katode caps

[francesco]

Can someone indicate me some thread,or post, which talks about the caps on katode tubes?
Thanks. Francesco

[Robert M. Martinelli]

Hi,
do you mean the bypass capacitor usually found between the cathode and GND on tube amps' preamp stages ?

If so, that capacitor is placed there to "fully bypass" the cathode, and allow the tube to release the designed gain over the designed bandwidth.

It keeps the cathode potential - usually lifted from GND by a cathode resistor to make the grid "negative" ( remember the cathode is a tube's reference point ) - constant, otherwise the current changes across the tube would change the cathode potential, altering the grid "negative" potential as well.

You can use the "search" button in this forum's menu bar, using "cathode capacitor" or "bypass capacitor" as your search's keywords.

If that doesn't suffice, here' s a very useful link to the Valve Wizard site, where a whole bunch of useful info and tube amp theory can be found - you' ll find the "gain stage" section to be particularly exhaustive ( it' s a 1.6 Mb .pdf file ), provided you have some electronic background ( Ohm's law and RC circuits ).

http://www.valvewizard.co.uk/

Hope this helps

Best regards

Bob

[francesco]

Thanks so much Voxrules.
I'd simply like to know ,if I replace them, wether I use polarized eletrolitics or non polarized,
Thanks again, Ciao, Francesco

[pdf64]

It doesn't make any difference. Just use whatever is appropriate such as electrolytic for 22uF, polyester for 0.68uF.
See also
http://www.aikenamps.com/CommonCathode.htm
for design method. Peter.

[Robert M. Martinelli]

As Peter says, there' s no difference from a functional standpoint, but if in need of a big ( value ) capacitor, choosing a polyester cap would result in HUGE dimensions....so my advice would be to use non-polarized caps for values up to 1 microfarad and "go electrolytic" for bigger values.

[stokes]

Polarized tantalum caps are a good choice,a 22uf is quite small compared to a 22uf poly.They will last longer than the electros as well.

[Robert M. Martinelli]

Hi Stokes!
I didn't suggest tantalum caps because I have had some reliability problems with them in the past, when, in my early working days, I used to repair industrial equipment cards; I agree that they have an extremely favorable capacity/size ratio though.....

Best regards

Bob

[mfratus2001]

I would use between 2uF and 10uF if you are referring to preamp stages and the resistor is about 1.5K. Less than that, and you get little low end, and more than that you get way too much low end.
In cases where the cathode resistor is low, like 820 ohms, use the 10uF.
Some amps have a problem with handling really low frequencies, and unless you are playing a violin, you need at least some low end for guitar.

Oh, at those values, use a polarized cap, as there is always a voltage of about a volt or more across the cathode resistor.

Mike.

[Gtr_tech]

Originally Posted by voxrules!

Hi Stokes!
I didn't suggest tantalum caps because I have had some reliability problems with them in the past, when, in my early working days, I used to repair industrial equipment cards; I agree that they have an extremely favorable capacity/size ratio though.....

Best regards

Bob

Right...they love to short when they encounter conditions that they don't like.