bypass cap values?

[mike_mccue]

Yesterday,
I was showing someone a champ clone I made 10 years ago.
I got lectured by a well meaning person who insists I should be using a 25mF cap on the cathode bypass of my 6V6 power tube rather than the 22mF I used on the build.

I have several Silverface Champs and enjoy them and this home built "champ" as well.

I was wondering if anyone here has such strong opinions? I was little put off by the insistence that, even without hearing the amp, he could predict I should switch out to a 25mF value to make it "better".

thanks,
mike

[MWJB]

I very much doubt that you would hear any difference between 22uf and 25uf. Someone with good ears could hear =/- 10uf (e.g. 10uf vs 22uf vs 33uf)? Once you get past 100uf, differences get harder to hear.

Some cheaper cathode biased amps were built with no bypass cap at all, folks still like the sound of them.

Bypass caps here affect the envelope of the note, smaller values compress more, higher values sound stiffer.

[satsloader]

Hi Mike,

The addition of 3uf would in theory lower the low frequency roll off of the circuit. This could be calculated if you want to do the math, and Aikenamps website has the info for doing that if you wish.

http://www.aikenamps.com/Equations.htm

Can you actually hear a difference between the 2 values? I would doubt it... I haven't done the math, but I imagine the guitar's lowest frequency is far enough above the roll-off point of what both cap's values would add in information that the extra 3uf provided wouldn't be dramatic enough, if at all.

Remember also,we're talking about a Champ, which by it's design compromises ( speaker size, power, etc. ), isn't reproducing a bunch of low end to begin with.

Long story short, I wouldn't worry about it.

Bill

[tubeswell]

Wello here's one thing I have learned about over the last few months fooling around with Cathode resistors and bypass caps - you don't need a bypass cap, but if you add a bypass cap, or increase its value (i.e. increase capacitance), the more bass and gain you get.

The formula for calculating the freq rolloff of a cathode bypass cap- cathode resistor combination is:

1/2pi (x) Resistance(in kOhms) x Capacitance (in uF) = frequency in hZ

The traditional fender values (i.e. a 1.5k Cathode resistor and a 25uF cathode bypass cap) gives you:

1/6.28 x 1.5k x 25uf = 5.97hZ

The alternative (if, like me, you find 25uF caps hard to find and go) for a 22uF cap is:

1/6.28 x 1.5k x 22uF = 5.25hZ

These are alot closer in value than (say) a 1.8k-1uF combination (used in the soldano SLO100) or the .68uF used in Marshalls or the .47uF used in Peaveys.

So a 22uF bypass cap will still sound 'fendery'. Even a 1.5K/10uF combination still sounds reasonably full and bassy IMHO.

[mike_mccue]

Thanks everyone for the replies and detailed info.

Just to be clear; Does the cap/resistor relationship in a Cathode bias circuit function as a "hi-pass" filter of sorts?

best regards,
mike

[Enzo]

That would be a bit simplistic. There are tutorial chapters in the RCA tube manual on how circuits work. You might look them up.

As to the guy who lectured you, you might point out that When Leo Fender made CHamps, he used resistors and caps with 20% tolerances. In fact the lytics may have had even worse tolerances. But in the modern world of today, you could grab a 22uf and a 25uf from the bins, and find both measure 23.5uf, and BOTH would be within their 10% spec. It simply is not a critical part value.

And as Tubeswell points out, the low end rolloff for the two values was 5.25Hz and about 6Hz. Less than 1Hz difference at a frequency far below human hearing, not to mention anything a guitar can produce. Plus the speaker in the amp could not come close to producing such a frequency. In short, there will be no audible difference.

next he will be griping that you used 0.02 instead of 0.022 caps somewhere.

[mike_mccue]

Thank you Enzo,
I do know I was being simplistic and although it seemed obvious, I wanted to make sure I was understanding that the bass rolloff was at the lower end of the spectrum.

I do have a reprint of the RCA maual... I'll read up some more.

best,
mike