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Choosing A Cathode bypass capacitor

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Hello everybody, this is an issue i've struggled with for a year.

The bottom line is that my Output transformers on my EL34 SET really like Cathode feedback. and I need to pick the best capacitor I can. currently I have two Rubycon 470uf non Blackgate caps in there, and I don't know what is ideal!

What is the science behind choosing a Capacitor value? I have never heard a good explaination.

With CCS Bias, should I change the Value of this capacitor?

Would it be better to use a 47uf Solen instead of the rubycon I have in there right now??

Any help would be appreciated. Thank you

-Moose
 
Keep the 2 470uF capacitors in circuit and try connecting an 80uF polypropylene in oil motor run capacitor from cathode to B+. This will cancel psu noise and give short loop path. El34 triode mu is around 10.5.

If there are blocking issues, these values are rather high.

This article gives the full rundown on choosing a cathode bypass capacitor. Of note is that you are bypassing the parallel combination of the tubes cathode resistance and the cathode resistor giving the low frequency gain shelf. This means a bigger cap for a given cutoff than the bias resistor alone would indicate.
 
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You can't use this hum canceling technique with cathode feedback as it is essentially a frequency dependent short across the secondary windings of your output transformer. I do use this same technique in the one solitary cathode biased se amplifier I have currently licensed, and it is very effective, but the amplifier does not use cathode feedback.

Your current values of capacitance are not unreasonable considering the impedance they are looking into. (cathode // cathode bias resistor)
You can shunt these with smaller black gates to good effect.

I use cathode feedback in some of my older pp designs, mostly over 100wpc, and here I use fixed bias - it eliminates the cap issue entirely.
 
So, for a 6550 SE the cathode bypass cap should be 7.86 uf? The primary inductance of my transformer is 9.44 henries. The ZA is 3000 and the cathode resistor is 400 ohms. So, 9.44/3000*400 = .00000786? Or have I got a decimal wrong someplace?
 
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Funny... all these comments so far have strayed from actually answering the technical question. The purpose of the cathode resistor is to create a positive cathode bias relative to the control-grid's nominal ground potential. If it were to have no bypass capacitor, it would also act as a very local negative-feedback element, much reducing gain (but ironically much increasing stage linearity. Ah, well...)

The point of the bypass capacitor is to let the A/C superimposed signal see the cathode as being at constant potential, eliminating the negative feedback, and simultaneously increasing stage gain. Yep. That's it. Nothing more, nothing less.

So... then the next tautology is then introduced: the sizing of the capacitor should be chosen to have a value where Z = 1 / (2πFC) is equal to 1/2 R (the cathode resistor value). It is up to you to choose the value for F (frequency). This is part of your design criteria. Say, for instance that you have a cathode resistor of 250 ohms, and you'd like fairly unattenuated response down to 20 Hz.

Z = 1/2πFC and

C = 1/2πFZ and we want Z = 1/2 (250) = 125 so...

C = 1/2π(20)(125)

C = 64 uF, where "closest standard value" is 68 uF

Obviously, if you look at the (Z = 1/2πFC) equation, the larger the C the lower the Z. For a given frequency, the lower the Z, the lower the frequency of discernible frequency cut-out of the stage.

LASTLY - it is something of an error to use the formulas above without regard to the accumulated effect of all the stages in succession. The thing is, that whatever roll-off of frequency that a particular R+C combination at a stage brings, it will ADD (in decibels) to EVERY stage's contribution. Thus, if one were to choose the -6 dB point (1/2 the gain) then by the 3rd stage, at 20 Hz, gain would be down -18 dB.

This last paragraph ( above ) is also somewhat simplistic, because the gain of each stage doesn't actually fall off per the -6 dB per octave 1-pole low-cut filter equation, but somewhat less dramatically due to the transition of the stage from mu dominated gain to one of degenerate cathode follower gain instead. The design consideration of "Z = R/2" consideration in conjunction with this degenerate cathode-follower transition yields about a -1 dB gain-point at the chosen R and F ... which is pretty much "good enough" especially at the low end where our darn ears are particularly insensitive to subtleties.

GoatGuy
 
GoatGuy said:
So... then the next tautology is then introduced: the sizing of the capacitor should be chosen to have a value where Z = 1 / (2πFC) is equal to 1/2 R (the cathode resistor value). It is up to you to choose the value for F (frequency). This is part of your design criteria.
Strictly Zc = Rk || rk, where Rk is the cathode resistor and rk is the impedance looking into the cathode (which is roughly (Ra +ra)/mu). However 0.5 Rk will be close enough in many cases.

oops: just read the end of your post, where you say much the same as this anyway.
 
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I read this in Russian Audio forum and I use it:
Ccathode = 160 000 / F low freq. x Ro.
/Ro - react res, of C that in parallel is = Rcathode/10.
For ex.....If Rcathode is 75 ohm, F need to be 10 Hz.....Ccathode = 160 000 / 10 x 0.1x75.....C = 2 133 uF.
 
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