Mk IIB reverb and loop mod

[TiPiMods]

Hi
I'm new here on the board, and did a near 2 hours search now on this topic, but didn't find a satisfying answer. Maybe somebody can help. I recently bought an 81' Mk IIB with reverb and EQ, which sounds fantastic. The reverb has the well known issues, its a bit noisy and quite weak, even when I turn up the pot on 10. I heard there is a mod, which was practiced by Mesa Boogie ( Mike himself ) to remedy this problem. Since I live in Europe/Austria I cannot send my amp to the factory.
Does anybody know more about this modification (schematics, or who could do this mod) What does the mod to the loop? Does the reverb get stronger? Is it worth the effort?
thanks
TP

 

[mark2boogie]

Hi TiPiMods,

I own a MKIIA and I solved all the noisy-hum-hiss issues on it, except the relay pops (which not the most annoying, in fact). It's a tough job, but you will find some help in my thread :

Boogie MKII 60/100 Rev Eq in the Mark Series.

Is it worth the effort?

Absolutely !!! My MKIIA is now dead quiet with an amazing sound - these amps are wonderful, but they need to have a cure on several factory misconceptions - you'll find these described in my thread. Please note that if your reverb is weak, there may be a faulty reverb tank, or a dying reverb 12AT7 driver, maybe. You should normally have a deep, smooth, rich and good sounding reverb.

A+!

 

[TiPiMods]

Thanks, I read your thread, but I dont think I want to go that far. I would like to leave the amp as original as possible.
As far as I know, Mesa changed quite a few things when they tried to cure some of the Mk IIA issues. On the reverb circuit there are some different resistors in a Mk IIB and they used a 12AX7 instead of a 12AT7. I tried both tubes, there is not much difference.
I just bought a new reverb tank, because the old one was broken and was the wrong type anyway when I got the amp. Tubes are all new. I also tried different reverb cables with no difference.
The reverb is very still weak, even on 10. There is no hum, but as I have to turn the reverb poti up nearly full, there is a lot of hiss.

btw: your amp looks great! I have an Altec Lansing laying around somewhere and thought about give it a try. But I suppose the alu-cone will make the leadsound sound a bit fizzy. And first I want to get the reverb fixed.

As I wrote before: I heard there is a reverb and loop mod Mesa used to do on Mk IIB amps in the factory to adress this problems.maybe somebody knows more about that.

 

[dino2sf]

Check out my thread :http://forum.grailtone.com/viewtopic.php?f=27&t=51959

Very happy with the mod Mike B. performed a few months ago on my mark ii b

 

[TiPiMods]

Man, sometimes things are much more simple, than expected: After I took out the chassis and compared the wiring with the schematic I found out the reverb output was simply in the wrong RCA jack. (There's a third jack on the chassis which is for switching reverb in and out probably with some Fender type switch)
Sounds great now, lots of thick, warm reverb, no hum or hiss.

So that's the deal, when someone on ebay writes "everything is in perfect working order" :lol:

Thanks for your help and your hints in the right direction.

 

[mark2boogie]

I found out the reverb output was simply in the wrong RCA jack

:mrgreen: :mrgreen: :mrgreen:

Ha ! I thought that this point was cleared when I was speaking about a faulty reverb circuit ! Now you've got the expected reverb sound +++. Therefore you may notice Larsen effect tendency when playing loud and at quite high reverb settings, if your reverb isn't installed in an acoustically shielded/insulated pouch. If it's a concern for you, here's what I have done :

Original installation, prone to Larsen :

Shielded installation, Larsen-free :

A+!

 

[TiPiMods]

Check out my thread :http://forum.grailtone.com/viewtopic.php?f=27&t=51959

Very happy with the mod Mike B. performed a few months ago on my mark ii b

Thanks, I read your thread, very interesting. I'm not that lucky living in California, so I have to find somebody in Austria or Germany who can do this mod.

The biggest difference in IIB and IIC in terms of reverb is:

On the IIBs the reverb input signal is taken from the clean channel right after V1b, so it sounds quite natural on clean sounds. On the lead channel it does not reflect the lead sound, since the signal is taken still before the gain stages. So you get a lead sound with the reverb of a clean sound.

On the IICs the signal is taken after the gain stages, after the lead master, so the reverb gets either the signal from clean or from the lead channel.
To change this you have to change the effects loop too, because it is also different on IIB and IIC:

On the IIBs V2a, which is the effect send, is built as a cathode follower, which is actually a good thing, because it keeps the impedance low and you can use long cables to some pedals on the floor without loosing treble.
On the IIC the loop is after the reverb and after the anode of V2a, no cathode follower.

So that's probably what Mike B. is doing: he changes the reverb and loop like they are on the Mk IICs.
Maybe I can find some authorised service over here, who can do the mod.

@mark2boogie: yeah, thanks, you gave the right hint
my reverb is in the (original) thick bag with some rubber underlying,. probably someone stole yours, but i think you can get this directly from Mesa Boogie. Its the same material as the original amp cover.
Haha, hope you didn't give your last pants to the boogie :wink:

 

[mark2boogie]

I have an Altec Lansing laying around somewhere and thought about give it a try. But I suppose the alu-cone will make the leadsound sound a bit fizzy

Sorry - Yes, when I took the pics, I installed an ALTEC 417-8H borrowed to a friend for trial (as Mr Santana used one in his MKI). Make the trial, and you will heard that yes, you're right : the lead sound has a small "grit" in the highs due to the alu-cone. It's not so bad, in fact, but in term of tone, versatility and dB efficiency, the warmer ALTEC finally can't compete with the EVM12L on A/B tests - so I shifted for an EVM12L Black Shadow, which sounds strictly the same as the old original EVM I also have.

But again, it's a matter of taste and I will never say that the ALTEC is sonically inferior : it really made me hesitate... I also made a trial with a Vintage 30, and I discarded it immediately. Another good sounding speaker was the Eminence ME-12-100-PE-8, the ancestor of the LEGEND 1218 of nowadays : a good compromise between the EVM and the 417, with the advantage of a reduced weight...

I'll try to find another MKIIA or B in the future, even only a faulty chassis...

A+!

 

[mark2boogie]

On the IIBs the reverb input signal is taken from the clean channel right after V1b, so it sounds quite natural on clean sounds. On the lead channel it does not reflect the lead sound, since the signal is taken still before the gain stages. So you get a lead sound with the reverb of a clean sound.

This is the same on the IIA, and it's what makes the reverb so good sounding compared to most of the other amps, Boogie included : it stays deep, harmonically even, not the metallic and often too blunt decay sound you have on lead mode. I am akeen to think that the reverb circuit of the IIC is probably less good sounding for that reason, if I remember how sounded the reverb of some MKIII I had for service, which is nearly the same circuit.

Haha, hope you didn't give your last pants to the boogie

:lol: I took some used-ripped-off blue-jeans from my 20 year-old son ! But it's only the external packaging : I first shielded the reverb tank with a carton plate with glued-on alu foil, then I put the whole inside a wool coat sleeve, and finally inserted the stuff in the bleau-jean pant leg. It is simply put in the the amp, and loosely maintained in position by the sides of the cab and the magnet... No screw, no scratch. That's odd, cheap, easily removable and openable, and it works very well ! Therefore, I am not sure that there was an original stolen pouch in my MKIIA : I saw other old Boogies without reverb pouches, so...

A+!

 

[TiPiMods]

I wrote to Mesa Boogie for the reverb and loop mod. Maybe they have some authorised repair shops here in Austria or in Germany, which would do the mod. I will report, as soon I get an answer.

I'm doing mods myself, mostly the famous Billm mods on Fender Blues Juniors, "blackfacing" silverface Fenders and so on, but on collector pieces like this Boogie, I dont want to degrade its value by doin some mods myself.

But the more I play this amp the more I love its amazing sound, both rhythm and lead. I can't understand people in this forum, which wrote, their leadsound was crap. My leadsound ist quite focussed, big and fat, not too much gain but fluid enough to feel comfortable. One of the most important and usable feature is the pull shift on the treble poti, which makes a huge difference on the lead sound, without altering the rhyhtm: fendery, crisp and shimmering when pushed in, more midrange focus and into Marshall direction when pulled out. Both very usable and great sounds.
I have a lot of great amps to compare: Marshalls, Hiwatt, Fenders BF and SF, Acoustic, Kitty Hawks (fantastic german Dumble inspired amps from the early 80ties) and the Boogie Mk IIB is a very unique character with his punchy and bold tones.

 

[mark2boogie]

I wrote to Mesa Boogie for the reverb and loop mod. Maybe they have some authorised repair shops here in Austria or in Germany, which would do the mod. I will report, as soon I get an answer.

They have, certainly. It's not like in here in France, where everything has to be done through the (lazy and uncompetent) importer.

I'm doing mods myself, mostly the famous Billm mods on Fender Blues Juniors

IMHO, it is not difficult to improve the stock sound of these amps, as it is so ugly...

"blackfacing" silverface Fenders and so on, but on collector pieces like this Boogie, I dont want to degrade its value by doin some mods myself.

If you don't consider SF Fenders as collector's item, then give it to me, to complement my collection :

I used to design and build tube amps, but I do not have the time anymore today :

But the more I play this amp the more I love its amazing sound, both rhythm and lead. I can't understand people in this forum, which wrote, their leadsound was crap. My leadsound ist quite focussed, big and fat, not too much gain but fluid enough to feel comfortable

I agree completely - I would add "smooth", "touch sensitive" to your tone description. These are sonic qualities that I barely find on multi-channel amps, which often sound agressive to my taste. Another rare quality of the MKIIA/B with EVM12L in 100W position/8ohms is if you increase the volume, the sound remains the same. Only my SF Twin Reverb can compete in that field.
In fact, the only drawback of the MKIIA/B is their anvil weight... but again, I barely saw lightweight amps sounding good on stage at loud volumes...

A+!

 

[TiPiMods]

yeah, nice collection. SF Fenders are great, but they made a lot of them in the 70ties, they are not that rare as Boogie MKI or IIs, so fortunatly the prices are reasonable, because collectors concentrate on BF Fenders. For musicans they are very collectible. And changing PI and bias supply back to BF specs makes them sound as great as their BF brothers and sisters.

 

[McBarry]

TiPi, interesting your note about Kitty Hawks..
I have one here Down Under and love it dearly (M1 head)
The Lead sound is my only complaint - just toooooo much gain with little control - bleeding some signal after the first lead stage helped, but it could do with some more bench-time, when I get a spare few days to tinker..
I added a fan and channel LED's on the faceplate - much easier to see what's going on..
Dave

 

[TiPiMods]

Kitty Hawk is famous for their first few amps: Junior (script logo), Standard and Custom. They imported Dumble Amps in the early 80ties and decided to make their own amp. These amps are copies with very few own features, with an amazing sound and built quality quite similar to Mesa Boogie at that time. All the amps came with EV12L speakers. There are also some very eary amps which had original Dumble parts inside cause Mr. Dumble sent his boards to them to finish the amps in Germany.
The Junior is a MK I copy with its two inputs:

The Standard is a Dumble Overdrive copy:

I never had the chance to play a original Dumble, but it is said by people who own Kitty Hawks and Dumbles (http://www.wolfgang-brammertz.de/index_brammertz_equipment.html) they sound quite similar.
The later models lost a lot of their reputation cause built quality degrade a lot and the sound changes more into Marshall direction. On his website Reb Beach said, they called them "Shi!ty Hawks" :wink: because they fall apart on tour: http://www.rebbeach.com/history_faq_gear.htm

BTW: very impressive collection of MK amps

 

[TiPiMods]

The Lead sound is my only complaint - just toooooo much gain with little control - bleeding some signal after the first lead stage helped, but it could do with some more bench-time, when I get a spare few days to tinker..

Another idea: maybe it helps to put in a tube with less gain like a 5751 or a 12AY7
Thomas

 

[McBarry]

Thx Thomas, I tried the 12AT and also a 12AU but the circuit has just ooodles of gain.. once the lead drive is over 3 it's pretty much saturated and only slightly changes from there up. Dropping some signal to ground before the inital lead gain stage helped, so I think when I get time I'll tinker with dropping the overall gain of the stage, and then possibly the final stage as well, before it leaves the preamp.
Those KH pics do look old, and agreed with build quality - mine is pretty good, and layout is neat and planned, but I've improved a few things. My fear is their reputation here in Australia for burning output transformers - I never found out exactly why..
And both the power and output are the biggest mothers I've ever seen...LOL
DB

 

[TiPiMods]

I never heard about burned output trannies, but there is some bad reputation because of weak power transformers.
I had a preamp in the 90ties, the "Kitty Hawk Quattro", which sounded really great btw. but nearly all of them burned their power trannies. I never heard about that issue of their bigger amps yet.