69 Plexi Gut Shots

Discussion in 'Let's Talk Vintage' started by Pageburst, Jul 22, 2019.

  1. Pageburst

    Pageburst Member

    Joined:
    Jun 26, 2017
    Messages:
    65
    Likes Received:
    66
    Despite what seems to be some cap changes etc and odd looking wiring, this is without question the best sounding Plexi I’ve owned.

    It took me until now to pull the chassis because I’ve enjoyed playing it for the past month.

    It’s incredibly dynamic, touch sensitive and cleans up as well as any amp I own including a completely original 1965 Vibrolux. It’s very lush with overtones and harmonics yet also very woody and crunchy depending upon the pickup and volume of my Les Pauls.

    I think it must be a bass spec as it’s not particualry bright and similar to my JTM45 in that regard. I definitely don’t need to jumper the channels and have the bass almost off.

    I bought vintage mustard caps 01/400 to change out the Wimas but right now I’m thinking that I should leave it be. Somehow it just works.

    The only changes I made was putting some good rca 12ax7 long black plates in the preamp section.

    Of course any insights from the knowledgeable folks on this board are always appreciated.



    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     

    Attached Files:

    TAZIN and gkelm like this.
  2. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    12,752
    Likes Received:
    6,635
    Location:
    Left Coast
    The photos are too dark to really see what's going on.
    But if it sounds good I would not do anything but change the bias filter capacitors. Those are the 2 caps that will cause trouble when the bias fails.
    How does it look?
    From the very dark limited view, so far it looks pretty good.

    "I bought vintage mustard caps 01/400 to change out the Wimas but right now I’m thinking that I should leave it be. Somehow it just works."

    The .1 caps appear to be factory original so I would leave those as is.
     
    Last edited: Jul 22, 2019
    Pageburst likes this.
  3. Pageburst

    Pageburst Member

    Joined:
    Jun 26, 2017
    Messages:
    65
    Likes Received:
    66
    Thanks ampmadscientist

    I tried to brighten up these photos.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
    TAZIN likes this.
  4. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    12,752
    Likes Received:
    6,635
    Location:
    Left Coast
    Way better thanks.
     
  5. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,280
    Likes Received:
    1,726
    That is my old amp!!
    It is an early 1970 JMP 50 Model 1989 (organ) I bought it with a blown PT and pilot lamp.
    I recapped it (LCR cans and RIFA bias caps).
    I also added the Welwyn screen resistors too. Someone has clipped the murata 100pF disc on volume pot and glued it out of the way (odd because I think it sounds better at lower volumes with that cap). Glad you like it. The PT is a replacement NOS off the shelf (total fluke to find it). I rewired the bias feed and found new snubber caps.
    I shipped it to the US several years ago (New York - maybe?)
    The new owner had his tech rewired the mains/neutral selector from original spec.
    Enjoy
     
    Kris Ford, ljs, TAZIN and 3 others like this.
  6. Antoon

    Antoon New Member

    Joined:
    Apr 10, 2019
    Messages:
    7
    Likes Received:
    6
    Neikeel, Can you remember if the mixer bypass cap (the flat RS cap) and the tubular treble cap is original to that amp?
     
  7. Pageburst

    Pageburst Member

    Joined:
    Jun 26, 2017
    Messages:
    65
    Likes Received:
    66
    Thanks Neikeel. I love the amp. I prefer it tonally to the other vintage Plexis I posted on this forum. Knowing that the OT transformer is original and the PT is at least original to the period makes the price I paid fair, afaic. I think vintage Marshalls on your side of the pond are priced more reasonably.

    I wanted a nice early JMP players amp with original/ period correct transformers and I think this fits the bill nicely.

    I had bought these mustard caps in advance of the amp getting to me. I was thinking of replacing the Wima caps with mustards but maybe I should just leave things are they are.
     
    Last edited: Jul 23, 2019
  8. StuC

    StuC New Member

    Joined:
    Feb 12, 2014
    Messages:
    14
    Likes Received:
    7
    Nothing wrong with the Wima caps and they're original to the amp. I'd leave them be.
     
    Pageburst likes this.
  9. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,280
    Likes Received:
    1,726
    It is all original apart from the bias caps, filter caps, screen resistors and the PT.
    I had to replace both V1 plate resistors and one other resistor (can't remember which).
    It had Mullard EL34s and ECC83s when I sold it.
     
  10. fifteenohms

    fifteenohms Well-Known Member

    Joined:
    Sep 30, 2010
    Messages:
    1,071
    Likes Received:
    534
    Location:
    Los Angeles, CA
    Now it has become amp porn.
     
  11. Micky

    Micky Well-Known Member VIP Member

    Joined:
    Apr 29, 2010
    Messages:
    20,108
    Likes Received:
    16,136
    Location:
    Vermont
    Wow. Lots of corrosion in that amp.
    Reminds me of my '77 Princeton Reverb.
    Glad it still works.
     
  12. GIBSON67

    GIBSON67 Well-Known Member

    Joined:
    Sep 8, 2008
    Messages:
    8,573
    Likes Received:
    4,244
    Location:
    Lakeland, TN
    Love the amp porn!

    V1 and V2 are clear top 12AX7'S/ECC83'S? What brand are they?
     
  13. BowerR64

    BowerR64 Well-Known Member

    Joined:
    Nov 23, 2014
    Messages:
    4,569
    Likes Received:
    2,485
    Doesnt that mean they are bad? I had a tune over heat and go bad and it did that exact same thing. I figured it was a way to tell when tunes wear out they start to over heat and that chrome stuff burns away

    I was just about to post that those tubes need replaced. DO they work?
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    21,246
    Likes Received:
    8,921
    Location:
    US of A
    No.
    They are side getter design. Look at the first picture where it can be seen.
     
  15. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,280
    Likes Received:
    1,726
    Iirc it came from a guy in Scotland, I scraped the loose stuff and dust off wiped it with light machine oil - nothing fancy.
    I’ll see if I can find the pics that I took when I sold it.
     
  16. Pageburst

    Pageburst Member

    Joined:
    Jun 26, 2017
    Messages:
    65
    Likes Received:
    66

    Thanks for the info Neil. Do you remember if those Mullards had double halo getters because the EL34s that came with the amp do not have any markings but do have double halos?

    I shocked myself good biasing the amp last night. First time I ever got shocked biasing. One of the EL34 was causing a buzzing or static sound during the note decay. The tubes were biased really cold. I think 28ma so I though upping the bias to 38ma might resolve the issue but the problem persisted so I replaced the offending tube with an Amperex MIE EL34 tube that had similar "output". Amp is working nice now but I actually preferred the clean tone when the bias was lower. Maybe I should resolder that cap you mentioned.

    All in all it’s truly a great Plexi and I am heartened to know the amp was owned and repaired buy one of the most knowledgeable vintage Marshall persons on this forum. It makes it that much better.
     
    Last edited: Jul 24, 2019
  17. LOVETONE

    LOVETONE New Member

    Joined:
    Aug 11, 2016
    Messages:
    9
    Likes Received:
    6
    Have you removed the images as I can't see them?
    Cheers
     
  18. Scott Johnston

    Scott Johnston New Member

    Joined:
    Jun 17, 2018
    Messages:
    13
    Likes Received:
    5
    "The .1 caps appear to be factory original so I would leave those as is." I agree. Although they are not the "bee's wax" paper caps, they still are paper. Paper made for poor capacitors, yet the bleed contributed to the old Marshall sound. In fact, I myself would make the effort to remove the new caps and replace them with oil in paper. I even remember reading that you can "bake" the paper caps to break down the medium similar to have the amp on for years and further break down the paper medium. Or you can find older tube radios with the superheterodyne design and salvage parts from them. Older resistors were also prone to tolerance errors beyond the specs and always worth salvaging as well.

    Do you know the exact model number and are you sure it's a 69? Oh and a shot of the cab would be cool!
     
    Pageburst likes this.
  19. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,280
    Likes Received:
    1,726
    I know those caps are original to the amp.
    Pretty sure the WIMAs are polyester film too?
    Oh and also pretty sure early 1970 but could be very late 69?
     
    Pageburst likes this.
  20. StuC

    StuC New Member

    Joined:
    Feb 12, 2014
    Messages:
    14
    Likes Received:
    7
    Yes, Wima TFF caps are polyester film and foil, the same as mustard caps.

    Edited to add: the Wima TFM caps (smaller body, black print) are metallised polyester.
     
    Last edited: Jul 31, 2019
    Pageburst likes this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice