Tuesday, October 18, 2016

1967 Marshall JMP "Bluesbreaker" 4x10" combo. One of the greatest amps of all time!

This was a fun project, and the 2nd most valuable amp I've ever worked on, the first being a Howard Dumble that belonged to a famous guitarist (not all that impressed with that amp, I would sell it and buy 5-10 of these!).

I've always wanted one of these. They are quite rare. I don't think any amp has surpassed the early 'plexi' Marshall era in terms of sound and style. Looks do matter and these amps are quite beautiful to look at. And of course, they have a special sound as well. Nothing quite like them. Clear, warm and beautiful overdriven sound when pushed. This amp is essentially a Tweed Bassman with tremolo circuit wise but with a much more present mid range.

When I received this amp it was quite a mess. It had been badly damaged in shipping. Poor packing? Maybe. Marshall combos are tricky to pack properly. These amps can command up to $15k so in my opinion it's best to ship the chassis separately. That is a weak part of these amps, how they are mounted.

It's heartbreaking when this happens. Fortunately, everything was fixable and my client decided not to send it back. The company he bought it from are good people and they payed the bill right away. No insurance battle for my client....

So this amp required quite a bit of gluing and clamping. Plenty of doweling as well. A couple filter cans were damaged and some components became unsoldered. The lovely Mullard 5AR4 tube was destroyed so I found a good deal on a NOS one. We're not going to load up a Rolls Royce like this with cheap new glass. No sir, not gonna happen!

Now the crux of this article. The unreasonable fear of 'originality' and 'collectibility' and what that does to a technician.......

Once I got into this amp, and it was gone through by what I assume is a good tech, I found a few issues. First, the Russian tubes were biased hot. 95ma each. These early models don't have a trim pot for bias. My opinion? Screw your 'originality' and put a trim pot in there! Running this amp this hot could kill the output transformer and possibly the power transformer! I was alarmed at seeing this but I've seen this many times before. The fear to alter something to make it ACTUALLY WORK properly. So I added a 22k trim pot. Took 5 minutes to install. Do it. If your customer freaks out about a change like this, you don't need to sell your amp to that person. He/she is unreasonable.

Had a private audience with George Gruhn last month. His words: "If it's old it's been worked on. Even a Stradivarius has a new fingerboard if it' been played." Enough said! My job is to keep important parts like your priceless transformers safe. In this amp the transformers and speakers are original and that is a miracle at 49 years.

Now the next problem. The speakers. At low volume you could hear this bzzzzzzzt as the note decayed. All 4 speakers were bad. One was not moving at all due to a frozen voice coil, the others were all rubbing. At higher volume one would think "oh this rocks". Inspect each speaker. It's important to do this! And it's worth having these re-coned. I had these done at Speaker Repair Pros in Los Angeles. I don't know of anyone here in New York that I trust to do these. One place I called after the third time the man on the phone said "you can just buy new ones cheaper" I finally said "you just don't get it, thank you for your time...." So off they went to Los Angeles. They did a fantastic job as usual!

So why was this amp sold with 4 bad speakers? I don't know. I attribute that to the same fear I wrote about above. In fact, in all honesty I refused to hear it myself. I was a bit afraid to tell my client he needed to get the speakers rebuilt cause I've dealt with many unreasonable clients who are so afraid their amp will lose all of its value. Bollocks to that! And fortunately my client knows better. He want's an amp that kicks it good, not one that will just be looked at!

I love the old logo on these speakers. Classy as can be.

Link to Speaker Repair Pros. They rock! https://www.speakerrepair.com/

So next was the output tubes. We went for Siemens EL34. This amp was built in the first year Marshall switched to EL34 tubes. My favorite year for the old Marshalls!

Biased this pair at 35ma each. Good safe place to run this amp. I also added a pair of 1k 5 watt resistors for the screen grid. The early Marshall amps didn't have them though curiously enough the JTM-45 loaded with 5881 or KT66 tubes had a 470 ohm resistor in this place. Trouble is when you blast this amp without those resistors in place the tubes draw a lot of current and you can pop a tube. You lose a negligible bit of tone by installing them but, it's worth it for the extended tube life. I think it was fine at one point when you had 800 volt Mullard El34 tubes available, but even the Siemens tubes should have those resistors in place.

The Siemens EL34 is by far my personal favorite EL34. Smooth and creamy, no harsh overtones like modern tubes. They were original equipment in 80's 800 Marshall amps and I've seen them go for over a decade being gigged with on a regular basis. And, amps back then were far more brutal on their gear......

Preamp tubes survived, and they are all Mullard long plate 12AX7. The finest for this amp!

One thing about the old Marshall amps the younger folks don't get is, they are 2 channel low gain amps. They evolved into the metal monsters we know and love or hate. These early ones are just as suitable for a jazz gig. In fact, they sound absolutely beautiful for those who want a nice clean tone. Jim Marshall supposedly hated his tremolo circuit but I found it to sound really nice.

These amps are probably the most guitar friendly versatile amps I know. A Tele, Strat, 335, Les Paul, big old Epiphone Jazz box, all sound great through one of these. The first Marshall I ever bought was a '77 JMP 2 input head. Got it for $200. Back in those days we didn't really have all this "true bypass' goodness. Pedals robbed your tone. But with that amp I didn't miss much. There was so much tone there I could stand to lose some. I was sold. My Fender Super Reverb was sold too. That amp was tough as could be. I wonder who has it now?

Amp sounds nice and tight now and is being loved in a happy home.


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