sub bass elimination

[daz]

Now that i've tightened up the lows things are much nicer except that the amp has always had too much low end. Actually, not really too much, but too low. It's like there are bass frequencies that extend much lower than typical guitar lows and muddy things up even after i got the bass tight with filtering. Is there any way to bleed off the lower portion of the bass besides the tone stack cap? (wich i already tried down to 5n w/o success)

[cbarrow7625]

Sure, the PI input cap and the PI output caps. Remember, with a 1M bootstrapped input resistor on the PI the actual input impedace is closer to 5-10M. Use 5 or 10meg when calculating the bass roll-off. That is why some Fenders get away with a 500pf cap going into the PI, there is still plenty of low end. The typical .022uF value usually just guarantees that the LF roll off is well below audio frequencies.

You can also change the grounded grid cap in the PI (usually 0.1uF) to something smaller so that only higher frequencies get full amplification.

[loudthud]

Any other place where you have used a .022 cap can be tried. At each stage where there is a coupling cap, there is a "High Pass" network made up of the output impedance of the driving stage, the coupling cap, and the grid leak of the next stage. The formula is the old 1/(2*pi*R*C). To do a good job, you need several of these high pass networks to break at the same frequency.

The input to the PI is a tricky one because the 1Meg is "boot-strapped", there is a signal at the bottom of the resistor so effectively the value is much higher. You can knock that cap down to .001 and not loose much low end. The first volume control and the next stage are good places to try, and at the EL34s.

Now the bad news. Some of these caps are going to mess with what you like about the low end. So the best thing to do is make yourself some little capacitor substitution boxes. I made mine with cheap 1 pole 12 position switches I got at Radio Shack. Select a cap at each location that gives you enough low end to sound good, but no more.

My cap boxes are setup 100pf, 200pf, 500pf, .001uf, .002uf, ... 0.5uf basically it gets you in the right octave. You can get inbetween values by using a shorting switch sometimes called "make before break" or just make finer steps by paralleling caps.

This is an age old method called "Cut and Try It". The best part is that there is no math

[cbarrow7625]

Now the bad news. Some of these caps are going to mess with what you like about the low end.

Good point, and one not to be taken too lightly. An option that is more complex but can help overcome this (but will have other drawbacks of its own) is to create a steeper LF roll-off in the areas in the amp that make the most improvement.

Typically, all high-pass elements in a guitar amp are 6dB / Octave. By cascading a few R/C combos in series you can create a 12dB / Octave or higher roll off. Thiss will allow you to extend the bass bown to the fequency you want but then get rid of the mud below there at a much quicker rate.

This is actually done in one of the 5150 amps right before the tone stack. I think there are two .047 / 470k R/C combos in series after the tube and before the tone stack. It essentially becomes a mud filter, combining with the bass control on the tone stack to get somewhere in the neighborhood of an 18dB/Oct filter for the sub bass.

Extra components can muddy the tone though. Cascading R/C constants willy-nilly can cause unexpected peaks & high-Q events right at the cutoff frequency if you don't know what you are doing. You should be safe by limiting yourself to 12dB / Oct at any one place in the signal chain.

[daz]

I tried the PI input cap, but it just doesn't do it no matter how small. I was thinking tho that it's been a long time since i tried a smaller gain pot. So i'll give a 250k a go and see what happens.