Heavily modded 76 Master Model new tubes and biasing.

Discussion in 'The Workbench' started by The Grim Peeper, Jan 24, 2020.

  1. The Grim Peeper

    The Grim Peeper Active Member

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    Ok so here we go. I’m gonna need A little help here. I got this thing on the bench now. Basically it was a four input 76 master model. Somebody did the cascading Randi Rhodes MOD and made it into an 2 input 800 block up two inputs. But before I even begin and hook up a speaker load I noticed there was another mod back here in the speaker input section. is this extra hole a effects loop?
     
    Last edited: Jan 24, 2020
  2. The Grim Peeper

    The Grim Peeper Active Member

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  3. The Grim Peeper

    The Grim Peeper Active Member

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    I fired it up using the in main speaker inputs I always have. I have my bias probe set to mA and my meter to 600dc. I got 0 milamps on the probe. And 0 volts anywhere. But on the filter caps which went to OL. Tubes were all glowing
     
    Last edited: Jan 24, 2020
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Then the HT fuse is blown?
     
  5. The Grim Peeper

    The Grim Peeper Active Member

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    I have an extra I’ll try it.
     
  6. The Grim Peeper

    The Grim Peeper Active Member

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    The old one was definitely blown. I put a new one in and turned it on. There was a loud hum and it blew again.
     
  7. The Grim Peeper

    The Grim Peeper Active Member

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    I just put the new tubes in my other jmp and they seem to work. Maybe the old tubes weren’t even bad after all.
     
  8. Matthews Guitars

    Matthews Guitars Well-Known Member

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    You say the tubes were all glowing. Do you mean filament glow or red plate glowing? If it's red plating then there's no bias current flowing, or not enough.

    Pull all four power tubes, replace the fuse (with another one of the correct value, please) and turn it back on. Probe for bias VOLTAGE. If there is none, your bias supply is gone.

    You say this is (was) a four input with master volumes? What model would that be? I'm unfamiliar with that combination. I know the master models (2203, 2204) and I know the four input 1959/1987, etc but I'm not familiar with a model that is both master volume and four input. Not as a factory model, but behind the faceplate of every 2203 and 2204 there are two more chassis holes for the extra inputs. They can be easily added by a tech.
     
  9. The Grim Peeper

    The Grim Peeper Active Member

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    Master model 50 W Mk 2 lead 4 input. Two of the holes are plugged. End it appears there’s some alterations inside that seem to show that it has been given the cascading gain mod like an 800. Every time I put an HT fuse in there it blows. Just by itself without the HT fuse all the tubes glowReading zero voltage.
     
  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Again, PLATE glow or FILAMENT glow? And reading zero voltage WHERE?

    Pull the output tubes. Does the fuse still blow? If it does, that's looking like a bad transformer. Check for shorted power supply filter capacitors, after being sure that they are fully discharged, of course.
     
  11. The Grim Peeper

    The Grim Peeper Active Member

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    I’m out of fuses lol.
     
  12. The Grim Peeper

    The Grim Peeper Active Member

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  13. The Grim Peeper

    The Grim Peeper Active Member

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    Not sure why it has four inputs and two are blocked off. I assume it was originally made with four. Most the ones I see are 2 input.
     
  14. The Grim Peeper

    The Grim Peeper Active Member

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    I’m gonna take it to a tech. This is over my head. How should I have it restored?
     
  15. David Szabados

    David Szabados New Member

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    That amp has had a LOT of work done to it! I suspect that the two additional holes on the front to make it a four hole were added probably for prior mod. The chassis of either a 2203/2204 or 1959/1987 will always have the four holes, but the faceplate will be punched for two or four. I suspect that someone drilled the faceplate at some time. The '76 50 watt 1987 wasn't cascaded so that's a common mod to make it consistent with later master volume Marshalls. I would keep that.

    As for "how to have it restored," you've got a LOT of really interesting components and parts that aren't original. That PCB with the square block caps are strange to see in 1976 as well. It wouldn't surprise me if that board isn't original to the amp.

    All said I wouldn't personally focus on trying to get the amp "correct" cosmetically or component-wise, but instead focus on having it done to sound how you'd like it to be. How does it sound compared to others you've played?
     
  16. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If you want it restored I can provide the correct front and rear panels for it. It is virtually certain that this amp is a 2204 that only had the usual two input jacks when it shipped from the factory. I have 2203/2204 front panels in stock and rear panels are coming soon.
     
    assquatch20 likes this.
  17. David Szabados

    David Szabados New Member

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    That's great you are making these panels! I'll keep you in mind when I get some amps in to work on in the future.
     
  18. assquatch20

    assquatch20 Member

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    Not to derail the thread but are you capable of reapplying the graphics to the front plate of my '76 SLP? Maybe simpler to just replace it, but it's in good condition aside from the numbers and letters being gone. I'd imagine it's a bit of a project to remove/replace though, not sure.
     
  19. neikeel

    neikeel Well-Known Member

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    Without wishing for this thread to go down the ‘spanner as a fuse’ route. I need to know why on earth people keep on sticking new fuses into an amp when one has blown before they either address the issue or do some simple tests?
    Ok rant over.
    If your HT fuse blows take the tubes out, then put new fuse in. If it is ok then it is your tubes. Check your pin out voltages against reference. If in range put in known good tubes (pref not your shiny new ones) and recheck.
    The amp looks like an early 80s version. I don’t see many mods (replaced jacks and added line out). I’ll have another look later on my lapto as they are blurry on my phone.
     
  20. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That extra jack on the back can’t be an effects loop, you need two jacks for a loop. It looks more like a direct out, takes signal out to a PA or for recording maybe.
     

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