Choosing cathode bypass values

[Gnobuddy]

rk is not equal to 1/gm

but rather rk = Ra/mu + 1/gm

When I worked out my equations, I completely overlooked the "leaky" nature of valves, especially triodes, in which the output electrode is also an input (low rp), and the input is also an output (Miller effect). With decent silicon transistors (and maybe pentodes) that Ra/mu term becomes negligibly small, because mu is so high. I was led astray by too many years of semiconductor experience, and relatively little valve experience.

I worked out the complex-number math for the shelving filter as well, but left it out of my calc, and just used a high-pass filter instead, because I didn't think it mattered. In hindsight, that was a poor decision, particularly for valves like the 12AX7, where gm is low enough that (1/gm) is not a whole lot smaller than a typical external cathode resistor. Nevertheless, the formula I gave is at least closer to the mark than f = 1/(2*pi*Rk*C), which is quite utterly wrong, and can be off by a factor of ten in some cases (if Rk is much larger than 1/gm).

The good thing is that my bringing up the original mistake triggered off lots of thought and activity here, and now those who don't want to mess with LTSpice or other full simulation software have a quick-n-dirty alternative in nickb's improved version of the calculator I posted. Yeah for Nick!

-Gnobuddy

[cjenrick]

you can also put a pot on the bottom of maybe a 25/25

[SoulFetish]

Yup. Wanted to stick to the RDH4 formula, to avoid confusion, funnily enough. Sorry about that.

Charts? Done! Give it a go Cap Calculator

I click the link, and I'm thinking "Man, Nick opted for a real minimalist approach on this interface, but hey, no one's cutting him a check here. He even left out the "calculate" button. What Where the hell is the enter button?"



On yeah, I probably need to enable java script on this site to run... a java script.
dude, this is a great page, man. I appreciate you putting the work into this utility. It was interesting to test it against values chose by ear in one of the stages of my amp.