Marshall 1930 Popular Clone

Discussion in 'Building the Classics' started by Timbo, Apr 28, 2018.

  1. Timbo

    Timbo Member

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    Hi guys, I'm doing a clone of this amp mainly due to it's unique design that MAY or MAY NOT sound good, as there is mixed opinions.
    I'm hoping it will sound like this.....

    The 6GW8 was used extensively in local built amps in the 60's and 70's here in Australia and there are plenty around, although NOS are getting very expensive.
    From the info on the net and the poorly drawn Marshall schematic I was able to piece together my schematic.
    [​IMG]
    Love reverb,
    so that was added.
    This local guy has one and put together this https://ivanrichards.com.au/2012/03/01/a-very-rare-marshall-jmp/ which was a great help to nut out the circuit board.
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    I did deviate from the drawn layout as a few changes came to me as I was marking the board.
    So i'll do a redraw once I'm done.
     
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  2. johan.b

    johan.b Well-Known Member

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    Interesting. ..but the "tone" pots must be drawn wrong in the schematic. Not that it affects your layout, but i thought i must point it out. .
    J
     
  3. Timbo

    Timbo Member

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    Thanks mate, Schematic fixed.
    [​IMG]
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    On the wrong side of the tracks.
    That's pretty cool, made your own board.
     
  5. Timbo

    Timbo Member

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    Hi guys, Circuit board is done.
    [​IMG]
    [​IMG]
     
  6. slagg

    slagg Active Member

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    Killer project man, yay! Speaker choice is a huge factor with that circuit
     
  7. Timbo

    Timbo Member

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    Thanks, I have a celestion G12m greenback as my first preference , but I also a couple of local made speakers that are greenback clones.
     
  8. Timbo

    Timbo Member

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    Hi guys, Cab is partly built and the chassis has been made...
    [​IMG]
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    So the fun begins............
     
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  9. Timbo

    Timbo Member

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    Hi guys, An updated board layout.
    [​IMG]
     
  10. Timbo

    Timbo Member

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    Hi guys, I have updated the schematic.
    I have added a switchable rectifier.
    I did this on another build and found it's great for adding some vintage sounds.
    [​IMG]
    Chassis wiring is just about complete.
    [​IMG]
    [​IMG]
    [​IMG]
     
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  11. Timbo

    Timbo Member

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    Hi guys, Circuit board is in and wiring checked.
    [​IMG]
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  12. Timbo

    Timbo Member

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    Hi guys, We have sound.
    And it is living up to it's reputation of noise and drive. :drool

    First up, the noise I'm mainly getting is from the fluro above my bench, so I'm hoping that will subside once in the cab.
    It does have a low volume hum that does not increase with volume, maybe from heater wiring.

    Sound is great, but I think some voltage dividers may tame some of the gain.
    Humbuckers really push the issue, awesome!!!

    Reverb needs some tweaking.
    I have seen it connected to the other half of the phase inverter???
    Otherwise the two mixing resistors 100k/330k may need to be adjusted.
    Possible a dwell pot (set and forget) mounted in side chassis.

    Issues....
    Low level hum.
    I'm thinking to elevate heaters.
    Can the heater Artifical CT be connected to just one 6GW8 cathode, is 8v DC be enough??
    Reverb pot scratchy.
    Thought it might be the .01u coupling cap leaky.
    Replaced with another, still scratchy????

    Probably need to address these issues before I proceed any further.
     
  13. johan.b

    johan.b Well-Known Member

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    Try keeping all signal carrying cables away from the heater wires. Especially near the input. . The high z signal will easily puck up the high current from the heaters
    J
     
  14. Timbo

    Timbo Member

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    Hi guys, I was able solve a few issues.
    Scratchy reverb pot.
    A .01u cap was placed between the 330k mix resistor and the pot lug, this has remove the DC and the pot is now clear.
    Micro phonic V1.
    I putting this down to the valve just operating on the low plate voltages.
    The 220k/100k resistors that are connected to v1 plates could be placed in a number of ways and from the photo's on Ivan's site suggests that the original Marshall circuit is how it should be, but finding that other amps may be wired differently as per some other photo's of the circuit board.
    I mucked around with this part of the circuit for sometime and had no real joy with sound or trem function.
    I was offered this as a solution http://sluckeyamps.com/18w/18w.pdf
    This worked a treat with a strong trem and NO micro phonic valve.
    One other issue I was having was the reverb didn't have much boing, but what was there I felt was sufficient, on inspection of the tank I found a piece of packing foam still wedge under the springs, removing piece of foam resolved this issue. :frowny
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    I liken the hum to something like the background hum that you hear from a PA system.
    I have grounded the 2x16uf filter caps to the ground buss that runs along the back of the pots. The low level hum is still there.
     
  15. Gene Ballzz

    Gene Ballzz Well-Known Member

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    You should re-do your heater wiring and it should look a bit more like this:

    [​IMG]

    I notice in your pics that your heater wires, besides not being well twisted are also quite close and/or in contact with other wires (possibly signal) and this is exactly the kind of thing that can cause the pow level hum of which you speak. Lead dress is crucial!
    Just My $.02,
    Gene
     
  16. Timbo

    Timbo Member

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  17. Gene Ballzz

    Gene Ballzz Well-Known Member

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    With heaters as you have them, all the chop sticking in the world won't likely show much difference, but doing them absolutely correctly "MAY" at least minimize a little bit! It certainly doesn't hurt a darned thing by having them right, except maybe bruising your pride at needing to do them over! Minimizing hum is a process of elimination and it's always best/advisable to address the most obvious and easiest first!

    Your Build, Your Hum Chase, Your Choice! :nutkick:
    Gene
     
  18. Timbo

    Timbo Member

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    Thanks mate, I'll give it a bit more attention.:thumb:
     
  19. Gene Ballzz

    Gene Ballzz Well-Known Member

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    My apologies sir, if I sounded like a jerk. Was just trying to be helpful and possibly save you a little bit of tail chasing. Been there, done that, got the Tee Shirt!
    Best Of Luck In Your Endeavor,
    Gene
     
  20. pedecamp

    pedecamp Well-Known Member

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    Gene you big jerk! :rofl:
     

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