removing the very top

[daz]

I'd like to hear some opinions as to the best place in the circuit and best value for a cap to drain of the very very top end. I made some preamp changes and it seems to sound fuller and better than ever so far. But along with that came a little bit of buzzy fizzy high end, that extreme high thats probably 10k or thereabouts. I want to cut that out w/o harming the rest of the high end. What would you use?? 100PF? 47PF? And where do you think it should go considering the fact it's origin is the preamp? across a plate resistor? Right from the signal to ground such as at the master ?

[wizard333]

For Value you'll have to experiment; those sizes look too small to me, at least for where I put them. Typically I use something in the 500pf to .022uf range to zap fiz.

The WHERE is a much more important question. As someone who has a fast pick technique, I'm very sensitive to anything that fights the leading edge of the attack envelope. Having said that:

The WORST place you can put a bleeder is right to ground from your primary signal path. I find even very small values like those you listed to be immediately noticeable in feel and extremely annoying.

I tend to like them across the plate resistor or between the bass and mid pots in your tone stack (at least with 1M bass pots). The fiz reduction is more subtle there but still effective, but it doesnt fight the pick attack like a direct bleed from say a volume pot or elsewhere in the circuit will.

If you aren't as picky about your pick attack (pardon the pun) then it may not bug you as much; but why make something that will annoy some players when you can make something that will annoy no one.

[jrfrond]

Across the plate resistor of one of the stages (not the first), or plate-to-plate if using a long-tailed PI.

I don't recommend cutting highs in the first stage unless it's a bass amp, because you gain some "snarl" when first amplifying the highs, then cutting them, rather than cutting them at the first stage.

[daz]

Any thoughts about value? I see some using as small as 39pf there.

[wizard333]

Depends on how much fiz you're trying to remove. Marshall uses .022uf frequently. I've used .022uf between mid and bass pots to ground, or between 500pf and .015uf across the plate resistor.

I doubt you'll even hear 39pf across a plate resistor.

And yeah never on the first stage, thats just common sense, because 1) you dont really have any fiz at the first stage, 2) even if you did, if you cancelled it there, by your 3rd stage it would be back in force.

I generally do all my fiz cancelling in the last stage, although Marshall often does it on the last 2 stages, and this can be a tool to tailor tone as well.

[daz]

OK, i'll look at the PI. But i don't mean to make you all think i haven't done this before...i have. I ask because like with a lot of things it is one of those which i have a hard time determining if it's working w/o harming the high end at all. One value may seem to work, bt later on i remove it to see if i was missing something and sure enough i was. So i wanted to see what values the general consensus here might be and which place is best. So far you all have made it obvious across the PI is best, and thats the place i typically have tried. But no particular value has been mentioned. I can say 500 pf is much too low because i have tried many values and that big a cap will affect the ranges i want to kill. Anyways, the PI is it. Any more thoughts as to the value?

[daz]

By the way, another question....i see a 47pf in a lot of marshalls. But in some i see it where it normally is put, between the PI plates. But in at least one marshall it's positioned AFTER the coupling caps to the output tubes. Whats the difference there?

[Steve Conner]

The cap does the same thing whether it's before the PI coupling caps or after.

It's there mainly to stabilize the power amp, stopping it from oscillating. Any effect it has on the tone is secondary, but making it bigger will cut highs sure enough.

If you know the frequency you want to cut, you can figure the value from the formula for a RC time constant. The "R" is, as far as I know, 2Rp in parallel with 2rp, which amounts to about 67k if the PI tube is a 12AX7.

The problem with RC filtering is that it isn't very selective. It always slopes at 6dB per octave. So if you want to cut 20dB at 10kHz, then you have to also put up with cuts of 14dB at 5kHz and 8dB at 2.5kHz.

A partial solution is, instead of using just one RC filter, use several. If you put a cap on the stage before the PI too, then you have two RCs, you'll get twice the slope, and that helps you be more selective about what frequencies you cut.

[tedmich]

don't some like conjunctive filtering (cap and resistor across OT primary) to tame this hash?

[wizard333]

I think you misunderstood.

I would definitely NOT do this across the PI. Especially in any kind of channel switching situation because you dont want to dull up your clean channel.

I'd do it at the last gain stage on the gain channel, either across the plate resistor for that stage or between the mid and bass controls in the tone stack (you are putting your tone stack at the END of your gain stage arent you?).

Sucking high end out at the PI (unless your just controlling super high oscillation) is bad juju.

[daz]

It's a single channel fairly high gain (about as much as the gain channel on a peavey classic 30) build, and i get my clens by rolling the start back. When you say between the mid and bass pots do you mean between each pot's wiper? Any schematics you know of that show this ?

[wizard333]

With a Fender style tone stack, your mid pot will be between ground and your bass pot. What I mean is, put the cap between the ground side of the bass pot and the mid pot.

Marshall sometimes puts a bleeder between the treble pot and the bass pot, where the mid pot is not between the bass pot and ground. The effect will be more dramatic there.

A treble bleed cap will have different feel effects and tonal effect depending on where you put it. Like I said, I can't stand the feel of a high end bleed cap between volume or treble and ground, etc. Putting it across the plate resistor on your last gain stage or between the bass and mid pots etc, doesn't kill the feel of the amp in the same way. The idea putting it after the bass pot is that the resistance of the pot is between the output of your tone stack and the cap.

You may also want to experiment with combinations of a cap across the plate resistor and one buried in the tone stack. or use a resistor before the cap if you're putting it elsewhere.

When I tweak those things, I either install little sockets for the leads of the caps so I can switch them in and out to experiment without soldering, or I CAREFULLY aligator clip them in and out of the circuit to find the value(s) I like best. If you are experimenting across the plate resistor in a live amp, remember, there is a LOT of voltage there, dont touch anything and remember your cap will be charged, discharge it before you touch it if you are switching them in and out. Even a small cap can give you a nice little zap. Use aligator clips with rubber over the clips and be careful not to touch the small explosed ends.

[daz]

It's a marshall tone stack. Between the wipers of the treble and bass pots?

[daz]

Well, i couldn't figure out where to put it in the tone stack, and each time i tried anything similar to what was said i heard no results. Odd. In any case i tried going across the PI plates with a 270PF. In the past i never tried anything over 100pf and heard no results. I never used anything bigger because i have never seen more than 100pf on any other schematic and i feared i'd ruin the top end. But the 270 seems to have worked. It's preliminary at this point and the way my ears seem to change from one moment to the next i can never be sure till i use something for a while. But it seems as if it chopped off only the very top that was irritating me and as a bonus it cleaned up the tone in a way that makes it more articulate. Faster runs are more distinct now. I like it so far, but time will tell. As i said, it's a single channel and the clean sounds that i get rolling back the guitar is where i was really bothered by that sizzle. This seems to have cured that. But the distortion tone is where i noticed that more articulate tone i mentioned.

I gotta say that after having left the amp alone for months here and there, i don't think i will ever stop tweaking this thing because when i think i've done all it's capable of, i always find something that make it better. Plus it's fun....sorta like a treasure hunt.

[yunger]

would a 68k-470k resistor before v2a's screen work to cut the fizzy highs at the source?