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Re: Accaptable ripple voltage

1/27/2005 6:43 AM
Wild Bill	Re: Accaptable ripple voltage Brett, there's more to the story than just ripple. And as for the amount of ripple, it's more important how much is in the output rather than how much you measure at the B+ pin. There are circuitry ways that are more or less susceptible to ripple. In your case, the filters are doing at least two other things than simply reducing ripple. One is decoupling the B+ string. This means providing a low ESR path for the B- and signal returns - the filter cap is like the negative lead that grounds your battery in your car. If that lead were to get a poor high resistance connection then everything gets screwed up. The filter value must be high enough to pass everything from all stages connected at that point easily to ground, plus a fudge factor. Otherwise you'll get squeals and motorboat sounds. The other is providing a reservoir of energy for when you're cranking the amp and drawing big "gulps" of plate current. If the "hot water tank" is too small then after even washing the dishes there's no reserve left for your shower. The effect with a filter cap is that the B+ sags in deep "nosedives" and highly compresses the signal. Not in a nice way either but excessively so. You've got the same problem every designer faces - how much is good enough? If the Firefly is a very simple and low power design with few stages sharing a common B+ node then you might get away with it. Unfortunately I know of no other way to find out than "trial and terror". Designers fudge on the heavy side to keep out of trouble. It's a lot harder to push the envelope the other way... You can try looking at similar classic circuits. Fender Champs got away with 8 mfd filters. Bassmans needed much more. One positive note - some caps are smaller than others. Illinois filters are much smaller than vintage Sprague Atoms. Might help to look around. HTH ---Wild Bill

1/27/2005 6:43 AM

Wild Bill

Re: Accaptable ripple voltage

Brett, there's more to the story than just ripple.

And as for the amount of ripple, it's more important how much is in the output rather than how much you measure at the B+ pin. There are circuitry ways that are more or less susceptible to ripple.

In your case, the filters are doing at least two other things than simply reducing ripple. One is decoupling the B+ string. This means providing a low ESR path for the B- and signal returns - the filter cap is like the negative lead that grounds your battery in your car. If that lead were to get a poor high resistance connection then everything gets screwed up. The filter value must be high enough to pass everything from all stages connected at that point easily to ground, plus a fudge factor. Otherwise you'll get squeals and motorboat sounds.

The other is providing a reservoir of energy for when you're cranking the amp and drawing big "gulps" of plate current. If the "hot water tank" is too small then after even washing the dishes there's no reserve left for your shower. The effect with a filter cap is that the B+ sags in deep "nosedives" and highly compresses the signal. Not in a nice way either but excessively so.

You've got the same problem every designer faces - how much is good enough?

If the Firefly is a very simple and low power design with few stages sharing a common B+ node then you might get away with it. Unfortunately I know of no other way to find out than "trial and terror". Designers fudge on the heavy side to keep out of trouble. It's a lot harder to push the envelope the other way...

You can try looking at similar classic circuits. Fender Champs got away with 8 mfd filters. Bassmans needed much more.

One positive note - some caps are smaller than others. Illinois filters are much smaller than vintage Sprague Atoms. Might help to look around.

HTH

---Wild Bill

Replies:

Brett Thanks Wild BillI'm surpris... -- 1/27/2005 4:59 PM