AB763 circuits - cathode bypass caps that are 47uf/100v.

[pjd3]

Hello,

I believe I "might" know the answer to this but to be sure.

While looking through my component stash for a single channel Deluxe (a Pro really) reverb build, I have a few 47uF/100v electrolytics but no 25uF

I've been under the impression that increasing the value of the 12ax7 cathode caps above 22uF, or 25uF will simply not change anything in terms of frequency response and I get that, but want to sure that there aren't any other electrical issues that would arise. I figure I may as well use these as I'm not finding any other current need for these cap value.

Here is one thought that arises. I do have two 22uF/25V caps . Perhaps use those in the first two stages and use the 47uF's in the rest? It probably doesn't matter. gust taking the least chance of allowing any "bloat" or "boom" in. But, I' pretty sure that at 25uF you are already bypassing low frequencies that my guitar will never even dream of producing.!

Good to go?

Thank you,
PD3

[maxkracht]

You can probably hear the difference, but it's usually not drastic. I think the really big caps are more of a feel thing. Pro jr I was working on recently used 47uf in the first position, plenty of amps use 220 or more.

[Raja_Kentut]

you may put two 47u in series, than you have half the value as a result - 23,5u. That would be electrically no problem and gives you the original behaviour of the circuit. The only problem may be the space you have…

[sluckey]

Many people, including me, are using much smaller bypass caps in the AB763 preamp for a tighter, less boomy sound. My last AB763 preamp is using 6.8µF bypass caps.

[pjd3]

Thanks guys,

I am in a bit of a struggle with the idea of using bypass caps that are even larger than what can be construed as that which is already risking the "boomy" syndrome - which I'm not necessarily a fan of.

I think I'll just order up a range of caps, 25uF and less, so that I can try a few values if desired. When I've looked at the response of using a 25uF cap, and how it would let even "thunder" through, it has occurred that trying smaller values could very well be a worthy idea.

Screw my nice new 47uF. I probably would prefer a smaller value. I like a solid low end, tight, but, not boomy.

Thanks guys, I believe I've reconsidered my stand!

Best,
Phil

[maxkracht]

I agree with Sluckey aesthetically, same pro jr that started with 47 I ended up with 4.7, but I do know people that really like larger values. One person's boomy is another's just right. Gotta listen for yourself to know how you feel. Tell someone you want to cut a little bass on their amp, and they say "oh no, I need all the bass, don't cut it". Say, I want to tighten the low end, and they say, "yes, tight is good, please do that."

[pjd3]

As far as I can tell, by varying the cathode bypass cap value you are moving the cut off frequency up and down, but with the actual bass control pot, you are either raising or lowering the gain (passively speaking) on a set frequency. Its bound to be a very different effect.

PJD3

[maxkracht]

Pretty much, until you get into more complicated bass controls. I just meant boomy to one person might not be boomy to another. The word implies too much, but it's a preference thing.

Pardon my, at least partially, inaccurate explanation. I am an idiot. Please correct me if I am way off, but maybe this will make oversized caps make more sense.

You have a negative feedback loop between the grid and cathode. No cap = more feedback and lower gain. Bigger cap = less feedback and higher gain. Smaller caps only boost highs. (like a presence control) The bigger the cap gets, the flatter the response, until you hit somewhere around 22uf and the boost is flat through the audible range.

 If you hit a big bassy chord, that cap discharges, and you temporarily lose a bit of low end. Start making that cap larger and larger, the harder it is to discharge, the steadier the bias supply, and the steadier the bass.
 Useful to some, boomy muddy mess to others.

[pjd3]

Thank you for stopping back to further explain things, I can use all I can get.

I understand the concept of feedback say, in the case of negative feedback where you are bringing a bit of the signal back 180 out of phase and creating more linearity, approaching a more hi fi production of the initial signal. But, I haven't wrapped my head around feedback as it occurs in a tube on its own. I've heard a few folks talk about it but, that aspect of it still alludes ne. Hopefully some fine day the lights will go on and I'll see it.

How I pictured a cathode bypass cap was that in the absence of it, the resistor is providing the sole element of opposition, both to AC and to DC (setting the bias).

When a capacitor is strung around the cathode resistor it provides a much lower impedance to AC signals than the resistor alone does. And as a cap does in a filter, it will allow frequencies to occur completely who's wavelength period is not yet long enough in time to fill up the capacitor to any huge degree. But, as the frequency gets lower, thus the period gets longer, the cap gets filled up before the wavelength period is completed, and it is that point (crossover) that lower and lower frequencies get more and more filtered out - like a cap does in an EQ, RC tine constant stuff.

Well, wrong or right, that's what I thought was going on but, still lots n lots to learn.

Thanks again for stopping by. I've been adding some various cap values to the wish list to try a couple of values to hear what they do. Its easy enough. I may find that I like the tighter leaner bottom or, decide I miss that good ol Fender fullness down there. It will be a good eye opener (ear opener).

Thanks and good night!

Best,
PD3

[chaccmgr]

Merlin Blencowe offers a ton of understandable documentation here.

see http://www.valvewizard.co.uk/localfeedback.html for calculating feedback and frequency response
see http://www.valvewizard.co.uk/OtherStuff.html for capacitor values and effect on the frequency response of a gain stage. Both 47µ and 22µ will let pass all frequencies your guitar will ever produce and you will not hear any difference at all as far as only guitar frequencies are present