Calculating cathode bypass cap.

[G]

Hi all. I have been designing a little direct coupled 12B4A amplifier this evening that I plan on using with a Fostex BLH. Since I am using direct coupling the bypass cap on the output stage cathode has to have a fairly high voltage rating. I would prefer to use a MKP cap there but obviously I don't want to use more capacitance than is needed to get the job done due to cost and size restrictions. Unfortunately I'm drawing a blank on how to calculate the minimum size needed to bypass the 12B4A cathode. I went through the Navy electronics documents but I could not find the precise info I need. Would someone be kind enough to give me a hand?

[dsavitsk]

It's on page 79 in Morgan Jones. Basic answer is to use the standard equation for finding the corner frequency.

F = 1/(2 * pi * C * R)

Into the resistance variable, you need to consider two resistances in parallel. These are Rk and

(Ra + ra)/(mu + 1)

[Tweeker]

Do you mean Rk || (Ra + rp)/(mu + 1)?

[dsavitsk]

That is what I understand MJ to be saying.

[Geek]

...then take resulting capacitance and double it for good measure in practise

[G]

Originally posted by dsavitsk
It's on page 79 in Morgan Jones. Basic answer is to use the standard equation for finding the corner frequency.

F = 1/(2 * pi * C * R)

Into the resistance variable, you need to consider two resistances in parallel. These are Rk and

(Ra + ra)/(mu + 1)

Ok I don't mind sounding dumb.

C=1/(2 pi F R)

and

R=(Ra+ra)/(mu+1)

In this case is Ra the DCR of the primary of the Output transformer? I believe that ra is the plate resistance at the operating points that I have chosen. Correct?

[SY]

ra is indeed the plate resistance. Ra is the AC impedance at the plate, not the DCR.

[Zen Mod]

Originally posted by G


Ok I don't mind sounding dumb.

C=1/(2 pi F R)

and

R=(Ra+ra)/(mu+1)

In this case is Ra the DCR of the primary of the Output transformer? I believe that ra is the plate resistance at the operating points that I have chosen. Correct?

R= Rk II ((Ri+Ra)/(mu+1))

where
Rk is cathode resistor
Ri is internal R of toob
Ra is load of toob (impedance (nor DCR) of OPT primary)
mu ........is mu

[5u4]

Been wondering about this myself with my Loftin-White. Circuit calls for 100uf. Tried several electrolytics for 2A3 cathode bypass & didn't like the sound of any of them. Requires a 250V rating, so pickins' are slim.

Now using a 30uf Solen (for max size reasons) to bypass 2.4K ohm. Overall sound is better.

Used the simple formula:

1/ 6.28*F*R to come up with 30uf = 3db@ 2.2hz, but guess there's more to it than that.

Please check my math using specs for Sovtek 2A3:

Ra 3K
ra 1K
mu 6.5

4K / 7.5 = 533

533 II 2400 = 436

1/ 6.28*5*436= 73uf

Used 5hz as this seems to be a good point to shoot for in a single ended amp. From this calculation, I should be using around 73uf?