Boogie MKIIB Rev Eq 60/100W head

[JOEY B.]

I am going to assume that you saw the incorrect screen grid resistor on the far left power tube socket.

Yes, and many other parts have been replaced too when it was serviced at the MESA factory in 2007. For the moment, I am tracing the "Fx Loop Mod" schematic. I'll try a search to see if I found some drawing to compare with mine.

A+!

That resistor is not part of the "mod". Well wishes on tracing the "FX Loop Mod", it really makes the Mark IIB sing. 8) Here is the link to the stock IIB schematic. http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/boogieamps/boogie_mkiib.pdf

 

[McBarry]

Gosh that's clean inside there Mk2B.

And the PSU caps all look nice and new.
Enjoy your journey with this new baby...
Dave

 

[mark2boogie]

Thank you JOEY B. & McBarry !

@ Dave : are you still on SBY ? Do you have found the time to CAD-trace your MKIIA schem as we discussed some time ago ? If yes, you can send it to me (.pdf or.jpg) so I can put all the values and mods of my MKIIA on it, and return it to you for info and/or comparison.

A+!

 

[mark2boogie]

Hi All,

Here is the schematic of the MKIIB Fx Loop Mod factory-released at MESA on my amp in April 2007 :

The main difference with the original MKIIB circuit is that the Reverb and the Fx loop section are connected after the Lead Master and Master 1 controls, which can act as a kind of Fx loop send level, while a 3rd pot has been added at the back of the chassis : the Post Fx LVL, which can be considered as a general master volume.

The Fx loop instruction notice mentions that if the loop is not used, stick the Post RFx LVL at 8, for a gain recovery of 1, and use the M1 and LM as usual master volumes, to retrieve the original performance of the stock MKIIB. My MKIIA is gain-boosted, but this MKIIB has slightly more gain : I'll try to see why, because the lead circuits (V1B, V2B, V3B) are not so different between the 2A and the 2B, even modded.

A+!

 

[McBarry]

Hi Bruno, apols no, not as yet.

Although I'm laid up with a broken foot, I'm trying to rebuild a small amp into a Fender 5E3.
With a couple of hundred other jobs on the side, the CAD project is a fair way down the list.

I don't have a CAD program so am going to have to find something freeware, but it's still firmly on my radar..
(Any suggestions for free CAD pgms folks????)

Best of luck with your new baby and your forensic interest in same. V inspiring..
Dave

 

[mark2boogie]

(Any suggestions for free CAD pgms folks????)

Sorry Dave (McBarry) : from me... No... As you can see, I use a PAD program (Pen Aided Design) :mrgreen:

I have made some first trials on the amp some days ago. Gretsch G5435, Hagstrom Viking, Gibson ES-3335TD, EVM12L open cab.
To sum up, compared to my "improved" MKIIA :

- the tone is the same in rhythm mode, a bit less smooth in lead mode, maybe, but the general character is identical +++
- the gain is a bit higher and more focused "lead/liquid", less "bluesy/soft". ++-
- The sustain is similar. +++
- the reverb is similar in clean mode, but more discrete in lead mode and its tone is less unique, less pleasant : more metallic, less decay, less deepness, due to the fact that the overdriven signal is reverbered. +--
- the amp is "noise ridden all around", as was my MKIIA, despite its recent and welcome servicing in 2007. ---
- the LM and M1 lacks spreading too. ---

In addition, the Fx Loop Mod involved some re-routing of traces on the original RP-8B preamp PCB, and some traces needed to be cut. Also, the section B of the relay is not used anymore. This mod is not a "2 wire / five minutes job", but a very well-studied "modernisation compromise" from Mr Bendinelli... A further accurate analysis will show me where is the difference in lead mode in terms of gain and tone when the Post Fx LVL is ont used and set "original MKIIB" as stated by the documentation.

I also have "for restoration to its original condition" a MKIIC imbuya combo from 1983 - so I think I will be able to make interesting comparisons, and perhaps try to record some samples :idea:

Wait and see...

A+!

 

[gonzo]

SO, i have a '82 Mark2b, that is a 60 watt head, with GEQ, but no reverb.

how would this mod effect my amp?

 

[mark2boogie]

SO, i have a '82 Mark2b, that is a 60 watt head, with GEQ, but no reverb.

how would this mod effect my amp?

Hi,

1 - about the reverb : the level of the reverb is more balanced between RHY and LD, even when you use the Booster. It is even much more balanced and good reverb sounding if you modify the reverb driver circuit for the use of a 12AT7, as I done it. But as your amp has no reverb, you're not concerned.

2 - if you want to overdrive another stage to have more gain, drive and sustain, you can, via the FX loop LVL used then as a "master of the master volumes". The rear placement of the pot is not practical, but it's nonetheless doable.

3 - it lets you manage the level of the signal send and return jacks to drive suitably your inserted effects.

I did not tested features 2 and 3 for the moment to confirm what I wrote, sorry, but I read somewhere that it was worth, even if I am not too concerned.

A+!

 

[mark2boogie]

Hi All,

Here is the complete traced schematic of my MKIIB 60/100 Rev Eq, including the 2007 MKIIB MESA Factory FX Loop Mod, plus the reverb driver and master volume mod I released some days ago.

The master volume mod consists in rewiring the M1 in pot, not in variable resistor. It gives much more progressivity in general volume level adjustment, and also much more progressivity in setting the levels between RHY and LD, while maintaining now a constant balance when M1 is changed.

The reverb mod returns to the 12AT7 driver instead of 12AX7, with change of the cathode resistor. The tone, deepness, and decay of the reverb is way better (specially in the LD mode) and also the reverb level balance between LD and RHY is now nearly equal.

This is not the end : wait and see... :mrgreen:

A+!

 

[McBarry]

Bruno, U are an inspiration..
U must have some serious time on your hands though !!
Dave

 

[mark2boogie]

Dave - I apologize but... what do you mean by :

U must have some serious time on your hands though !!

tu dois avoir quelque sérieux temps sur tes mains toutefois :!: :?:

Yes, these modifications are probably "inspiring", but with the exception of the reverb and master mods (easy to do / try), the Fx Loop Mod is not so evident to realize as it involves PCB trace cuttings and components re-routings...

A+!

 

[McBarry]

probably lost in the translation..
I mean all the work that you've done would take a lot of time, eg: looking around inside a chassis, re-routing leads, planning modifications, checking circuits against the schematic and them modifying the schematic.. That sort of stuff.
In isolation, each little thing would probably not take too long, but collectively, much time would have been spent.
For me to achieve this, I'd have to totally clear my bench of a "hundred" other works in progress, as well as have a good general tidy-up as well (which is always a good move I reckon)..

Bonsoir from DownUnder - I'm off to bed (c'est 1.45am et je suis tres fatiguee...)
ZZZzzzzzzzzzzzz... Dave

 

[mark2boogie]

I mean all the work that you've done would take a lot of time, eg: looking around inside a chassis, re-routing leads, planning modifications, checking circuits against the schematic and them modifying the schematic.. That sort of stuff.

In fact, I just traced the schematic of the Fx Loop Mod as I found it in the amp itself, and as it has been done at the Mesa Factory in 2007, which was ordered by the previous owner of this amp, along with the check-up (I have the invoice). In this field, I changed nothing, with the exception of the reverb driver and the masters mods - two easy things to do.

I don't know if you have the same problem, but the most difficult now for me is tied to my ageing sight, so I am forced to check, re-check and cross-check... For example reading the difference between a 33K and a 330K resistor is sometimes tough on some models under average light, even with glasses.

This MKIIB amp indeed needs other improvements as my MKIIA needed, and I will do the same : the job is long, you're right, but it is fully rewarding in the end... Imagine your amp singing with a near to SBY S/N ratio. That's why I wrote that it was not the end :mrgreen:

A+! from l'autre bout du monde

 

[McBarry]

Yes, agreed re eyes.
I have these bifocal glasses but prefer to take them off completely for close work.
Resistor Grey band is what usually trips me up, then sometime the red - as U mentioned it can often fade and look orange..
When snooping around in chassis', my trick is a cheapo LED torch ( http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=280707493562&ssPageName=ADME:L:OC:AU:3160 ). The light power is incredible.
DB

 

[mark2boogie]

Hi All,

The work is in progress - almost coming to the end - for the electronic section :

- installation of an export power transformer, with bold/tweed "variac" facility,
- shielding of many sensitive connections,
- rewiring of the master1 in true pot,
- re-routing and insulation of the GND line,
- re-routing and balancing of the heater circuit,
- additional PSUs for fan (12VDC) and lead/rhy relay (5VDC),
- installation of a bias control,
- miscellanous & issues... fixed or to come, more or less like on my MKIIA...

Tomorrow should be the 1st sound test of the modified version...

A+!

 

[TiPiMods]

wow! looks great and a lot of work ...just came back from my holidays and returned to the forum after some absence. Thanks a lot for posting all this schematics. Did you order the export tranny from Mesa? And what does this Bold/Tweed switch? Is it your own invention?
greetings

 

[mark2boogie]

Hi TiPiMods,

Did you order the export tranny from Mesa? And what does this Bold/Tweed switch? Is it your own invention?

I got the export transformer from TAD (Tube Amp Doctor / Germany) :

Some years ago, when I worked at ALSTOM, I would have it built to my specs in the highest quality grade... The TAD Xfo is Italian-made, the quality is good BUT you will COMPULSORILY need to insulate the tie-rods to avoid the magnetic short-circuit of the lamination stack, otherwise the transformer won't work, will grunt loudly, and in a few seconds the tie-rods will get super-hot !!! Yes, you read it : it was faulty "factory stock right out of the box"... Here's the tie-rods insulation mod, using PA washers and heatshrink :

Now the transformer works correctly... It has additional features : 5V and 12V independent windings, perfect for powering a 12VDC fan and the Rhy/Lead relay correctly (on 5VDC). The BOLD/TWEED facility was included in this Xfo, at the primary : it is an extra winding which reduces the power by increasing the transformation ratio. Doing that, voltages decreases at the secondary, and so the power, for a "tweed" or "spongy - brown" sound... This feature exists on some MESA models nowadays. I invented nothing...

A+!

 

[TiPiMods]

Unbelievable, I never heard about this issue with the skrews. Isn't it the same with all kind of Fender Amps?

 

[mark2boogie]

Unbelievable, I never heard about this issue with the skrews. Isn't it the same with all kind of Fender Amps?

Indeed, unbelievable... But true here. So if you buy an export Xfo, inspect it carrefully for the tie-rods... If you want to use an old transformer, check that for damages, too.

On old Fenders, and on good replacement transformers, you will find at least bakelite, cardboard or nylon washers on one side of the tie-rods assembly : that's usually enough. I just wanted to go further with total insulation, which is indeed better in the time (and used on critical transformers applications).

For the same reason, the iron core is laminated and sometimes thinny varnished in-between : this is to avoid magnetic short-circuits (by cutting Eddy or Foucault currents). Installing non-insulated tie-rods would be sacrifying all that, you understand it, now...

In the same range, rusty laminations edges can more or less suffer magnetic short-circuit between them, too.

A+!

 

[TiPiMods]

Very interesting. I knew, that the reason for core lamination and isolation is to cut eddy currents. So probably the tie rods form shortcut, if the holes in the core are too small and the tie rod can touch the core inside. And I never thought about the rust thing. Maybe thats the reason why in one of my old Kitty Hawk Amps the Xfo gets quite hot after a while: it is indeed a little rusty outside.
thanks for sharing your knowledge