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Old 5th April 2007, 06:53 PM   #1
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Default Choosing A Cathode bypass capacitor

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
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Old 5th April 2007, 07:18 PM   #2
kmtang is offline kmtang  Canada
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What's the value of the cathode resistor???
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Old 5th April 2007, 07:34 PM   #3
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Right now, 500 ohms, but will drop to ~250 with the switch to KT88s, or will be an LM317 with a Zener in front of it (just to be safe)
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Old 5th April 2007, 07:40 PM   #4
Tweeker is offline Tweeker  United States
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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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Old 5th April 2007, 10:21 PM   #5
kevinkr is offline kevinkr  United States
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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.
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Old 6th April 2007, 09:53 AM   #6
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The aiken-amp article deals with bypass capacitors on the cathode in gain stages.....
To find the right value for bypass capacitors in the end stage (eg. connected to transformer) one has to take in account the transformer:

The optimum cathode bypass capacitor for SE is:

ck= Lp / Za.Rk
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Old 6th April 2007, 10:53 PM   #7
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Za*Rk

Transformer impedance * Cathode Resistor

What is Lp?
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Old 6th April 2007, 10:59 PM   #8
kevinkr is offline kevinkr  United States
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Lp: primary inductance
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Old 6th April 2007, 11:00 PM   #9
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of the Output Transformer,

also, Za is impeadance on the Secondary of the Transformer? or Primary?
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Old 6th April 2007, 11:05 PM   #10
kevinkr is offline kevinkr  United States
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IIRC Za is the plate resistance of the tube? Someone will correct me if I'm wrong, but it is relevant to the discussion at hand.
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