Converting Marshall ptp PA to Lead

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by neve1073, Dec 13, 2005.

  1. neve1073

    neve1073 Member

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    I have an early '72 pristine 50 watt PA head. I'm recapping it with elnas. I'm also thinking about converting it to the specs of a guitar head. What are the least invasive mods to the front end that I should consider doing?

    Thanks
     
  2. Reeek

    Reeek Member

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  3. VintageJon

    VintageJon Member

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    Mother Of Gawd Man, you've got a valuable piece of gear there! DON'T CONVERT IT, JUST RESTORE IT!!!

    -Jon
     
  4. neve1073

    neve1073 Member

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    Aside from caps, it doesn't need restoring. The problem is this: I'm not running an amp museum; I'm a guitar player looking to add more sounds to my studio and gig arsenal.

    Do you think I'd be better off selling this as a PA on ebay and buying a ptp '71/'72 50 wt marshall guitar head? My sense is not, but I'm open to being educated.

    Thanks for all replies
     
  5. Swarty

    Swarty Member

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    Heck, just turn it up and enjoy it. Stock it should smoke and not have that overbearing top end, more like a SuperBass; rich, thick and samooth! FWIW, many players, including myself much prefer the PA and Bass models for guitar. You've got a great guitar amps there! It is essentially a JTM45 w/ a solid state rectifier.
     
  6. daneswede1

    daneswede1 Member

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    Im not sure of the year of it, Lynch uses a PA Marshall head as well. DOnt know if it needs any modding to become a lead amp. Maybe cut the bright cap but thats for your ears and your tech to decide / talk about. What you need to do is bridge the channels like you would with any good non master volume head and then buy yourself either a ToneBOne Hot British 12ax7 pedal to kick it in the ass or somewthing similiar. THe best way to drive those old amps other than turning it up to 10 and pissing off everyone is to boost the front end. A typical TS9 IMO wont get you where I think you should be. Listen to mojaveampworks.com. The JTM45 clone they build has some soundclips on the site. CLip #9 and #10 is the amp bridged (I think) With a tube driven pedal (DOnt know which one) and it sounded amazingly fluffy and very very toneful. I bought a Valvedriver from pedaldoctor.com which is a revamped CHandler TUbe Driver without the noise. Good build qaulity. WIll set you back $400 unless you want to get the Tone bone Hot British which is another good choice IMO to push your PA head over the edge. Ive had some success with this setup. I own a 1981 2203 and reissue 2203 and am mostly into Master VOlume Marshalls but ive tinkered with my buddies 89 reissue JTM45 and the Tonebone as well as my Valvedriver. Im looking to get either the JTM45/100 66 replica that George built at Metroamp.com or a Marshall non master head. DOnt know yet?? HOpe this helps. I sont think you need to really moid the crap out of it though. I think you can get to where you want simply driving the front end. A 12ax7 pedal wil lget you this.
     
  7. neve1073

    neve1073 Member

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    The mod we have been discussing would be to change the cathode bypass capacitor and increase the value of the resistor on the preamp (with NOS parts). What do you think?

    I'm not sure what I'm going to do. Sounds like the best thing is to take it slow and just get the thing recapped with elnas and hear how it sounds for me.
     
  8. neve1073

    neve1073 Member

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    BTW are elnas the best choice?
     
  9. bob-i

    bob-i Member

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    I just did a mod on an old Fender 100 watt PA head. Basically it didn't need anything to use it for guitar, but it could be improved.

    The Fender used lower impedance input than the Marshall so I modded that, but the big mods were that I took channels 2-3 and 4 and cascaded the gain stages. I added some voltage dividers and coupling caps, plus a few snubbers and the thing wails, like a Twin on steriods.

    You might try the same approach, or you might just plug in and play.
     
  10. Swarty

    Swarty Member

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    hmm, the input impedence for a Marshall and Fender should be identical.
     
  11. Ian Anderson

    Ian Anderson Supporting Member

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  12. hdiddy

    hdiddy Supporting Member

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    I did this to a 69 and 70 Super PA; I got the info on the Plexi Palace board
    The mods are reversible and basically all you are doing is adding some caps or bypassing them on the Volume Pot
    Mine is as Follows
    1. Early Super Lead
    2. Later Super Lead
    3. Super Bass
    4. JTM 100 (Clean Sound)

    IT rocks; I am keeping the 69, but unfortunately selling the 70 now for some cash
     
  13. daneswede1

    daneswede1 Member

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  14. tocs100

    tocs100 Member

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    I did a search for Marshall AND input AND impedance, but coudn't find anything.

    According to Shure's website, for correct impedance bridging, audio input should be 10X imp. of audio output. So a 10kohm Fender or Gibson pickup should go into a minimum 100kohm input.

    Is that the typical input-impedance of guitar amps,, or are they even higher, like 1 Mohm?
     
  15. Beam Tetrode

    Beam Tetrode Member

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    If that Marshall is original, leave it stock!

    It should sound awesome just the way it is. The PA heads are one of my favorite amps for guitar. They are fantastic with pedals. The PA and Bass heads are of the same design; they are quite similar to the famous tweed bassman in terms of circuitry and voicing.

    The Super Lead circuit is way too bright in most cases...
     
  16. John Phillips

    John Phillips Member

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    No offense intended, but this isn't possible. The Lead models use different tone stack and PI coupling cap values to the other models, and these components are common to all the channels, so you can't truly put Lead and other versions in the same amp.

    But I completely agree that fitting Lead-model channel network values to a Bass or PA while leaving the other stuff alone actually sounds fantastic - in fact, the 'best amp Marshall never made'. You get the extra cut and aggression of the Lead circuit but with the fatness and warmth of the Bass/PA tone stack.

    The very simplest mod you can make to a PA model to make it a little more 'leady' is completely non-invasive. Just get a 470pF cap and twist its wires tightly around those of Channel 1's 470K resistor where the channels mix. It will make a good enough connection for that job and doesn't even involve soldering if you don't want to.

    :)
     

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