decoupling caps

[9 volts]

Hey there, I'm wondering if someone can help with a basic explaination as to the benefits of raising the value of decoupling caps. Eg My preamp (2 12ax7's) has two 8uf decoupling caps, what happens if I raise them to 20uf. Does this increase the voltage going to the plates or does it provide a bigger 'reservoir' of power for the plates to draw from..?..I noticed most fenders amps use 20 or 22uf in this position. Also what influence does this have on sound?
I've had a scout and can't find the answer. Any links or comments appreciated. Thanks

[stokes]

I have never experimented with raising the value of the preamp caps,I was told a long time ago it could be detrimental to the tone,but raising the value of the main and screen supply caps will give you a more solid bass response and tighter overall headroom.I usually will use a 40 or 50uf in place of 20uf,if you have a solid state rectifier you can go 220uf with no problem.With tube rectifiers there are limits listed in tube manuals for the input capacitance,but I have found these to be very conservative.For instance I have seen tube data for a 5R4 listing 4 to 20uf as max,but right now I have a 5R4 in a 5E3 with 80uf at the main and it has been working fine,the amp gets used on a daily basis and is gigged with by my son at least once a week with this rectifier/cap combo for at least 6 months.

[TD_Madden]

maybe I'm confused......"coupling caps" or "decoupling caps"..?

[Enzo]

Coupling caps would be the ones from a plate to the grid of the next tube. Decoupling caps are the filter caps that serve the individual preamp stages. Those prevent a large signal in one stage from coupling into another via a shared B+ node.

[9 volts]

decoupling caps

So what influence do these decoupling caps (filtering) have on the the sound? Does increasing them make a difference to the sound at all? I can't seem to find any info on it. Lots on cathode caps and coupling caps but these.....Thanks
ps Thanks Stokes, I have a solid state rectifier and am at present using 100uf caps there. Will look into increasing them in the near future

pss I just found the following site which answers some of my questions and also provides a warning for those who may be unaware that electro caps hold alot of charge (don't rush into this!)

www.triodeel.com/capnotes.htm
*fixed the link.

[Enzo]

Increasing those filters will not increase the voltage, and since the preamp tubes are all running class A, there is no reservoir of power needed.

[Tom Phillips]

...pss I just found the following site ...http://http://www.triodeel.com/capnotes.htm

That link jumps to something unrelated to caps when I try it
Does it work for anyone else?
Tom

[9 volts]

link fixed