Is bigger better?

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Steptoe
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#1 Is bigger better?

Post by Steptoe »

When using bypass capacitors, eg with the cathode resistor, the recommendation is for up to 200uf. However I often meet much higher values, up to 6,800 uf at the correct voltage and on the basis that if some is good, more is better, I tend to use the biggest I can find. Is that ok, or are there any reasons why they can be too large? Thank you for your assistance, Steptoe.
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ed
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#2 Re: Is bigger better?

Post by ed »

mh audio has a calculator, very near the bottom of the page:

http://www.mh-audio.nl/tubecalculator.asp

for a full explanation of the function this is comprehensive:

http://blog.audioworkshop.org/the-catho ... 4BrSD-C5kh

or Morgan Jones page 78 second edition
There's nowhere you can be that isn't where you're meant to be
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Mike H
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#3

Post by Mike H »

It also forms a series resonant circuit with the OPT, so will produce a peak at some low frequency (the resonant frequency).

So I have found that I always need to choose a value that achieves a flat freq. response at the bottom end, in other words, with no peak or boost.

However this resonance is also very useful in that if the OPT response rolls off higher than you would like, you can tweak the cap value to boost it's bottom end and flatten it out better.

For stages not having a transformer in the anode (purely resistive load), the cathode cap can be used to control the LF repsonse for whatever bass roll-off you want.

So no, bigger is not necessarily better. If purely arbitrary that is (IMHO).

HTH
 
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IslandPink
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#4

Post by IslandPink »

The calculator in the 'mh-audio' link is handy . I hadn't seen that before.
Just bear in mind the entry 'Rp ( plate resistor ) ' should really read 'Rp (plate resistance) ' - ie. of the valve
.
I put in my numbers for the 300B :
mu : 4
Cathode R : 940
Rload : 3000
Rp : 700
Target -3db at 3Hz
and got 100uF almost bang-on . That's reassuring.
I try & target 2 or 3Hz for any of these bass roll-offs, as there are usually several in any valve-based system . Higher, & the dynamics and bass tone start to suffer .
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Steptoe
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#5 Is bigger better?

Post by Steptoe »

Dear all, thank you for taking the time to reply to my query. Unfortunately, however, you have once again overestimated my understanding of the subject and the phrase ' please don't confuse me with the facts' springs to mind. Although not yet incapable of learning I have a long way to go and what I would like to find out at this stage is whether there is a downside to greatly exceeding the calculated value of the bypass capacitor and if so, why? With best wishes, Steptoe.
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Paul Barker
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#6

Post by Paul Barker »

You're fine Steptoe.

Your huge oversize simply means all bass gets through. You would be outside the resonance Mike brought up.

The usual reason to calculate size is to obtain correct performance with least. Size for cost Purposes.

However. Important to realise supposedly more choices. Of quality. In the more typical values. Found.
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#7 Is bigger better.

Post by Steptoe »

Dear Paul, thank you for that info. As you are probably aware, I tend to use whatever comes to hand and modern equipment is long on high value caps, up tp and including 10,000 uf. The question of quality does not arise as they all cost the same to me and I can't hear any difference anyway, which is very sad but something I just have to live with. Best wishes, Steptoe.
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