Marshall JMP 1987 50w tube guitar amplifier 1960A 4x12 speaker cabinet

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Last Update 12/09/2017

My guitar playing brother - who is very good at spending other peoples' money - had been encouraging me to get a Marshall amp.  A vintage Marshall Major turned up at the store and my brother encouraged me to buy that amp.  He used to own a 100 watt Marshall head and by the time it got a good crunch it was unbearingly loud.  One of the reasons I sold my 60 watt Mesa Boogie MkIIa was it was too loud.  Reminding him of those amps, I told him that a 200w Marshall amp was impractical.  Those things were good for concert arenas but way overkill for a recording or club gig environment.  Some years later, this Marshall 1987 50w head with 1960A 4x12 cabinet turned up at the same store.  The small Marshall logo and tethered power cord gave away its vintage.  Trying it out, it got a good crunch at a more reasonable volume than the way-too-loud amps we both used to have.  The asking price wasn't a bargain but it was reasonable.

The store explained to me that two of the speakers in the cabinet had been reconed, but they couldn't get the cabinet to sound right.  I had a good idea why it didn't sound right, and sure enough after I got it home I found that one of the speakers in the cabinet was wired out of phase - yeah that will screw up the tone.  When I corrected the wiring, the cabinet sounded MUCH better.  I knew those guys at the store were competent and was surprised they didn't check for phase.

Going by the serial number, the amp dates to 1975.  It's a PC board amp and not point-to-point (PTP) like the earlier Marshalls.  The guitar community prefers point-to-point Marshall amps due to the hype that PTP "sounds better".  Frankly, the guitar playing community takes far too many myths like these as gospel.  Being an EE, I'm well aware that PC board design done the wrong way will ruin the sound of the amp.  But done right the amp will sound great.  While Fender and Vox in the 60s and 70s did a PC board amp design the wrong way, no one ever complained about Mesa Boogie amps that didn't have PTP wiring.  Marshall PC board amps sound great and were a bargain compared to earlier PTP amps but as this is becoming more common knowledge the value of these amps have crept up.  Put a crappy guitar through this amp then through a crappy speaker cabinet, and yes it will sound like a dog.  By this time I had good guitars in my possession and was gaining experience with different speakers.   So I had little trouble getting the guitar tones associated with this amp - texas blues, southern rock, early Bad Company, 70s hard rock.

When I bought the amp, it already had three modifications:
  1. Original 6550 power tubes replaced with EL34s (requires a circuit change)
  2. Post Phase Inverter Master Volume (PPIMV)
  3. Two channels converted to single channel with cascaded input triodes
The amp also had the original speaker impedance selector which has a history of going bad, which risks running the tube power amp without a load and POOF the output transformer is toast.  The original OT was intact so I quickly took the amp to have a new and better speaker impedance selector installed.  The amp tech doing the work recognized his work on the modification - he had modified the amp.  His memory is hazy but he believes the PPIMV circuit is the Ken Fischer design.  The amp is easily reverted to stock as the tech carried out the modifications without altering either the front or back panel.

When Marshall amps were imported to the US, EL34 tubes more prevalent in the UK were in short supply in the US back in the 1970s.  It was not possible to import tubes between two countries due to rough shipping, which can break the vacuum seal in the glass dome and ruin the tube.  With the 6550 power tubes more readily available in the US, they were installed in Marshall amps in the US with minor circuit changes.  There is a tone difference and players preferred the tone of the EL34s in the UK amps.  The power tubes weren't directly compatible without changing the circuit, but many players opted to have their amps modified to accomodate the EL34 tubes, which became more available as time went on.

The unmodified amp was a non master volume (non-MV) amp, and many players liked the sound of the amp turned up but the volume was way too loud for anything but large halls and concert arenas.  Marshall introduced master volume amps during 1976, but their implementation wasn't ideal as the tone suffered.  Techs like Ken Fischer later uncovered a better solution in the PPIMV.  The reason why the non-MV amps sounded great when cranked was because the distinctive Marshall crunch was a product of overdriving the power tubes and the phase inverter (yes the phase inverter DOES impart its tone when overdriven).  Marshall's master volume solution inserted a volume control BEFORE the phase inverter, yielding a wimpier tone even with the preamp tubes overdriven.  Ken Fischer's solution placed the volume control between the phase inverter and the power tubes, so the phase inverter continued to receive a hot signal thus better tone.  This solution however does require more parts.

The final modification involved cascading the two input triodes to achieve a high gain sound.  This duplicated the configuration in later Marshall amps.  The downside is the amp is no longer a two channel amp so you can't use the "jumpering channels" trick, but the gains are better.  Two of the four input jacks were disconnected with the removal of the second channel.  The remaining two were wired so one jack gave you the stock input configuration, while the other jack gave you the raunchier high gain sound.  It isn't the atypical Mesa Boogie high gain sound, but I actually like this modification better.  Unlike the Mesa Boogie high gain architecture, you can get different tones on this amp that are not just high gain.  I found a setting that is ideal for a touch player, which is my style.  Hard playing breaks the amp into crunchy overdrive, while lightly stroking the strings cleans up the tone.  The dynamic timbre is very controllable with just fingers, and the relative volume stays the same.  It's even better with the 65w speakers.  No need for channel switching!  When I was performing ZZTop's "La Grange" with the southern rock band, I could play the clean tone during the intro then go to that raunchy sound without channel switching.  That was convenient.  And not enough players have an appreciation for touch playing.  This modification also gives a nice articulation with muted and "chicken" picking.

It wasn't until later that I learned what that 4x12 cabinet really was.  Checking the datecodes of the speakers, I determined that the cabinet dated to 1970.  It did have the metal handles that identify these early cabinets but I wasn't aware of them at the time.  Checking further, I discovered that the Celestion speakers - 25w "greenbacks" - were the highly sought after "pre-Rola" speakers.  Rola was a plant where all Celestion speaker manufacture was moved to by 1971, and the label on the back of the speaker magnet identify them as being made at the Rola plant.  Players noticed a change in tone, and found that speakers in the older cabinets sounded different (I won't say "better" because this is highly subjective).  "Pre-Rola" speakers can be easily identified by the label on the back of the speaker, which is missing the "Rola" in the title.  In addition the construction of 4x12 cabinets back then was different, which impacted the tone.  So I found myself with a highly desirable vintage 1970 Marshall 4x12 cabinet with highly desirable "pre-Rola" greenback speakers.  The store didn't know what they were selling.

At first I thought it was one of those custom color cabinets with its dark green tolex, but I later learned that was the stock color tolex back in 1970.  The tolex on the amp is black and doesn't match, but at a distance they are so close in appearance that they look the same.

Needless to say, that Marshall 4x12 has lots of friends.  Every guitar player I knew who owns a newer Marshall 4x12 and has tried my cabinet has loved the sound.  When I was in the southern rock band, our guitar player borrowed it for a week and he begged me to sell him that cabinet.  By then I knew what it was and I politely told him that the cabinet was not for sale.  He sought something similar; he landed a cabinet made in 1972 but it didn't sound like my cabinet - different construction and different speakers.  He's still looking, but they're hard to come by as players hang onto these cabinets.

Later I realized that my brother's 4x12 cabinet - which was long missing its original grillcloth and piping - had metal handles and grooves where the original piping used to be - same as my cabinet.  His cabinet was also an early one like my 1960A slant cabinet except his is a 1960B straight cabinet.  Between my brother and me, we now had a full stack of highly desirable 4x12 cabinets!

The southern rock band I was playing with was playing a large venue with a stellar in-house PA system.  I had been playing guitar through my Vox Tonelab SE amp modeller in my keyboard amp/speaker (I like a simple setup, one amp/speaker saves time on setup and cartage), so for giggles I decided to drag out the Marshall head and 4x12.  That was a mistake, as everybody raved about how much better they could hear me and they wanted me to use that setup at every show.  I didn't really want to gig that vintage 1970 4x12 cabinet and scruff up the cabinet more, and I didn't want to risk blowing those irreplaceable speakers (the original supplier of the speaker cones - Pulsonic - lost the "recipe" to the paper cones when their factory was destroyed by fire in the 1970s).  I had two other Marshall cabinets with 65w and 75w speakers but they didn't get the southern rock tone like the greenbacks did.  Neither did my brother's Marshall cabinet loaded with vintage 30s.  The 75w Celestion speakers sounded so close to the 65w Celestion speakers in my 1960B cabinet that the 75w speakers were redundant.  I wound up putting the 75w speakers in my Rhodes piano (why should guitar players have all the fun?), and decided to load the now empty cabinet with reissue 25w greenback speakers and gig the newer cabinet instead of the vintage one.  Comparing the vintage and reissue cabinet/speakers... there is a difference in tone.  The difference is slight but the reissue speakers still got that southern rock tone.  Why do they sound different?  Could be the speakers, could be the cabinets (old vs new are different construction) but I haven't gotten around to swapping speakers between cabinets yet.  I definitely prefer the pre-Rola speakers but for club work the reissues are sufficient.  Many players complain that the greenback 25w speakers lack some bottom end, but that can be an advantage depending on the rhythm sound to fit the song or making the guitar lead be heard in a mix without turning it up.

One other accessory proved very useful.  My Groove Tubes Speaker Emulator did an uncanningly scary good job of emulating the sound of my cabinet through a flat frequency amplifier like my keyboard amp.  Like my Hughes & Kettner Red Box, I had options for recording or for leaving the guitar cabinet behind for a gig.

A very fine vintage package...  this one isn't going away.

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