MP-1 full refurb and restoration

[Systematic Chaos]

Alrighty, this is a follow up to my previous thread about my 1.38 that had some issues > V1.38 gain drop/loss just seconds after startup / warmup

Disclaimer: There is absolutely no scientific/by-the-book background on what I did and came up with. I dial sh!t in with my ears rather than my eyes and most of the time I couldn't care less about what some spec or data sheets state - if it sounds good and works fine I'm a happy camper.
I absolutely love the combination of preamp tube coloration + power amp tube coloration + speaker coloration.

But let's get started....
This v1.38 came with a faulty Vactrol, just seconds after power up the gain dropped to barely crunchy levels. As stated in the other thread I sorted that out by swapping said Vactrol for a new one. Next was a blown resistor in the EQ circuit resulting in the treble control being useless. Sorted that out as well.
Thanks to MJMP I got one of the old go transformers that provide a tad bit more of V to the 5V regulators and tube heaters than later ones, but still less than an MDRT (I already mentioned that I actually like the more compressed sound with the OG trannies and these old ones really tick off all my boxes).
After putting it in a taking the preamp through its paces I thought I was done, wrap. After about 30-40mins noodling the gain dropped again - wtf!?!?! After some real thorough inspection it turned out that the trace connecting the + lead of C6 to R15 on the tube board lifted after everything had "warmed up". Rectified with a point-to-point wiring:

You'll notice that the tube board now has all new components.....Super Stock Mod with increased gain.

While I was looking for the aforementioned culprit I also noticed that some of the small electrolytic caps had leaked to some extent....Coin-toss: started replacing them. That's when I found another blown resistor:

Cleaned:


And this is where I arrived at crossroads...leave it be and flip the unit "for repairs" or go full circle (...you know what's coming). Everything worth doing is worth overdoing. Days are getting shorter and I decided to put in some more hours on this one and do a full make-over.....All new resistors (higher rated/spec'd higher quality metal films), all new electrolytic caps (higher rated/higher spec'd, better quality), all new WIMA Caps (whats good enough for Diezel amp sure is good enough for the 1.38), all new Diodes, Zeners,...swapped the input jack for a closed/switched Cliff Jack, rewired one of the display ribbon-type cable which had broken leads (faulty display).....here she is:




In addition to the Super Stock Mod with increased gain and the noise mod I also applied parts of the EQ mod of the 3.666, basically just the resistor change as stated in the 3.666 instructions without putting in trimpots and not changing the .047µF caps for .033µF (tried both and prefer the .047). I also swapped the TL074 OpAmp for a BB OPA4134PA which sounds better (to my ears)

So the big question: Was it worth it??
Abso-effin'-lutely!!! This MP-1 is dead quiet noise wise, quieter than my buddies two year old Engl e530 Preamp!
Everything works like charm. I had a loop playing for 2 1/2 hours with zero issues or irregularities.
How does it sound? Awesome! Juicy, creamy, chewy with that nice bit of compression courtesy of the OG transformer.....
Here's some sounds....1 HB Ibanez RG-Mutt (DiMarzio ToneZone, Pickguard-mounted on a basswood body, rosewood neck, EDGE Trem with brass block, Volume Control only, no caps) direct to the MP-1. Output A through a Mooer Radar Cab-Loader with an OH IR (blend of Mesa 3/4back 2x12 with MC90 speakers and Mesa Trad 4x12 with 70w V30) direct into two cheapo old Alesis M1 Active 520 monitors. No fancy playing....Tubes are PM 12AX7HG (which is the same as Ruby 12AX7AC5HG (so basically the last iteration of the Shuguang Chinese 12AX7). OD1 and 2 at 5 on the first patch, OD1 at 5.5 and OD2 at 6.5 on the second, OD1 at 6 and OD2 at 8 on the third...

[rnolan]

Nice SC,  :whoohoo!: :headbanger:

[MarshallJMP]

Sounds really good, must been a lot of work replacing all those parts. Well done :thumb-up 

On my 3TM unit I also changed all the electrolityc caps, that changed the sound also quite a bit.

[Systematic Chaos]

Sounds really good, must been a lot of work replacing all those parts. Well done :thumb-up 

On my 3TM unit I also changed all the electrolityc caps, that changed the sound also quite a bit.

I didn't approach this as a rush thing, so yeah, one section at a time. Since the schematics and load charts are blurry in places I also had to cross-check some resistor and especially zener values by checking the color codes or part numbers.
Replacing those display cables was a PITA!!
The adjusted EQ in combination with the increased gain Super Stock really is awesome, sorta more modern José modded 2204 meets Friedman HBE (with a hint of 5150 when going past 7 on the gains)

[MarshallJMP]

Yeah those display cables can be tricky.