I want to Tweak the BASS on my Mark IIB (follow-up 7/8)

[gonzo]

are there any suggested mods, or values, for tweaking the bass pot on the 2B?


i can barely bring it into play, 1 or less, anything higher, it gets flubby.

i use my GEQ now, to compensate, but i also would like to bring some BASS level into the signal, because i can tell it rounds the sound out, and adds a touch more gain in the low mids, which is what i like.

low end gain, but no flab.


in my totally NON-tech mind, i'm thinking a BASS pot, that has a really wide range from 0 to 10, where only a little bit of something happens.

LOL


do any of you know what i'm talking about?
am i crazy?
wait, don't answer that.

 

[gonzo]

oh c'mon, now........

NO ONE.....
has tweaked their bass control?

even after years of folks simply not using the bass knob at all, and complaining about bloated low end while using it?


man, i'm just going to have to dive in, and do something, i guess.

if it don't work, i'll rip it out, and put the old stuff back in.

film at eleven.

 

[kaluablue]

Hi gonzo

you might get lucky by not looking at the preamp, but the poweramp.
One thing I noticed on my MKIIa (where the preamp is quite similar to yours) when playing with the negative feedback (aka Presence) circuit: There is only little nfb for low frequencies, which means the poweramp is much more sensitive for bass then for treble. Raising the nfb there made the bass pot work more even.

I suppose you have a schematic for your MKIIb.
I suggest you try to remove the .01µ cap (should be an orange drop) parallell to the 56k resistor near by the presence pot in the schematic. Both should be near the presence pot on the back of the amp aswell.
This will increase the treble response in the first step, but if you turn down the presence pot you now lower the bass response aswell. This won't damage anything, if you don't like it, put the cap back. If it's the right direction, but not quite 'it', you can start to play around with these two parts, lower that resistor to lower the bass response and try different capacitors to keep the treble where it was before.

 

[Boogiebabies]

I have had some success with using a lower value cathode cap than the 25uf on the V1A. Anything from 1-15uf should help as 25uf is boomy to begin with.
You can also try a small cap in the tone stack to replace the large .1uf. In some of my higher gain Fender type circuits I use a .047 or .022.

Also, Herr kaluablue is correct. Mesa's later designs reduce the negative feedback from 680 Ohm to 3.3K and the .1 bypass cap
is lowered to 4700pf/.0047uf to reduce some bass frequencies. They also reduce some flub on the second 3.3K by again replacing
the .1uf/3.3K to ground with a .047uf cap. The end result is punchier and slightly less flubby with the bass around 2 or 3. It's really
difficult to work around the bass frequencies with the tone stack being so early. Mesa's answer was the GEQ and Fender used a .001/.1
coupling to the phase inverter. That .001 does not allow very much bass though. The .1 does and they blend together for a chimey, plucky
and brighter tone.

 

[gonzo]

kaluablue, Boogiebabies

thank you so much.

awesome information, exactly what i was looking for.


i have a local guy, who is a boogie-authorized repair guy, but who is also basically real sharp with tube amps.
he's done one mod for me already, and did my full tune-up on my '81 MKIIB a couple of years back.

i'm quite sure he could do this mod, it seems pretty straightforward.

i WILL do this, and report back.

it will be about 2 weeks before i can begin.

i've never had any issues at all with my 2B, other than the bass control issue, so if these mods leave most of the mid and high end alone, and the gain structure basically alone, i think this will dial in what i want.

 

[gonzo]

FOLLOW-UP, THURSDAY 7-8

well, i called the repairman, made the appt to drop it off, when i got there he said 'if you got a second, i'll do it right now'

:mrgreen:


so, out comes the chassis, in goes the hot solder iron, out comes the .047 cap, in goes the .022 cap, voila.


a very nice change, indeed.

cost me $10.

more on the sound, when i get back from my road trip to denver..........

 

[RR]

Yeah I notice my Mark IIB doesn't have the bottom end like my Dual Rectifier but if you play through 4x12" you'll get some bottom end.

gonzo, good for you, some technician will charge a bench price of $65+ whether they're looking at your amp for 5 minute or an hour.

yeah, report back to us if you have improved bottom end ... like if has great bottom end comparable to Kim Kardashian's.

 

[gonzo]

it has definitely moved more in the direction of kim kardashian.
:mrgreen:

 

[gonzo]

i had been looking for the right word to describe what i wanted..

the tech said it right off the bat...

"resolution".

i wanted finer resolution of the bass.....


now, i can actually dial up to about 3, before it starts to get into flub territory...
before, it was no higher than 1.5 on the dial.


and now, as it approaches the 'sweet spot', it doesn't get there as fast.



we decided to wait, to see how that one measure of modification works out.

if i end up wanting more, we'll open her up and do the phase invertor mod.

i haven't fully tested it yet....
maybe tonight....

will report back.