crunch versus mush circuit experiments - slightly related to the "Marshall Thing" thread - warning! AMP CONTENT!
- From: morris.slutsky@xxxxxxxxx
- Date: Thu, 27 Nov 2008 11:38:23 -0800 (PST)
Dear all,
I hope everyone has a happy Thanksgiving planned. I just thought I'd
write this one post before getting on the road.
I really enjoyed the "Marshall Thing" thread, for the most part - and
it seems that there's a big debate here about the low end. A Fender,
with a wide low end, has a big fat sound. But when cranked and
heavily distorted, that bass can just make a mushy mess. The classic
"Marshall" sound seems to have a serious component of low-end rolloff
to improve 'crunch', although this is not necessarily present in
everything Marshall ever made, and not unique to Marshall amps, it's
probably what people are talking about - you get crunch and nice upper-
mid definition, but it makes the sound thinner overall. As pointed
out, if you were listening to Angus alone, Back in Black would sure
sound pretty thin! So it's a trade. And I don't know if there is one
right answer.
Anyway I've been tweaking my amp project, trying to make this low-end
thing work right. The clean sound works wonderful, and cranking up
the master volume does distort pretty nicely. But for true preamp-
gain crunch, it was lacking. Pedals work better. So I decided to
play with low-end rolloff in my preamp stage. The original circuitry
follows, for the first 12AX7 (2 gain stages + tone stack):
http://img140.imageshack.us/img140/2668/nocrunchfilternz5.png
Here it is. Just your basic standard gain stage, followed by a switch
which can either take the first stage's output straight to the tone
stack, or take the tone stack input from the second stage. The
cathode bypass cap on V1A is there for just general purpose gain. The
bypass cap on V1B is there to sort of punch up the highs - which I had
hoped would provide good crunch. Problem is, turning up PRE too high
would just go mushy. So I was just playing with low-end rolloff and
came up with the following:
http://img87.imageshack.us/img87/6259/crunchfilternc8.png
So now I've added a 1M grid resistor and a cap after the PRE pot,
instead of running that straight into the V1B grid. My first attempt
was a 1n capacitor. This gave a rolloff below about 300 Hz, or so I'd
expect it to - it sounded very very crunchy. But just a little thin.
Would end up slamming the Bass control up all the way. I then went up
to a 2n capacitor. This makes it sound bigger, but it can get
slightly mushy when PRE is maxed out. I did worry, also, about this
RC network sucking highs out of the 'clean' setting by loading it
down, but that isn't really a problem. Wasn't enough of an effect to
notice.
I think this is sort of there, but what I really need, I think, would
be some sort of variable cutoff filter. Maybe a buffer stage followed
by a similar hookup with a cap and a variable resistor. But that
could just as well wait until the next amp, maybe. For now, there's
pedals. And the dilemma - keep it at 2n, for the bigger sound, or put
it back to 1n for more crunch? The problem is that I really want my
distorted sound to clean up easily with the guitar's volume knob, and
I don't really like it being too thin when I do that. I guess I'll
just keep it this way for a few days and make a decision then. At
least, I've still got pedals.
Comments and suggestions welcomed. And happy Turkey day everyone!
.
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