Change coupling cap from .47uf to 2.2uf

[andy2]

I just recently replace the coupling cap of my 300B SE amp to a .47uf Auricap but it seems like the bass suffers a bit. The coupling cap I replace is between the 12Au7 and 6SN7 stage. There is another between the 6SN7 and the 300B that is a little hard to get to but they will be replaced too.

I am thinking of ordering maybe a pair of 2.2uf to replace the .47uf. Do you think this will improve the bass without any serious effect on transient response?

[DigitalJunkie]

2.2uf seems a bit large to me,for a tube circuit.
Maybe try a .68uf if you can find one,or maybe a 1uf,if you have to..seems kinda large to me though..
Do you have a schematic we could see?

[andy2]

Originally posted by DigitalJunkie
2.2uf seems a bit large to me,for a tube circuit.
Maybe try a .68uf if you can find one,or maybe a 1uf,if you have to..seems kinda large to me though..
Do you have a schematic we could see?

I don't have the schematic. I bought these used on the web.

[audiousername]

Andy, the low frequency -3dB point is determined where the reactance of the coupling capacitor is equal to the resistance of the grid leak resistor. i.e. f3=1/(2*pi*R*C) where R is in ohms and C is in Farads. So, the amount of attenuation you get in the low end depends not only on the capacitor's value, but that of the grid leak resistor of the next stage. For instance a 0.47mF capacitor into a 470K gridleak gives a -3dB point at 0.72Hz, while the same capacitor into a 47K gridleak gives the -3dB point at 7.2Hz.

As you can see, even with a small valued grid leak resistor, the -3dB point is low - far lower than the speakers can reproduce. Normally problems with bass will be to do with the output transformer and/or too high an output impedance.

Using a larger than necessary coupling capacitor is detrimental to the hip pocket, and makes overload recovery slower.

[analog_sa]

The coupling cap I replace is between the 12Au7 and 6SN7 stage.

The measurable bass performance will depend on the coupling cap value and 6SN7 grid resistor. I suspect it is 470k-1M but you better check. In this range of values increasing the cap value will have nil effect upon the audio range. My guess is that you're observing the capacitors sonic signature or lack of break-in.

[kmtang]

I am rather sure that you won't get any improvement by increasin g the capacitor value from 0.47uF to 2.2uF.

The strength/dynamic in bass are associated with various components in Power Supply, and Cathode Pass capacitor of the 300B. Also, the OPT is an extremely important factor as well.


Johnny

[jeff spall]

Hi, I have a similar question about coupling/signal caps.
I've just been rebuilding some vintage amps, replacing the original caps with the nearest modern values, so 0.5 becomes 0.47. 0.25 becomes 0.22 etc.
Is it worth making them up to the original values by adding small bypass caps or would I just be wasting money and solder?

regards, Jeff

[percy]

given the tolerances generally availabe in capacitors(5%-10%), in the real world, chances are the old .47 measures .5 and the new .22 measures .25 .