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jmp P.A. heads

5/25/2000 7:33 AM
jmp P.A. heads
whats the deal with the jmp P.A. heads?  
are they similiar to regular jmps?  
any info or insight would be appreciated.  
5/25/2000 6:51 PM
Jim Jones

Hi Rob,  
As far as I know, the early Marshall PA heads are very similar to the lead models. You're talking about the 4-inputters, right? Of course the PA models have an extra preamp tube and another bank of 4 inputs...  
As far as the actual circuits go - I believe that the lead, bass and PA heads are essentially identical save for a couple capacitor differences.  
Hope this is of some help,  
5/27/2000 7:34 PM
Peter S

Aside from the extra tube which gives the extra 2 channels the circuitry is identical. The only difference is on the lead amps there are bypass capacitors to increase the treble response. I usually end up removing these anyway because with a strat or a tele the amp can be too bright with them on and without 'em the amps sound just least to me they do.  
Peter S
5/30/2000 4:01 PM
Ted Breaux

You've all forgotten that in addition to several pertinent small parts values which affect the signal, the post '67 Super Lead amps also have a split cathode arrangement at V1, which gives considerably more gain. The Super Trem, Super Bass, and Super P.A. have a shared cathode arrangement at V1, and have less preamp gain.
5/31/2000 2:54 PM

The Super Pa and Super Bass are identical other than the 500pf bypass cap on the mixer resistor on the 'guitar' channel of the super bass, whereas all 4 channels on the super pa are like the bass channel  
Differences to the guitar heads: (other than the obvious extra 2 channels ;)  
1) .1uF(pa/bass) coupling caps to the power supply as opposed to .02uF(lead)  
2) Combined Ck/Rk combo, vs the split/brighter combo on the guitar amps  
3) no .68uF Cathode bypass on the second stage cathode., lead amps had this, pa/bass didnt'  
4) .02 uF (PA/Bass) coupling cap on all channels, whereas guitar amp had .002uF coupling on the bright channel  
5) 500pF(lead amp, this is not there on the PA amps) bypass of the mixer resistor on bright channel for guitar.  
6) 56k(bass/pa) vs 33k (lead)Slope resistor on the tone stack  
7) 250pf (bass/pa) vs. 500pf (lead) treble cap in tone stack  
8) finally, the .05uF bright cap on the bright volume is not present on the PA amps (since all channes are created equal)
6/1/2000 11:40 AM

excellent information.  
7/6/2000 3:42 PM

A friend of mine picked up a Super P.A. and asked me to convert one channel to a Superlead. I was quite surprised with the outcome. This amp kicks!!! Probably one of the best Marshalls I've heard. He uses a power brake most of the time unfortunately but I convinced him to try it without the PB and stand in front of it and a 4X12 with the volume on 6....Heaven.....( although I woulnd't recommend this kind of ear torture for too long...) And yes I did give him back the parts in case he ever wants to sell it.....( I doubt he will. I GAVE AWAY a broken JTM 45 when I was a kid...kickin' my ass ever since!!!!)

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