Yet Another Roberts 770 mono block conversion

Discussion in 'Tube Audio' started by Omegaman, May 3, 2013.

  1. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Actually these are amps from a Roberts 990 before sound on sound and headphone jack

    Awhile back, I got a low hours, Roberts 770x to pull and convert the mono-block EL84 amplifiers. The restoration went so well that I could not bring myself to put it under the knife.

    I lamented this sad story to an audio pal of mine and he pulled these amps from under a tarp in his pickup and gave them to me. Thanks Mark.

    Roberts 990 EL84 mono-block amps, older than my restored keeper and not as swanky but good enough for free

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    No tubes and in barn found condition but all there and clean but dusty, hey its Arizona, electronics preserve here very well
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    I have read all the threads on the conversion , well most, and I got the instructions to do it

    I'll do them one at a time, then probably go back to the first one to do what I learned on the second one

    I got all the caps and I read that it is best not to change the resistors if they are good

    For some reason it feels like rebuilding a small block Chevy
     

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    Last edited: May 12, 2013
  2. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Amp one recapped (mostly)

    The first amp is now mostly recapped

    I didn't have a .005uf nor a .001uf but I had everything else "in stock" (edit: don't need the .005, its part of the now disabled tone function)

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    It cleaned up pretty nice
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    I found a nice set of tubes in my collection.
    Mullard EL84's
    Mullard 12AX7's

    EXPENSIVE EF86 tubes (not needed)

    I do need to get two 6X4 rectifier tubes, but I have some loaners for now. AES is only 20 minutes away

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    I have some questions?
    Why is the .01 cap wrapped in the GREEN wire?

    I would wrap the new .01 "orange drop" in the green wire same way? right?
     

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    Last edited: May 19, 2013
  3. Palustris

    Palustris Well-Known Member

    Messages:
    694
    "Why is the .01 cap wrapped in the GREEN wire? "

    I remember building my Dynaco SCA80 years ago and having to wrap a certain number of turns of wire around the filter caps to form an inductor. I can't remember where the inductor went in the circuit. What are the green wires connected to?
     
  4. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Hmmm...I think its part of the now, disabled tone control
     
  5. RWood

    RWood The future is not what it used to be Subscriber

    Messages:
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    Location:
    Richmond, VA
    Nice job on the restoration; keep the pics coming. I like the Roberts logo on there.

    Can you run a magnetic cartridge through these amps? I was wondering if that is one purpose of the EF86 tube.

    Thanks, I've been saving a few of these for eventual use.


    RWood
     
  6. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Left amp is running

    Victory! I recapped it and it did NOT blowed UP!

    I just left the green coiled cap and wire in place as "abandon-in-place"

    Turns out the amps are from a Roberts model 990, but basically the same for the conversion as the M7 and 770x, just less styling. I wonder if that grill would buff out shiny?

    I got the 6X4 rectifier tubes from AES in 2 days. Still waiting on caps for the right amp

    Don't need the EF86 tubes so back in the stock they go

    This version of the 990 does not have a mag phono stage just a line level input. I think the 770x might have a mag phono stage? maybe?

    The left amp is still waiting for the last three capacitors, but I couldn't wait and I fired it up.

    Sounds clear and dead quiet at idle and I'm glad to see the lamp illuminate. Full volume was still very clear on my crappy, but functional test speaker

    I haven't done anything with the meter yet, but I see it move with hi volume

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    The instructions are plentiful and the conversion is hardly brain surgery

    Most of these units were not used often and are in great shape

    I think I will just try different EL84's and even a 7189 I got

    And different 12ax7's too

    a test bed kinda like

    Now for a case
     

    Attached Files:

    Last edited: May 11, 2013
  7. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Easiest mono-blocks I ever had

    I finished the second amp and listened to it with nice crappy speakers. I love it. It just works great and I couldn't be happier.

    Very impressed in stereo mode with both mono-blocks very close in volume balance.

    No headphone circuit in the model 990

    I want to make cool cases for each one
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  8. cubby01

    cubby01 aka Buck

    Messages:
    9,746
    Location:
    Kansas City area
    I've got an M8 and a 770x in the garage waiting for me to work on. I think I'll convert one pair or the other to mic preamps (for a singer friend) but haven't decided which yet. The 770x has 12ad7 instead of ax7.
     
  9. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    I just used a 12ax7 (per recommendation) in place of the 12ad7 for more gain
     
  10. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
    Hmmm... I just picked up a Roberts 720, because it looks so much like the Akai I have that my father brought back from his tour of duty in Vietnam. It runs pretty straight and solid, but I think I might still hack it up for the amps. This thread has me excited!
     
  11. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
    So, I've found some of the Roberts R2R threads and a couple Akai threads that talk about pulling and using the amps. I found one detailed thread about a build, but he totally tore them apart and went full custom. I'm looking for something a little less grand. I want to keep them in tact, build a power supply, move the input and output to the rear and build some cases for them. Any good threads that walk through a very basic repurposing of these things?
     
  12. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
    These are mine, freshly pulled from the 720:

    [​IMG]
     
  13. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Those are very similar to the units I have. Yours have the early sound-on-sound duet mode switch

    Recap the unit, new power cords and fuses, lock the records mode and add the 375pf cap and enjoy
     
  14. dread31

    dread31 Addicted Member

    Messages:
    5,207
    Location:
    The Big Blue Ball
    I've got two pair of those inthe garage, one set an Akai M8, the other a set or Roberts 770. I started on the Akais and got one working and the other Akai working, but noisy as hell. Messed around with it for a month or so with no success and moved on to other projects.

    Dave
     
  15. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
    Yeah, I was wondering what that duet mode was?!? I felt bad tearing this thing apart, as it played like new. However, it was this one (picked up off CL for $35) or the Akai my father bought while stationed in Vietnam. I want to keep that one intact for sentimental reasons, so... chop, chop, chop on the Roberts!

    I am currently reading this thread to help me figure out what all I should and shouldn't do:

    http://www.audiokarma.org/forums/showthread.php?t=79384&highlight=akai
     
  16. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
  17. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    hey janikphoto, nice guitar amp setup. Did you do anything to the meters to get them to dance?
     
  18. janikphoto

    janikphoto Lunatic Member

    Messages:
    10,457
    I've used my Akai (the Roberts is basically a re-badged Akai) for a guitar amp with great success, but these Roberts amps will be for hi-fi use!
     
  19. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    I made cases for my amps

    Rained today so I had to stay in and do crafts

    I call them "Bookshelf Amplifiers"

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    Jig saw
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    Last edited: Jun 20, 2014
  20. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,271
    Location:
    Front Range, Colorado
    That's very clever and looks great too! A perfect fit. :thmbsp:
     

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