1970 Spec Ceriatone 50 Watt Jmp Clone, Doing Some Mods, Need Advice!

Discussion in 'Building the Classics' started by jman271, Nov 13, 2017 at 7:25 PM.

  1. jman271

    jman271 New Member

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    Hello all, Im new here and wanted to post and share a project I am working on and wanted to see if I posted pics and such if some would be able to offer constructive advice for what I want to do.
    It is a 1970 spec 50 watt JMP clone made my Ceriatone, the only extras it has is a post phase master volume, other than that it is stock.

    One tube is getting pretty red, I think it is old and maybe suffered internal issues so I bought (2) new ones that will be here this weekend. The manual states a 35ma plate reading so I will check the old tubes first and see whats going on there.

    Some of the other pics are the board and what is in there for caps and such.

    My end goal is to get this to sound brown, warm, punchy, and raw.
     

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  2. bill bokey

    bill bokey Active Member

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    What's missing ? With this circuit you should get all that... (I have never played a stock Ceriatone amp but this looks like a standard 69 lead circuit)
     
    Last edited: Nov 13, 2017 at 11:59 PM
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
    On the wrong side of the tracks.
    If one tube is red,
    The speaker load is incorrect, wrong impedance, bad speaker cable, etc...the output may be wired wrong.
    You cannot run a tube amp without the correct speaker load....or...it blows chunks.
    OR
    One tube is not getting bias voltage. (*if bias voltage is coming from the master volume control, there may be a wire missing.)
    Check DC voltage on pin 5 of each output tube. This should be the negative (bias) voltage on pin 5 of both output tubes.
    If negative voltage is not there on pin 5, the tube will turn red.

    *
    If bias comes from the master volume, (if master "is" the bias splitter...)
    bias voltage must go to both halves of the master volume...and I think you may have one half hooked to bias voltage instead of both halves...(need a jumper wire on the MV control?)

    The photos do not show the entire amp...I am not looking at all of it.
     
    Last edited: Nov 14, 2017 at 12:15 AM
  4. jman271

    jman271 New Member

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    Well right now it sounds very "plain" and toneful but when pushed on the volume channels and the master down, it sounds "flubby" and the distortion seems to squeal, the red tube is probably smoked..(2) new ones here soon.
     
  5. jman271

    jman271 New Member

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    OK, I will get more pics of the amp circuit and post it. I also did install the V30's a few days ago, I could have very easily done a wire wrong..they are 8 ohm 30 watt V30's on top, and 60 watt V30's on the bottom. I ran them for an 8 ohm load so series para I believe, I will check! thx!
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
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    I think you need a bias voltage jump wire on the master volume...

    See where the yellow wire is hooked to 470K / 2.2M ?
    I think you need another wire from that point, to the other 1/2 of the pot - where the (other) 2.2M 470K is, on the front section of the MV.
    Because that is missing, 1 of the output tubes has no bias voltage.
     
  7. jman271

    jman271 New Member

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    OK, re-connected all the wires in the cabinet, but have to run out there to take more pics of the chasis, I thank you for your insight!

    ps- so if I am reading you correct, you are saying that since it is a dual pot MV, there is a yellow cloth wire on the front pot and it also needs to be jumped to the "rear" pot, (in the same spot), so now both bias voltages are going to the MV?

    I will go see if there is a jumper hiding under the chasis.

    *edit* there is a bare jumper wire from the front of the pot to the back of the pot so it should be good.

    It there currently a "cheat sheet" diagram that outlines WHERE some of these areas on the chasis are. For example, I hear, "take the V1a to the cathode bypass",,,(i made that up lol), but you get the point. I can see that the circuit is simple in that it runs from one end of the amp down to the other, but as a newbie, it would be helpful to know if somethign like this exists so I can stare at it.
     
    Last edited: Nov 14, 2017 at 5:49 PM
  8. neikeel

    neikeel Well-Known Member

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    I have not seen that ppimv set up before. I presume that is a dual 500k pot and the 470k are in parallel with the pot for approx 220k. The 2M2 are for safety if the pot were to go open circuit.
    Normally a I would shield the ppimv wires. They are neatly twisted etc. But you may be getting PO from them particularly sitting so close to the presence pot!! Sometimes adding grid stoppers to pin 5 of the outputs sockets helps too (4k7 to 10k) they may be there already - can’t see.
    Of course you need a working pair of basically matched tubes and if one has been subject to stress may need to be replaced.
     

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