cathode bias question

[blues man]

Standard 25uf, 1.5 resistor vs 5uf, 1.5 resistor on a pre-amp tube. Does the 5uf increase gain or is it just a frequency change? Is using a 2.7 res or 3.3 res better with a 5uf?

[Willabe]

Standard 25uf, 1.5 resistor vs 5uf, 1.5 resistor on a pre-amp tube. Does the 5uf increase gain or is it just a frequency change?

Yes and No.  

The value of the cathode bypass cap sets -- were -- the gain increase is frequency wise. The bigger the bypass cap value, the lower the frequency boost will be. So by increasing or decreasing the bypass cap value we can "tune" the amp for more or less bass/treble responce.

Some of the other guys here will give you a more detailed explenation math wise.


             Brad          

  

[Fresh_Start]

IRRC the formula for the "cutoff frequency" (bad misnomer) is:

1 / (R * C * 2 * pi)

remember that 1uf = 10^-6 Farads

Keeping it simple, if you double the resistance to 3K, you would halve the capacitance to 2.5uf to get the same frequency response.

All other things being equal, increasing the value of the plate resistor or decreasing the value of the cathode resistor increases gain... but not in the same way.  The first chapter of Merlin's Preamp book is available here:  http://www.freewebs.com/valvewizard1/gainstage.html.  Discussion of cathode bypass caps starts on page 25.  He also covers the impact of the plate (anode) resistor, cathode resistor, grid return (or "grid leak") resistor, and how to draw load lines.  Excellent introduction IMHO.

Chip

[blues man]

Thanks for the tip on the book, I downloaded it to read. I just heavily modified my 5e5 and have one problem with it I am trying to resolve and then I want to tweak the tone.

[blues man]

I have another question. If you have a resistor right before a standard master volume on the input side of the pot like a 220k in series with a 1M pot but you want to reduce the signal and used a 250k pot instead woudn't it also act like voltage divider. Or like a 470k resistor right before a 500k pot.

[jojokeo]

I have another question. If you have a resistor right before a standard master volume on the input side of the pot like a 220k in series with a 1M pot but you want to reduce the signal and used a 250k pot instead woudn't it also act like voltage divider. Or like a 470k resistor right before a 500k pot.

Yes, you're forming a voltage divider network. Check this file out and d/l it to help you w/ % of output, etc... that Doug has here on his site. http://www.el34world.com/Forum/index.php?topic=10884.0