Old Console Stereo Conversion Question

Discussion in 'Amp Tech Center' started by mikestearns, Sep 27, 2019.

  1. mikestearns

    mikestearns Tele-Meister

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    I got the bug to tear apart an old Motorola Console stereo my girlfriend had laying around collecting dust. Inside the amp works, and has 6v6's and 12ax7s, plus a spring reverb unit. (I pulled a 12ax7 to put in V1 of my Orange OR15 and it screams). Would I be able to use the tubes, chassis, and transformers to make 5e3 style build, or would I need a different transformer/chassis?
     
  2. mikestearns

    mikestearns Tele-Meister

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    Here's a schematic if that helps.
     

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  3. gusfinley

    gusfinley Tele-Holic

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    Do you have a schematic for the power supply or power amp?

    The schematic listed doesn't show wether the power amp is push-pull or single-ended. It also doesn't show the power supply for use to check for proper voltages, rectifier type, etc.
     
  4. mikestearns

    mikestearns Tele-Meister

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    Sorry, I uploaded the wrong one.
     

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  5. mikestearns

    mikestearns Tele-Meister

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    Some more info on it -


    The Motorola SK79W is an AC operated 21 Tube Stereo BC/FM band receiver, Phono Amplifier and 4 Speed Automatic Record Changer. Has Vibrasonic system and tuning eye indicator. Chassis HS-969 has 11 tubes used for the BC/FM receiver, the vibrasonic preamp and the main preamp. Chassis HS-968 has 10 tubes used for the 3 channel power amplifier and power supply. Cabinet is Walnut. This model has Qty(1) 15" PM speaker for bass channel, Qty(2) 8" speakers and Qty(2) 5" PM speakers. Has separate bass channel that drives the 15" speaker. Uses Record Changer VM59RC..

    A "FM Stereo" option was available for this model, which used Stereo FM chassis HS-996.

    The Motorola Tuner-Chassis HS-969 (AM/FM receiver with preamplifier) and the Amplifier-Chassis HS-968 have been used in Motorola models SK56MB-1, SK57W-1, SK77WA, SK78W, SK79W, SK79WH and SK80CW. This is what can be seen on SAMS Photofact set 600, folder 11 from Oct 1962. Some of the sets like the SK77WA were on the market late 1960. Beitman volume 22 names the models without suffixes: SK56MB-1, SK57W-1, SK77WA, SK78W, SK79W, SK79WH and SK80CW.- but also misspells to 7S instead of 77 and HS-96S instead of HS-968. It also writes: "HS-963 in Models SK73, SK74, SK75, SK76, is very similar." That is another story.

    The first 11 tubes on our list are for the Tuner-Chassis HS-969 - and the tubes for the 10 tube stereo Amplifier-Chassis HS-968 are 3 x 12AX7A, 6 x 6V6GTA plus 3DG4 as rectifier. All of these models also contain a 4 speed automatic record changer. Some have 5 loudspeakers (left and right 5" and 8", center 15"). For service information on record changer VM59RC see similar V-M1571 at Photofact set 527 folder 14. AM Tuning range is 540 - 1640 kc and FM 88 - 108 mc. Rating is 175 watts, 1.6 amp.at 117 volts ac 60 cycles.

    Motorola calls these models "Three amplifier Stereophonic High Fidelity Radio-Phonograph" with trademarks like Golden Duplitron, Feather Trac, Golden Stereo 1000, Vibrasonic (is a reverb unit) and Golden Voice. The suffix W = Walnut, WH = White, CW = Cherry, M or MB = Mahagony. Probably not all models have legs.
     
  6. muscmp

    muscmp Tele-Afflicted

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    i'd just do whatever simple mods you can to use it as a guitar amp. it has a ton of great tubes and speakers in it. otherwise, do whatever it takes to bring it up to speed as a stereo system.

    play music!
     
  7. muchxs

    muchxs Doctor of Teleocity

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    There is almost enough stuff in there to build three amplifiers.

    Satellite speakers with a center sub are nothing new. All the major manufacturers were doing that circa 1960.

    I see three output transformers, three pairs of 6V6s, three 12AX7s and one power transformer.

    You could be doing it a terrible dis- service to trying to gut it and make it into a guitar amp.

    I'd buy a 5E3 chassis, a 6G3 chassis and a Princeton Reverb chassis. I'd use the appropriate power transformer for each. I'd build three amplifiers.

    I'd use the Motorola power transformer for a low plate JTM45. So make that four amps.
     
  8. mikestearns

    mikestearns Tele-Meister

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    That was kind of what I was thinking, I just wasn’t sure if I could use those transformers for that. There’s actually 6 12ax7s, the preamp section had 3 also.
     
  9. Nickfl

    Nickfl Tele-Afflicted

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    Wow, thats a hell of a find! Like much says, there are at least three amps in there. Usually these old consoles are stereo single ended, its a gold mine to find one thats not only actually PP but loaded with three PP OTs.

    Someone is going to chime in with "please don't destroy this, let someone use it for its intended use". Meh, stereo collectors don't want these and while it probably would sound good if you serviced it and got it working, if you'd rather have three tube guitar amps than one stereo I say gut it (its definitely what I'd do).

    The first thing I would do is pull all those OTs out and measure their winding ratio, based on the winding resistance readings on the schematic, the bass transformer will be different than the left and right transformers. From there you can figure out what primary impedance options you have and what circuits the transformers might be best suited for. While you are at it, pull the PT and check its unloaded voltages.

    Here is a link to information about testing an unknown OT. It uses an autotransformer (a variac) as the voltage source, but you can use a low voltage transformer winding (such as a filament winding) instead. Make sure you use a lightbulb current limiter for safety when testing transformers and measure the actual voltage across the OT primary (Vin on the diagram) as it can sag when connected to the OT being tested (due to magnetic flux or some such thing I guess).

    Good luck!
     
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  10. mikestearns

    mikestearns Tele-Meister

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    Thanks! It worked fine as a stereo, but way too big for a small apartment, and I definitely didn’t want to move it, so I have the power section, preamp, reverb unit and 12” speaker all pulled out for when I get settled in the new place. The other speakers didn’t seem all that useful.
     
  11. rolandson

    rolandson Tele-Meister

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    Does the reverb tank have any interesting language printed on it...say something about who made it?

    And...I see no reason why the chassis can't be repurposed, though one might need to construct a custom cabinet to accommodate it.

    Nice find/acquisition...!
     
  12. Nickfl

    Nickfl Tele-Afflicted

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    If you haven't already thrown them out, those 8" speakers might be worth keeping too. I have a pair of 8" alnico speakers pulled out of a voice of music stereo console and they turned out to be surprising good when I used one in the vox AC4 inspired amp I built with the PT and one of the single ended OTs from that same console.
     
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  13. mikestearns

    mikestearns Tele-Meister

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    Initially planned to keep them all, but my finger slipped when I was taking out the smaller speakers and went right through it. They were all pretty brittle, so at that point I figured they weren't worth salvaging.
     
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