Pros and Cons of this technique?

[fdegrove]

Hi,

Now I see that you were just working with what G had already ordered.

Yes, it just seemed odd to me...In the end you end up NOT doing any maths but go by what you've seen done before I reckon??

When in doubt I do resort to the maths...
Guess I'm not a fly by numbers kind of person...

I use 1/ (2*pi*R*C) I think that's the correct one in this case.

Yes, that and Ohm's law is all you ever need most of the time.

I think it's a life-long thing..learning that is.

One more yes, it makes it almost as fascinating as horseback riding ...

Cheers and be patient while you do the ropes...it's well worth the
effort.

[G]

"I use 1/ (2*pi*R*C)"

I assume that when using this formula for calculating a bypass cap you would use the total R of the tube and cathode resistor. Correct?

G

[45guy]

Frank, correct me if I'm wrong, but you would only use the actual resistor in the equation, since that's what's being bypassed.

Just don't ask me how the math works when calculating the cap in an Ultrapath type circuit!

[fdegrove]

Hi,

I assume that when using this formula for calculating a bypass cap you would use the total R of the tube and cathode resistor. Correct?

In this case R = Rk the cathode resistor.

since that's what's being bypassed.

Exactly.

Cheers,

[G]

Thank you Frank.

G

[PRR]

> you would only use the actual resistor in the equation, since that's what's being bypassed.

No, it is also bypassing the tube's cathode impedance!

Rk is the same as 1/Gm, transconductance. I don't have a spec-sheet handy, but for this type tube working about 50mA I would guesstimate about 200Ω to 300Ω.

In most cases of self-bias, you will find that the cathode resistance is around half of the necessary bias resistor.

Take the low guess of bias-resistor and cathode impedance, 500Ω and 200Ω. Parallel them. 143Ω.

Ignoring load effects, for 17Hz response with 143Ω I would grab a 70µFd cap.

Since weak bypassing raises plate impedance in the bass, where load (transformer) impedance is falling, a "just big enough" cap will sometimes give a bass-bump, and sure is not helping damping. And -3dB at 17Hz is barely "hi-fi". Also the "grunge" of electrolytics is less if you over-size them. And electro caps at this voltage level are not that expensive. I would look at at least 4 times "just enough" or 280µFd, something like that. 220 or 1,000 should be fine.

Going back to the power supply on page 2 message 21: without doing an analysis, my pants-seat guess is that 32µFd - 100Ω - 64µFd may not be enough filtering for a transformer-loaded triode with no feedback. You could get a little less hum and much less buzz if you split the 100Ω into two 50Ω resistors and moved one of the caps: 32µFd - 50Ω - 32µFd - 50Ω - 32µFd. However 32µFd may not be enough supply bypassing for two channels, there will be some crosstalk. One way or another, I would use more capacitance.