Mesa Boogie Mark III - How to get more touch response?

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Darkbluemurder
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Mesa Boogie Mark III - How to get more touch response?

Post by Darkbluemurder »

I asked the same question at the Boogie Board but it seems that there are not many who have tweaked a Mark III's lead channel to have a more harmonically rich sound and better touch response. I don't think the amp sounds bad but the lead tone is very middy and flat. From what I read elsewhere (I believe it was Gil Ayan's article on lead dress) the challenge will be to avoid oscillations once the highs are no longer dumped to ground (or through plate bypass caps).

Any ideas where to start? I would first rip out the 1000pf cap that is in parallel to the 68k to ground after V3a. Same with the 1000pf cap that is in parallel to the 220k to ground after V3b. Finally there is a 1000pf cap in parallel with the 270k plate resistor on V3b. My feeling says that 470pf should be enough.

Any input would be greatly appreciated.

Thanks Stephan
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Darkbluemurder
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by Darkbluemurder »

That seems to be a tough call.

This is interesting as the basic architecture of a Mesa and a non-HRM ODS - apart from the reverb topology - is similar: 1st gain stage - tone stack - volume - 2nd gain stage - 1st OD stage - gain - 2nd OD stage - OD Level/lead master. The purpose of the 68k resistor to ground before the 1st OD stage must definitely be to cut bass response so I see this as a parallel to the ODS's input trim.

Anyway maybe I should just try and see. I will let you know about the results.

Cheers Stephan
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bnwitt
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Build something else

Post by bnwitt »

I hate to say it but I'd build something else and sell the boogie on ebay.
Great things happen in a vacuum
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bnwitt
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Build something else

Post by bnwitt »

I hate to say it but I'd build something else and sell the boogie on ebay.
Great things happen in a vacuum
fred_farkel
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Re; Touch Response

Post by fred_farkel »

Darkblue....if you get an answere would you please share with another Mesa owner? Thanks!
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Darkbluemurder
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by Darkbluemurder »

Of course I will. It looks however that I would have to be the guinea pig in this case. I hope that taking the amp out of its case will not be as difficult as my Fender 75 otherwise I may never know.

Cheers Stephan
JayB
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by JayB »

Darkbluemurder wrote:Of course I will. It looks however that I would have to be the guinea pig in this case. I hope that taking the amp out of its case will not be as difficult as my Fender 75 otherwise I may never know.

Cheers Stephan
For what it's worth, remove any cap from signal to ground. The .001 cap in parallel with the 270k plate resistor you will want there. It's what smooths that spikey top end sizzle you get from to much gain. Remove the 47 pf from grid to ground on V2A. Remove the .001 from grid to ground in parallel with 68k on V3B and the .001 from in parallel with 120k to ground after V3B. If it starts oscillating like a pig, I suggest starting with 250pf caps from plate to cathode on V1A, V2A, V3A and B. Keep increasing them till it stops. It's be some where between 250pf and 500pf. Plate to cathode is so much better than from grid to ground in my opinion. A cap from grid to ground just makes it dull.

Something to experiment with is decreasing that 680k on the grid before the gain pot on V3A. Then that 68k to ground can be increased to 100k atleast. The resistor from the effects return jack to ground can be played with also.

I'm interested in what you end up with. Let us know what you ended up doing.
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KGW
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by KGW »

Hi,

I was perusing the archives and came across your Boogie post. I also have a Mk III (bought in 1988) and have been fixing/tweaking it. So far I have:
  • * adjustable bias pot
    * changed the preamp to the Mk IIC+ version
    * added a rhythm II level control
    * rewired the power tube filaments (using shielded cable)
    * changed the MF resistors to Vishay/Dale
    * new PS caps
    * Mallory 150 coupling caps
    * changed ceramic caps for silver mica
I've also played with the reverb circuit a bit (trying to get it deeper) and tried to make V1A a bit cleaner by changing the plate resistor to get B+/2 on the plate. I'm not sure about the last 2 mods yet. I'm also not sure if I like the Mallory 150s or OD caps better.

I like the MkIIC+ and rhythm II mods for sure.

Please keep us informed of your findings.
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KGW
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by KGW »

Hi,

I was perusing the archives and came across your Boogie post. I also have a Mk III (bought in 1988) and have been fixing/tweaking it. So far I have:
  • * adjustable bias pot
    * changed the preamp to the Mk IIC+ version
    * added a rhythm II level control
    * rewired the power tube filaments (using shielded cable)
    * changed the MF resistors to Vishay/Dale
    * new PS caps
    * Mallory 150 coupling caps
    * changed ceramic caps for silver mica
I've also played with the reverb circuit a bit (trying to get it deeper) and tried to make V1A a bit cleaner by changing the plate resistor to get B+/2 on the plate. I'm not sure about the last 2 mods yet. I'm also not sure if I like the Mallory 150s or OD caps better.

I like the MkIIC+ and rhythm II mods for sure.

Please keep us informed of your findings.
amguitar
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by amguitar »

What did you do for the MkIIC+ mod?
Also, if your amp has Simulclass, how did you set the bias?
I set mine rather hot. I have a set of STR 415's from the mid 80's, and the amp runs much hotter with those than with Boogie's newer tubes. In fact, when I first got the amp in '85, I had trouble with the outside-socket tubes burning out quickly. I needed to add the 2.7k resistors to the grids.

Now I am using JJ 6L6's. The trick with biasing the Simul-class, is to get the middle tubes biased hot enough without toasting the outside tubes. The inner pair ends up being a little on the cold side for 6L6's.
amguitar
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by amguitar »

What did you do for the MkIIC+ mod?
Also, if your amp has Simulclass, how did you set the bias?
I set mine rather hot. I have a set of STR 415's from the mid 80's, and the amp runs much hotter with those than with Boogie's newer tubes. In fact, when I first got the amp in '85, I had trouble with the outside-socket tubes burning out quickly. I needed to add the 2.7k resistors to the grids.

Now I am using JJ 6L6's. The trick with biasing the Simul-class, is to get the middle tubes biased hot enough without toasting the outside tubes. The inner pair ends up being a little on the cold side for 6L6's.
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lastwinj
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by lastwinj »

Darkbluemurder wrote:
Finally there is a 1000pf cap in parallel with the 270k plate resistor on V3b. My feeling says that 470pf should be enough.
might find removal of this cap to cause an oscillation.

germ
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Darkbluemurder
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by Darkbluemurder »

I know this is an old thread but I finally got around to progress. I did not yet attack the Mark III but I implemented the Mark lead circuit in another amp and then started to mod it until I got what I liked. What I came up with was the following seemingly minor changes to the basic circuit (BTW if any of my technical reasonings are incorrect, please step in):

1. I removed the 120pf between the grid and cathode before the first gain stage of the lead circuit. The issue I have that cap is that it adds to the Miller capacitance of the tube which makes the amp sound quite dull at lower gain settings (since the series resistance gets much larger with the 1meg log pot turned down plus that 680k series resistor in front).
2. I removed the 1000pf in parallel to the 68k resistor to ground in between the two lead gain stages and added that in parallel of the 82k plate resistor on the first lead gain stage. Again, this cap adds to the miller capacitance but this time the effect is less severe since the second stage (when unbypassed) is lower gain than the first and the series resistance before the second lead stage is lower (270k) and fixed. Across the 82k plate resistor it cuts highs, too, but not as much as it does not add to the miller capacitance of the next stage.
3. To cut some bass, I added a 100k resistor to ground after the coupling cap off the first lead gain stage. This is something I have seen in an older Kitty Hawk amp scheme. Good move IMHO because that allows for more bass in the clean mode without muddying things up too much in the lead mode.

So I basically followed the recommendations given by JayB earlier in this thread. I left the 1000pf cap in parallel to the 270 plate resistor on the second lead gain stage. My amp does not have the "Bright" function on the lead channel which adds a 220nf cap to the cathode. I did not implement this as I do not like the function on the original and it's always off.

These changes result in an amp that is much more touch responsive and easier to play because it's no longer killing the pick attack. The gain knob is useable over a much broader range than before. Before I always kept it way up - now I can set it anywhere from 10.00 to full on with pleasing results. The tonal color is still Mark-like but more open.

The only playing style I liked the caps to ground with was slide playing - with that style I felt the pick attack killing properties of the caps to ground were actually beneficial and made sliding easier - exactly the opposite to the regular playing style. I have an unused switch leftover which I will use to switch in a 120pf cap to ground for that. I think this is much more useable for me than the lead bright function on the original.

Many thanks to all who helped. I will now implement the results to the Mark III and see what they do to that amp.

And BTW: the Mark III is a 60 watt model, so no Simul-Class whatsoever.

Cheers Stephan
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Darkbluemurder
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by Darkbluemurder »

Meanwhile I cracked the Mark III open to compare to the schematic on schematicheaven. My amp is a Purple Stripe 60 Watt model with reverb but w/o the graphic EQ. There are some differences to the lead circuit:

- the resistor between the lead drive pot wiper and ground is 330k, not 150k as in the schematic, and there is a 120pf cap in parallel to that which isn't on the schematic
- there is no .001 cap in parallel to the 270k plate resistor on V3b. Instead there is a .002 (!) from the output of the 270k to ground
- after the coupling cap from V3b (which is .047 like in the schematic) there is the Mark II C+ network of 220k II 250pf series into 100k II 500pf to ground

The presence cap in parallel to the 56k is .01 and not .005 like on the schematic.

I will first halve the value of the presence cap and also halve the value of the snubber cap on V3b. That should already get me in the ballpark of II C+/Red Stripe tone. I will probably not stop there but I want to hear how that sounds.

Cheers Stephan
fenderbender
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Re: Mesa Boogie Mark III - How to get more touch response?

Post by fenderbender »

Hey Stephan, how did your mkIII turn out? Did you end up doing all the mods that
You did on your Homebrew amp?
I had a blue stripe that I did a resonance mod to. A 3n3 cap in series
With the neg. Fb circuit. I put it on a p/p pot (r2 master) so that I could bypass it
If I wanted to. This made channel two much more useable (REALLY good heavy
Rhytm tones available here). It turns out that I could turn the 80hz slider down on the GEQ and turn the bass down in the front end, for a tighter, yet less muddy lead tone.
Also, make sure you have gridstoppers on the power tubes, neither my mkIII or mkIV had them. I got serious blocking distortion when turning them up.
Best, Tommy J.
It sounded so sweet... And then it caught fire!
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