Mesa Boogie SOB to Mark 1 rebuild

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Mark
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Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

EDIT: As the Mark 1 Boogie is a modded Fender amp. I have posted here.

Years ago I bought a butchered Mesa Boogie S.O.B. It had channel switching, solid state effects loop and a line out added to the amp.

The amp was not working very well when I got it, it turned out that the cathode bypass capacitors had gone short circuit in the preamp. Once the caps were replaced, the output from the P.I. was an issue, thus a new P.I. stage was required. At this stage I removed the channel switching, effects loop, and the line out.

The amp has been rebuilt with the Tweed style P.I. which is stock in the original Mark 1 amp which sounds good though the amp was a one trick pony, the gain in the Tweed style P.I. had much greater than the Blackface style P.I. stage, it was then rewired as a reissue Mark 1, but I found the amp to sound very flat, it was then rebuilt as a Dumble Bluesmaster, it was okay but lacked bass response. At this stage it was going to be a hassle implementing all the switches into the chassis so I decided to rebuild the amp into an early Mark 1.

There are a few resistors in the circuit that I don’t understand. There is a 100K resistor between the treble wiper and the top of the volume control this 100K resistor was in my late 70’s Boogie, there is also a 220K resistor from the reverb recovery stage to the pre P.I. master volume.

The hardest part of the circuit to get right is the P.I.

https://el34world.com/charts/Schematics ... _marki.pdf

https://el34world.com/charts/Schematics ... eissue.pdf

The circuit by Gil Ayan is definitely right, though I wonder about the 10K tail resistor, 56K feedback resistor and 10K presence control being correct. Any thoughts on this one?
Last edited by Mark on Thu May 25, 2023 5:48 am, edited 2 times in total.
Yours Sincerely

Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

A few pictures of the amp in it's current state of rebuilding.
IMG_1365.JPG
IMG_1366.JPG
IMG_1367.JPG
IMG_1368.JPG
IMG_1369.JPG
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Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

The power supply has had diodes and caps replaced.

The diodes were changed to UF4007's, the filter caps were replaced and the 110uF main node has been reduced to 50uF as the amp sounded quite stiff. I'm awaiting two 47uF/100v caps in the bias circuit.
IMG_1370.JPG
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Mark Abbott
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clarkleach
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by clarkleach »

Mark wrote: Sun May 03, 2020 7:40 am EDIT: As the Mark 1 Boogie is a modded Fender amp. I have posted here.

...snip...

There are a few resistors in the circuit that I don’t understand. There is a 100K resistor between the treble wiper and the top of the volume control this 100K resistor was in my late 70’s Boogie, there is also a 220K resistor from the reverb recovery stage to the pre P.I. master volume.

...snip...
The 100K resistor creates a voltage divider with the 1M pot to ground. I suspect this is to attenuate the signal a bit to not overdrive the following preamp stage.
Not so sure about the 220K, maybe it helps remove ice-pick frequencies.
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

Thanks for your reply. I have found in Dumble style amps that the preamp doesn't distort after the first stage due to the loading of the tone controls. The addition gain stage would definitely change that though. I suspect many of these mods were common knowledge amongst L.A. amp repairer as Rivera's mods and the Mark 1 have some similarities.
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Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

I'm trying to work out a pot value for the bias network. It is done the dumbest way on the power supply pcb. They are shunting it at the first node where it has the most regulation. I thought I'd be smart and use a 1 M pot to determine where the resistance starts taking effect. I basically blew holes in the Alpha pot. The maximum resistance needs to be 50K and I'll have work out what resistance is required to get it to -37v for possible use of EL-34s.

The other issue is where to mount the PI stage in lieu of the wacky S.O.B. PI stage. The PI is on the far left side of the main PCB. All suggestions welcome.
IMG_1373.JPG
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Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

Early Mark 1 shots versus SOB gutshots. I find it very hard to read the colour codes on the resistors though.
IMG_0808_zps1aqwzgxz.jpg
IMG_0809_zpsmjp76fia.jpg
SOB gutshot 1.jpg
SOB gutshot 2.jpg
EDIT: this amp appears to have a 820 ohm feedback resistor and the 100 ohm tail resistor.
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Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

I powered up the Boogie yesterday and I rather liked the tone of the amp with the Mallory 150 caps. I also put a Dumble value 0.0022uF mid boost cap in. When the amps normal input (not the extra gain stage volume) was dimed and the mid boost engaged it distorted quite nicely.

I have decided to stick with the Mark 1 reissue P.I. as it sounds good and it is bright enough. The circuit in the original Boogie drawing would be unruly to use and gain would be too much. I think I'll use the Limit pot position for a LeMar post P.I. master volume. This will allow the P.I. stage to be overdrive and the volume reduced. The amp is really loud, and it needs to be made manageable.
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Mark Abbott
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

I have the gain stage in the amp now. The gain stage is pretty stock with the exception of the 22uF cathode bypass cap is now bypassed by a 2.2uF cap. My Les Paul likes this set up and the amp gets a reasonable faux Marshall tone (gain 1 on 10, gain 2 on 2 with the bright switch on, the pre P.I. Master volume is in 10 and the LaMar Master volume is set to two or three). I'll have to listen to some Santana to see if it can pull those tones.

Getting good tones from my Strat is still an issue, it is quite hard to just plug in and play with a good sound coming out. It takes a bit of knob tweaking to get something acceptable.

I did look on Mesa's manuals and I was surprised to see the P.I. stage uses a 12AX7, looking at Gil circuit diagram I would have thought the P.I. valve would have been a 12AT7 as I'm using. I seem to remember my old Mark 1 using a 12AT7 in the P.I. stage instead of an 12AX7.

I'm still at the tweaking stage with the amp. I'm not in love with the output transformer which is the same one as used in the Mark 2 series of amps which have a following, I think a Bassman style transformer would work best in this amp.

I'll post a circuit diagram of the amp very soon.
Last edited by Mark on Sat Jun 13, 2020 7:07 am, edited 1 time in total.
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Mark Abbott
Doug R.
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Doug R. »

I have built several amps using the Mark II B circuits...they have always sounded good, IMO.
Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

I have the holes in the back of the amp to put in an effects loop. My friend has used the Valve Wizard one and says it is quite transparent.

http://www.valvewizard.co.uk/fxlooppcb.html

I'm still at the tweaking stage with the amp. I'd report back more later.
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Mark Abbott
southbound suarez
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by southbound suarez »

I know I am late to this party but curious as I have found this thread one year later as I hunt down info on Early Boogie amps.
I suddenly within one week have had four separate Boogies appear at my door. A Lonestar for basic maintenance, a cute little lunchbox with a quad of mighty KT88s that blew a tube and took out plate resistors,,,,, just regular stuff but still a whole wave of Boogies in a single week.... Last week was three separate Egnaters...
Anyways the interesting stuff is an original Mark1? This was brought to me by the brother of the orig. owner who bought it new in 1973. No idea what the older brother had done to it or by whom? This was a hidden closet gem find where it had been lost for decades. Not familiar with these amps but I presume it is a Mark1 ? Unsure what mods are done. Interesting to find that both inputs have been internally jumpered Ala plexi style with both gain channel controls interactive regardless of which input. Also noticeable is that the first preamp tube socket has no heater voltages and instead of tube there is a solid state tube replacement device? Three pots occupy the back panel along with addition of two in and out 1/4" Jack's next to the first pot labeled slave. The next pot labeled presence and third pot is labeled reverb. It seems as if extra speaker outputs have been added yet no additional impedance taps and all jacks are mono non . shorting. The center top of the chassis has in and out RCA phono jacks for reverb and centered between them is another 1/4" phone jack of which I am clueless? It appears that the switched leg of the jack is tied to the left leg of the treble pot and probably mutes signal to gnd when switches with plug insert?
I am not too familiar with these early Boogies. But if that isn't crazy enough,,,,, Yet another old Boogie is brought to me! I think this is a Mark 4 that is a true basket case. The owner blew it up and took it in for repair a dozen years ago but the PCB had a hole burned all the way thru near a tube socket and the tech he brought it to returned it totally disassembled while declaring it a basket case.
I am just starting my research and perhaps I might start a new thread elsewhere.
I hope your Boogie project was a success and would enjoy your update! I am going to download those el34 world schematics linked elsewhere in this thread. Thanks!
Here are some pics:
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southbound suarez
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by southbound suarez »

Oops !
I wanted to add pics of the Mark-IV too.
Probably less confusing to post one last posting on the thread here showing the Mark-IV basket case that was brought here by yet a different person . Yet another Boogie and it's an oldie too, like the presumed Mark 1
This is what was received. Unsure how long it will take before I can get to this one....
16277144994064194450475397214983.jpg
16277146799194846135060685080900.jpg
16277147519865827354461598313433.jpg
Well, kinda interesting stuff whadya think?
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Mark
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Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

Thanks for the pictures.

The Mark 1 looks interesting. It looks like the Fetron is still working or if it’s not the diodes should be taken out and replaced with 1.5K resistors.

The PI is similar to to reissue amp. On You Tube I saw an amp that used the circuit for the PI that’s in the oldest circuit diagram. It sounded different.
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Mark Abbott
Mark
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Location: Sydney Australia

Re: Mesa Boogie SOB to Mark 1 rebuild

Post by Mark »

I end up removing the La Mar master volume, I didn’t like the way the P.I. stage broke up. Going from PPIMV to pre PIMV resulted in a change in timbre. I like the sound of the stock master volume.

I have found that there was a lot of signal going into the P.I. stage. I was using a 0.01uF cap going into the P.I. and decided to use the 0.001uF this seemed to help matters. EDIT: I end up using 0.003uF per the reissue Mark 1 amps. I found that ceramic caps sounded better to my ear than silver mica. The treble was harsh sounding and ceramic caps in the tone stack and input to the PI sorted that out.

I also found there is something in the treble frequencies that I don’t like. I have decided to use ceramic caps for the treble cap and the P.I. entrance cap. I left the 10pF silver mica cap in place as it was hard to find a 500v 10pF cap,
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Mark Abbott
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