• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Which cathode bypass capacitor?

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The bypass capacitor may be calculated knowing the lower audiofrequency to be amplified without attenuation, and the resistor value which will be bypassed.

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

where F min is the frequency at -3dB, R the resistor above mentioned. Choose almost a 10x value if you want no -3 dB loss. The working voltage, the more closest up to the expected DC voltage across it, ej, for 100V use 150V more or less. Not use 400V !! in this case.

Good luck.
 
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Hello Gregg, what would be the difference in sound of Elna cerafine vs. Silmic II?

I think its more a case of how each interact with the rest of the circuit..and tube you are using..taste etc.. One is warmer the other is brighter..<<detail is different in how its presented..if you click them in my post they are linked to HIfi collective just for an eg of price (I have no connection) there is a link on the page (small writing) that will take you to manufactures info and a short blog..

They are so cheap you could try both and see...just remember that a 1 hour test is no good give it a week for each.. :)

Black gate would also be a good choice the trouble is getting them and cost..

Regards
M. Gregg
 
Cathode bypass cap

Hi Marko.
I saw that you are fan of exotic components with astronomical prices. You should ask C. Van Haus to make CuTf or TfTf capacitors for you :)

Seriously, 47uF/160V/SET.... output tube cathode bypass, I guess.
For me, forget electrolytics. Also forget any MKPs, because it sounds too harsh (Solen), harsh (Ansar) or too "sssssssss" (Supreme).

My recommendation:

1 - Siemens MKV 125Vac (B25811 type). I use NOS 2x15uF in parallel. If I know well, today Epcos B25811MKVs products in Russia for the Russian market only.

2 - Bosch/Siemens/SEL MP type. These capacitors are huge and need more space for them.

3 - Russian MBGO 160V (only MBGO, not others).

4 - No four.

Best diying
 
As dhaen says..

EG mount the cap on its own tag board connections and link to the resistor (even a short link is still better than direct on to the resistor leads) if the cap acts as a heat sink its life will be short.. + a hot cap can have a different sound..

Regards
M. Gregg

Fair point. Everything I build is breadboarded so I never really consider the long term issues, plus everything's open so the heat escapes. One day I'll use a box though so it's well worth remembering, for output valves at least.
 
Hi,

...would you use in SET amplifier if you had voltage of approx. 100v and 47uF as requirements?

I would suggest to convert to fixed or semi-fixed bias instead and put a high quality capacitor in the final power supply position.

If for reason self bias must be kept I would use a high quality film capacitor.

However I would connect an additional capacitor with a value of appx. Ck / (Mu-1) between cathode and the +B connecton of the output transformer and I would a resistor of approximately the DCR of the output transformer between the power supply and the +B connection of the Output transformer.

Precise values are best fine-tuned in circuit, with the extra C adjusted for minimum output noise from the Powersupply (this means a very quiet DC supply must be employed) and the Resistor should be tuned for a low frequency response without Bump (output stage only, not complete amplifier).

Ciao T
 
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