capacitor value question

[MBD115]

Hi guys

My project is calling for a 1uF@50V Radial Electrolytic Capacitor. Can I use a 22uF@50V instead?

What is the different in these?

Thanks

[Structo]

What is the different in these?

Thanks

About 21uf!

Best not to mismatch that much.

A few uf is ok but not 21uf.

[iknowjohnny]

Depends what context you're using it in. In a signal path the 22uf will allow much lower frequencies thru than the 1uf will. If it's a cathode bypass the 22 will allow most of the frequency spectrum thru while the 1uf will only allow high end and upper mid frequencies thru. So in that context the 22 with give you a fuller sound and more gain therefore. The 1uf will give you a gain boost in only in those higher frequencies.

[MBD115]

Its a bypass cap around a cathode resistor to ground. Pin3 of V1 tube. The resistor is 820 ohms if I remember right.

[collinsamps]

You can play with that value on a cathode bypass cap. 22uf 50vdc is fine and very near Fenders 25uf, you won't hear the difference. With an 820 resistor and a 22uf your low frequency response will be about 150HZ. The original 1UF would have been about 1000hz, which begs the questions...........what are you working on for the recommended frequency to be 1K?

[MBD115]

I'm sorry, that is 1uF @ 25V, not 50V like I thought. The 820 ohm is correct. I checked that also.

I was considering the AX84 build but now you have me wondering also!!!!!

It is showing those 1uF bypass caps used on 2 stages and a .68uF on the other.

Thanks for the help.

[collinsamps]

No worries with that build. The values are all explained here.

http://ax84.com/media/ax84_m35.pdf

[MBD115]

Thanks everyone for your help

So a smaller value, like 1uF, will give me a brighter tone with the loss of a little gain where a larger value, like 22uF, gives me a darker tone and a little more gain. I think I understand how that would work.

So the answer would be 'yes', but the tone and gain is going to change. Right?

[Andy Le Blanc]

As a bypass cap, you are correct. But keep in mind that what your controlling
with a bypass cap is degenerative feedback, by going with a larger cap your
changing where the usable gain of the low end rolls off. This is also controlled
by the value of the grid resistor and also the coupling cap at the plate.
If your stuck with a component value like a bypass cap, that will critically change
the voicing of the stage, you can compensate, by choosing a smaller coupling
cap at the plate and end up with a voicing that is closer to the design intention.

[martin manning]

Bypassing the cathode resistor increases the stage gain, typically about 5-6 dB. It creates a shelving effect where a transition from the un-bypassed gain to the higher bypassed gain occurs over about a decade (a factor of 10 in frequency). The value chosen determines the frequency range over which this gain boost occurs.

For a 12AX7 stage with a 0.82K cathode resistor, the mid-point frequency of the gain boost will be:

0.68 uf, 318 Hz; 1 uf, 216 Hz; and 22 uf, 10 Hz.

(calculation based on Merlinb's work)

You can roll the low end off with the following coupling cap, but that won't produce the same effect.

MPM

[MBD115]

I did a little searching for Merlinb and found some of his writings. Pretty good stuff. He seems to know whats going on. His formulas are over my head though.

I had no idea a cathode bypass cap has so much control over the tone of the amp when I started this thread. I seem to be learning about bias resistors at the same time also.

Thanks everyone for helping me out. I'll have more questions soon I figure