Mark IIa Lead Channel - Can It Be Modded?

[Rick Mark2A]

Does anyone know if it's possible to mod the lead channel on a Mark IIa? I find that my lead channel is a bit darker than the first stage, and while I can use EQ to get the sounds I want, there is always a compromise if i want to switch back and forth from first channel to lead channel. Wouild it be possible to brighten the lead channel in some way?

 

[bendervish]

Maybe a similar issue here. I have a Mark IIa export head dated 1979, she's got a pretty big looking transformer. I'd like to know if anybody has advice, and/or experience with modifying the Gain Boost feature. I love the character and the nuance in the breakup when the Gain Boost is engaged, but when it's on it seems to not respond much to changes in the geq or face eq knobs. Very dark and mushy. Just to complicate the experience, I tune down to C# which can make for a real tubby sound. It can be fun to mess around with, but some more control would be nice. It's interesting, because when the Gain Boost is not engaged, the lead channel can be bright as hell, icy even. It's a very dramatic tone shift. Would doing something to the Gain Boost alter anything that makes this amp the killer old-school beast that it is, as it is? Or would it just be isolated to that feature? Anybody ever try anything with that?

There is some talk here about the IIa Gain Boost feature:
http://forum.grailtone.com/viewtopic.php?f=27&t=50731&hilit=Mark+IIa+pull+gain+boost

and this is a discussion about a similar deal on a Mark IIc, but maybe something to think about too.
http://forum.grailtone.com/viewtopic.php?f=27&t=62830&p=435914&hilit=Mark+IIa+tone+stack+gain+boost#p435914

 

[Rick Mark2A]

Bendervish, from what I have read over the years the gain boost feature is supposed to totally bypass the tone controls. My IIa does the same as you described. It gets very dark and undefined when I pull the gain boost out. I never found that feature to be of any use so it's something that I never use. I agree though, it might be cool if there was a simple mod to perhaps brighten it up.

Let me take this opportunity to ask you a question about your IIa if you don't mind. On your amp, if you leave all tone settings the same and switch from clean channel to lead channel do you notice the sound getting darker and less defined? I was just wondering if this is typical of the Mark IIa or if mine is different than most.

 

[bendervish]

Hi Rick,
That was a fun question. I played around with switching back and forth between Ch1 and Lead and a few other settings a bit last night. I'm sure differing combinations of guitar, pickups, and speakers could present a pretty broad band of tonal impressions in the switch, but this is what I heard. I noticed that when the Treble Pull Shift is not engaged there is a perceptable deepness that happens in the switch from Ch1 to Lead, it could come across as darkness. It's definitely honky and a little rouded off, but the high end of the clipping kind of makes up for it so it doesn't feel real muddy. When the Pull Shift is engaged though, it is actually more crispy or present when I switch from Ch1 to Lead. I'm playing a couple of SGs with moderate-high output pickups straight into the MkIIa with Vol 1, Lead Drive, and Lead Master all dimed. I had Master 1 in different levels between 0 and 3, enough to be loud but not quite performance level. I'm also running through a MB 3/4 back 2x12 (C-90s I think). I'm probably starting out from a pretty rich, dense position to judge the high end response broadly. Hope that helps some.

 

[igfraso]

The Mark I and the Mark II-A are dark amplifiers. That is why you have to use a bright speaker, like the Altec or the JBL.

 

[Rick Mark2A]

I can get either channel to brighten up through the use of EQ. My problem is that the lead channel is noticeably darker than the clean channel and since I am sharing the same EQ settings with both channels I end up compromising each channel to make them sound closer to each other. In other words, when I dial in my lead channel to sound excellent, the clean channel sounds too bright, and when I dial in the clean channel to sound right, the lead channel sounds dark and murky. Overall I really like the clean channel on this amp, its probably the best clean I have ever heard. If I could figure out a way to make the lead channel a bit brighter I think I would have a killer combination. I might try some more experimenting with preamp tubes and see how that goes.

 

[Heritage Softail]

I can get either channel to brighten up through the use of EQ. My problem is that the lead channel is noticeably darker than the clean channel and since I am sharing the same EQ settings with both channels I end up compromising each channel to make them sound closer to each other. In other words, when I dial in my lead channel to sound excellent, the clean channel sounds too bright, and when I dial in the clean channel to sound right, the lead channel sounds dark and murky. Overall I really like the clean channel on this amp, its probably the best clean I have ever heard. If I could figure out a way to make the lead channel a bit brighter I think I would have a killer combination. I might try some more experimenting with preamp tubes and see how that goes.

Have you called the peeps at boogie for ideas? They do mods and of course repairs to get you back to original spec.

To accomplish your goal of tone shift, try an EQ in the loop. It is very effective. Cheap too.

 

[mark2boogie]

Does anyone know if it's possible to mod the lead channel on a Mark IIa? I find that my lead channel is a bit darker than the first stage

Yes. It is possible to mod it quite easily, if you have the competence, but Daniel "igfraso" is quite right :

The Mark I and the Mark II-A are dark amplifiers. That is why you have to use a bright speaker, like the Altec or the JBL.

Personnally, it's what I love on my MKIIA, feeling that post models are more aggressively voiced...

You can benefit for some extra-brilliance (on a push-pull switch), more gain and smooth sustain by reworking some values on V3B, the lead stage. You need to replace a push-pull pot, some caps. If you want, I can draw the schematic, scan it and post it here. It is what I have done on my MKIIA.

A+!

 

[Rick Mark2A]

Have you called the peeps at boogie for ideas? They do mods and of course repairs to get you back to original spec.

To accomplish your goal of tone shift, try an EQ in the loop. It is very effective. Cheap too.

Hi, yes I have been considering calling Mesa, was going to try that yesterday but found out they are closed on fridays.

I already have a 15 band GEQ in my loop, it does work fine and I can get almost any sound I want from clean to dirty. The problem is that I would like to be able to switch back and forth between clean and dirty without having to re-adjust my EQ settings. My thoughts are that I can do one of the following to accomplish this:

1. Modify the dirty channel to be more bright

2. Use multiple EQ and use some kind of switching automation (midi perhaps) to handle my channel switching and simultaneously switch presets on a digital EQ. This would most likely cost more than I would like to spend but it is an option.

You can benefit for some extra-brilliance (on a push-pull switch), more gain and smooth sustain by reworking some values on V3B, the lead stage. You need to replace a push-pull pot, some caps. If you want, I can draw the schematic, scan it and post it here. It is what I have done on my MKIIA.

A+!

Yes sir, I would be very interested to have a look at that! Please post if you would be so kind.

 

[mark2boogie]

Yes sir, I would be very interested to have a look at that! Please post if you would be so kind.

Here it is :

Also have a look to my thread about my Boogie MKIIA, for the occasion :

http://forum.grailtone.com/viewtopic.php?f=27&t=54614

A+!

 

[Rick Mark2A]

mark2boogie, thanks for posting that. I am looking at the original mark II schematic and the lead stage appears to be based around V3a instead of V3b as indicated in your diagram. I would assume I could just change the pin designations and it would work fine. I think I am going to try and locate the parts for this and try it out eventually. I also have another idea which I am going to try. I am going to see if I can get some help designing a rack mounted remote switching box which would enable me to switch between channel A and channel B on my stereo equalizer and also change channels on the boogie with one click of a footswitch. My thoughts are that I might be able to use something like this http://www.smallbearelec.com/servlet/Detail?no=1253, either one or two of them combined in the rack box.


Edited to add: I enjoyed your Mark II boogie thread very much, lots of good info there.

 

[mark2boogie]

I am looking at the original mark II schematic and the lead stage appears to be based around V3a instead of V3b as indicated in your diagram. I would assume I could just change the pin designations and it would work fine.

Yes, Rick Mark2A. I don't remember if the schem is correct or if I exactly noted what was in my amp, but nonetheless you're right.

I am going to see if I can get some help designing a rack mounted remote switching box which would enable me to switch between channel A and channel B on my stereo equalizer and also change channels on the boogie with one click of a footswitch.

It should be doable, as the channel switching requires only one dry contact to be activated.

Edited to add: I enjoyed your Mark II boogie thread very much, lots of good info there.

Thanks !

A+!

 

[Rick Mark2A]

Update: I just got the amp back from my local tech. I had brought it in for a cap job and he offered to mod my amp so I told him to go ahead and do it. The amp now sounds AMAZING. He said it was a fairly simple mod, he added two hi pass capacitors in the lead channel and also removed a small capacitor elsewhere. The lead channel is much more articulate now and the frequency response is pretty much identical to the clean channel. Exactly what I was looking for. He said the early Mark amps are pretty much notorious for the dark and muddy sound on the lead channel. Finally I am totally happy with my Mark II.