R2 gain reduction mod?

[boogiemon]

According to this: http://homepage.mac.com/mesaboogie/dot.html, the gain on R2 was increased as of the red stripe. i'd like to tone this down/reduce the gain on R2 for my blue stripe. Could anyone offer any clues/info on how to go about it?

If the solder-work is too complicated, maybe i could just put some lower gain tubes in the appropriate places. Anyone know which sections of which pre-amp tubes affect the gain in R2?

thnx.

 

[swbo101]

Sorry...but this might sound like a stupid question, so I'm apologizing up front..lol Just lowering the gain doesn't do it?

 

[boogiemon]

R1 & R2 share the input gain. Changing the relative gain is going to require a mod.

Sorry...but this might sound like a stupid question, so I'm apologizing up front..lol Just lowering the gain doesn't do it?

 

[okgb]

correct me if i'm wrong but the mk III's had a 3 meg or so resister
between stages and v1b and v2a
the R2 ch shunts around this for extra gain
with another 3.3 meg that switches in paralell to increase gain
so maybe a 5meg trimmer instead of the 3.3meg
[ that is attached to a 180pf cap , don't get the wrong one ! ]

looking at poor schematic i see it also switches a 6.8k to ground after
the V2A tube stage or switches a 330k before the v2a grid to ground ,
you could think of the first resister as drive and the second as master .
I wish the schmo's had parts designation numbers

could try changing v2 to a lower gain tube BUT you're driving that tube
not using the gain from it so may not make much different in that

 

[boogiemon]

thanks for the informative post. i think the task is a little beyond me though. i guess i'll just take it back to MB @ some point.

correct me if i'm wrong but the mk III's had a 3 meg or so resister
between stages and v1b and v2a
the R2 ch shunts around this for extra gain
with another 3.3 meg that switches in paralell to increase gain
so maybe a 5meg trimmer instead of the 3.3meg
[ that is attached to a 180pf cap , don't get the wrong one ! ]

looking at poor schematic i see it also switches a 6.8k to ground after
the V2A tube stage or switches a 330k before the v2a grid to ground ,
you could think of the first resister as drive and the second as master .
I wish the schmo's had parts designation numbers

could try changing v2 to a lower gain tube BUT you're driving that tube
not using the gain from it so may not make much different in that

 

[Restless Rocks]

The available schematic for the mark III is accurate to a large degree for the Black Stripe Mark III. IIRC the black stripe has the RP30 board and the purple stripe thru green stripe has the RP31 PCB. The R2 circuit in the later stripes ads an additional LDR which switches in a .47uf cap to the cathode of V2a increasing the gain. Also in the blue stripe and green stripe there is a jumper wire from the relay which engages the 750pf cap into the tone stack as does the lead channel when pull treb is out. If you were to lift one side of the .47uf cathode cap you would get closer to the black stripe R2 gain (which IMO is much more natural sounding than the later stripes with more gain and darker texture) Maybe I'm just old school but the black stripes, with the right tubes and a little tweaking could be the best of the III's. Particularly that the R2 is much closer to a cranked fender/marshall sound. Here's one more trivia tidbit: One of my clients has an original no stripe he bought new. He had so much trouble with blowing output tubes that mesa actually replaced his "105" tranny with the later style to lower the voltage and stretch tube life. He says the amp was never quite the same as it lost some of the clean channel head room.

 

[boogiemon]

Thanks so much. i have a no-stripe (60 watt, rev) w/the smaller transformer & it sounds great to my ears. i sometimes wonder what it would sound like w/the "+" mod and/or a larger transformer, but i'm afraid to lose what i've got.

i picked up a blue stripe because i liked my no-stripe so much but it's definitely a different beast. it's got the "+" mod & the el34's are wired in pentode but even w/the same preamp tubes it still doesn't sound quite as smooth as the no-stripe. i noticed early on that ch2 seemed to be trying too hard: too much gain of the wrong sort. i had no problem w/the relative volume of the channel, just the voicing/gain character, hence my post. Maybe i'll pop the hood & take a peek this weekend. If it's just lifting one end of a cap i might try it. (if i can find it)

btw, can you recommend a replacement for the 60 watt transformer? i know mercury magnetics has them but i'm not comfortable w/the fact that they have three taps: 4, 8 & 16. Does that mean that using the 8 ohm tap will bypass some of the windings? i notice that both my marks don't sound as good when i've got a 4 ohm load plugged into one of the 4 ohm jacks.

thnx again.

The available schematic for the mark III is accurate to a large degree for the Black Stripe Mark III. IIRC the black stripe has the RP30 board and the purple stripe thru green stripe has the RP31 PCB. The R2 circuit in the later stripes ads an additional LDR which switches in a .47uf cap to the cathode of V2a increasing the gain. Also in the blue stripe and green stripe there is a jumper wire from the relay which engages the 750pf cap into the tone stack as does the lead channel when pull treb is out. If you were to lift one side of the .47uf cathode cap you would get closer to the black stripe R2 gain (which IMO is much more natural sounding than the later stripes with more gain and darker texture) Maybe I'm just old school but the black stripes, with the right tubes and a little tweaking could be the best of the III's. Particularly that the R2 is much closer to a cranked fender/marshall sound. Here's one more trivia tidbit: One of my clients has an original no stripe he bought new. He had so much trouble with blowing output tubes that mesa actually replaced his "105" tranny with the later style to lower the voltage and stretch tube life. He says the amp was never quite the same as it lost some of the clean channel head room.

 

[Restless Rocks]

I now sell MM xformers and as I've yet to have to replace any boogie iron I'll look at that for you. There are more than one way to reduce the perceived gain of the black dot R2 but it's something I would want to work very closely with the owner/player on. Everybody has different taste's when it comes to the elusive Mark R2. Most want more gain hense it's evolution thru the stripes. Obviously the easiest way to get started would be choosing a V1 tube with a little less gain. Not a 12at or 12 au, but there are lots of variences among brands of 12AX7's and gain. For example the mesa branded JJ 12AX7 has noticably less gain than say an off the shelf Sovtec or especially many of the NOS goodies. And then you really can reduce the preamp gain simply by adjusting the input volume setting. On my III's the R2 pretty much sounds like a darker R1 when I reduce the input below 4-5. And funny as it would seem; if I turn up the master volume a little the R2 starts to really shine as the touch sensitivity becomes more apparent and it actually begins to sounds less dark. When at bedroom level I tend to pick harder loosing the dynamics. Then too we want to consider whether you're using hot humbuckers or vintage single coils.

 

[Restless Rocks]

Okay there's a "No Stripe" on my bench right now for a new standby switch and the board is CP10. Sorry for my bad memory before.

 

[boogiemon]

Had a little chat w/MB a couple days back & apparently (in agreement w/the post by Restless Rocks, above) the later MIII's (blue stripes, anyway) automatically engage the treble shift when ch2 is in use. On the black/no-stripes, the treble shift is active only when pulled in ch3.

Another thing he mentioned (as i understood it anyway) is that the Marks up through the IIC & the later MIII's have an additional hard-wired tone stage after the final gain stage in ch3.

i'm *so* glad to live w/in driving distance of the factory.