Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: navdave on March 19, 2012, 04:17:20 pm
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I wanted to add a depth mod to my amp like the one used in the Saldano Decatone.
But instead of using a 4.7k shunt resistor and a 39k series resistor. My amp is
using a 100r shunt resistor and a 1k series resistor kinda like in a Twin Reverb.
I figure the standard 1m/4700pf combo is way to big because of the size
of my series resistor. So I did a little math and figured since a 1m pot is
25 times bigger than the 39k series then a 25k pot would be the correct
value for my 1k series. Now my problem is what size capacitor to use with
my 25k pot? I did some math on that to my 25k pot is 40 times smaller
than the 1m pot. So would a cap 40 times smaller than 4700pf a 120pf cap do the trick?
I don't know what kind of math to use to figure out frequency response
when bypassing resistors with capacitors?
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Opposite.
The time constant (basis of the frequency response of the RC combination) is 2 time Pi times the product of R and C. If you want to maintain the same time constant, and you make R 25 times smaller, then C must be 25 times bigger.
.0047uF (or 4700pF) * 25 = .1175uF. 0.1uF is probably close enough.
Check: 1M * .0047uF = 1,000,000 * 0.0000000047 = 0.0047
25k * 0.1uF = 25,000 * 0.0000001 = 0.0025
We have a cumulative rounding error; no problem, use a 0.2uF (or 0.22uF cap). Don't confuse with a 0.022uF cap...
With the stock circuit, take the inverse of 2*Pi*R*C; you'll get around 33Hz as the turnover frequency. With the new, non-corrected values, it will be an octave higher at ~66Hz. If you don't do a lot of drop-tuning, you may not need the bigger cap. If you drop-tune, or really like the original sound, go for 0.22uF.
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Thank you very much HotBlues!! What book do I have to read to figure this stuff out on my own?
I understand part of your explanation on what value to use but I am lost when it comes to algebra.
I do play a lot of drop tuned stuff so I probably am gonna have to use that .22uf cap.
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The Navy Electricity and Electronics Training Series (NEETS) is a pretty easy-to-read primer on basic circuit concepts. Look for it online (not in print). It does not really touch on audio stuff, but provides basics of how d.c. and a.c. circuits work.
The trick is you can learn this stuff, but not realize the easy way to apply it, until someone else points it out.
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I believe I have a copy of NEETS on a disk from Lord Valve.
Never paid it no mind now I'm kicking myself in the butt for doing so.
Thanks again!
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Not a problem.
I started reading portions of RDH4 about 15 years ago (can't believe it's been that long). There are parts of it I'm just starting to get now, even though it's all there in black 'n white. Understanding it, and knowing how to use it to make life easier are two different things.