just found it to be an *odd* formula.  I assumed -Vg to equate to bias, which is there with or without Vk, but Vk can only be there IF there's a Rk.

Quote

I suggest you learn to do these problems

Solving formula's is not my problem, remembering the formula is.  So with a spreadsheet they all live in the same house.  I've also found if I take a handful of measurements, plug them in the sheet, then all the formulas can take the data and give me all the unknowns at once.