Re: crunch versus mush circuit experiments - slightly related to the "Marshall Thing" thread - warning! AMP CONTENT!

On Nov 27, 8:41 pm, RS <R...@xxxxxxxxxx> wrote:
On Thu, 27 Nov 2008 11:38:23 -0800 (PST), morris.slut...@xxxxxxxxx
wrote:

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.

Yep.

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

Marshalls typically use Celestions, too. In general, not the most
mellifluous of speakers when played clean.

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

The .1 against 560 ohms has a very high corner frequency though. It's
above 20Khz.

- 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.

The -3db corner for 1nf (.001) with 1Meg will be around 160Hz (that's
2nd octave E on a guitar.  2nd fret on the D string).  You'd only be
down about -6 db at low E.

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.

Roll the bass before the OD stages. Bump it back up as late in the
circuit path as possible.  Of course if you're OD'ing the power
stages, that's not possible. You could use a very bass-heavy speaker
and attenuate the bass throughout.

Another key to keeping tight sound is the coupling caps into the
output tubes. Those determine the recovery time when the power tube
grids conduct. The caps will charge when the grids break over, and
will store the artificial low bias point. Time-constant is determined
by the value of the coupling caps working against the power tube
grid-leak resistors.  The down side, of course, is that it will roll
off bass for the clean channel as well.

This is why a lot of players just use two different amps.

 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.

Not sure how that would happen. You don't have a value for the volume
control on your diagram. But even if it's 1 Meg (higher than it should
be), the max 'forward' impedance through the pot would be when it's
set at 50%, yielding two 500K 'virtual' resistors. The impedance seen
by your bass cutoff circuit (the .001/1Meg) would be the two 500K's in
parallel...250K.  So attenuation ratio would be 1Meg/(1Meg+250K).  Not
even 2db due to loading.  (BTW, the plate resistance of V1a would come
into play, but can be ignored)

Comments and suggestions welcomed.  And happy Turkey day everyone!

Happy Thanksgiving, Morris.

I like the 2 amp solution -- with some 15's for low end - Ed S
http://www.geocities.com/edshamble/guitar_rig.jpg

just dont move it..... hahaha
.

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