Old tube hifi mod into guitar amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wavey63, Dec 26, 2009.

  1. wavey63

    wavey63 Member

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    I just got an old tube hifi that is a bit dirty but orherwise seems fine. It powered up after a fuse replacement. Any tips on info to turn this into an amp or mabey a preamp/overdrive?
     
  2. Trout

    Trout Member

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    A lot depends on the Make & Model. Some HI-Fi amps have a good compliment of tubes that make it easy, others not so much.

    What did you get?
     
  3. wavey63

    wavey63 Member

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    I think its a Zenith stereo. I got it late last night but did notice it has 4 el8s and 5 12ax7. The 84a are shot but a AX's look fine.
     
  4. hasserl

    hasserl Member

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    5 12AX&'s gives you a lot of possibilities, I'd be thinking of doing a two channel amp, each with its own output, run that out into two cabs, or a stereo cab, you could do something really cool, something like a 5E3 on one side and a plexi channel on the other. Or, use one channel for your main tone shaping and amplification, and use the other channel strictly for reverb, that way you're not having to use reverb on a dirty signal, it could have it's own complete amplification, again, run the output into a stereo cab, maybe even something like 12" for the main amp, and a nice full frequency 8" driver for the reverb.

    Lot's of possibilities, think about what you'd use most.
     
  5. Prairie Dawg

    Prairie Dawg Member

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    What I would do to start with before you completely strip it out is to measure all the voltages-high voltage AC, B+, vP, vG, and vFil with the tubes installed, and measure the impedance of the speakers at the amp. Then, take that information and file it away someplace safe. You'll need it again.

    Even if you decide not to convert it I'll bet there's some dandy iron on it. Zenith did use some funny 3 volt rectifier tubes (3DG4, about like a 5R4 with a different pinout) so if that's what it's got let me know as I have a few new ones that I can contribute to the cause.
     
  6. wavey63

    wavey63 Member

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    It actually has a 5AR4 for recto tube. When I fired it up the original EL4's flashed and popped pretty bad so I removed them toot sweet. I put in another pair I had lying around and no problem. Are there any guides or resources for modding this as I am new to creating my own amp. I have built a few kits and have the builds down pat by now. Thanks for the help guys.
     
  7. Trout

    Trout Member

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    Get the Chassis number, there were a few variants, I have a Zenith Service manual that covers most of the chassis in the 50's 60's. They give voltages on the power supply section which will be useful.

    What Rectifier? most of the 60's models used a 5U4
    Alot of those transformers had dual 6.3V taps as well as 5V and HV.

    Those chassis with the Quads of EL84's have a very robust power trans, The OT's are rather shity though.

    Heck, with 4 el84's and 5 12AX7's, you should/could be able to build a 2 channel 40W Marshallesq amp, just get the proper PP OT, and have a blast.
     
  8. oldhousescott

    oldhousescott Silver Supporting Member

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    Not that you won't find lots of helpful ideas here, but several guys at the Hoffman amp forum have done and documented these types of conversions. Look in the schematics section for several different plans.
     
  9. wavey63

    wavey63 Member

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    Man i was way off. It is a Realistic SAF-40B. Gonna do some research now. ALl ideas all welcome. Thanks!!:Devil
     
  10. wavey63

    wavey63 Member

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    Not much info on this thing. I found 1960's and that it gets hot...not much to go on. WIll try more...
     
  11. Trout

    Trout Member

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    If it were me, I would restore it, especially if I was looking for a tube stereo. These things are RARE & Sleepers. A collector might go some serious coin for a good one.
    That thing has some serious OT iron on it!



    Check out the online 1962 Radio Shack catalog .
    Here is what is says about the "New Design" SAF-24:
    "A Top Buy! Proivides 24 watts of Stereo Power!" Provides dual 12 watt stereo channels, separate stereo controls and a multitude of "important 'extras'". Sold for $54.99.
    Specifications: 24 watts, 48 watts peak. Frequency response - +/-1db 20-20,000 cps. Distortion: total RMS 1.8%. Hum: -52db phone, -72db tuner. Sensitivity: .85volts tner, .004 mag. cart. Feeback: 18 db. Outputs: 4,8,16 ohm.

    Page 80/81
    Or
    Page 70 as a Combo Deal

    Edit, there is a Sams Photofact file for it at the Sams website $7.00. Sams # 550-12
     
    Last edited: Dec 27, 2009
  12. brad347

    brad347 Member

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    I think you should leave that amp alone and/or restore it, and find an old MONO or PA head to convert. Converting a stereo amp is more trouble, AND there is a market for them and the are desirable.

    The 'flashing' of your EL84s on startup is totally normal for Mullards, Amperexes, Philipses, and other Euro manufacture tubes. Totally normal. I'd power up with them again and assess, or put them on a proper tester. DO NOT throw them away.
     
  13. SatelliteAmps

    SatelliteAmps Member

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    Realistic used to re-brand tubes with their logos. A lot of times they were European manufactured tubes. Some of the highest quality stuff. You might want to look into what they really are.
     
  14. wavey63

    wavey63 Member

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    Thanks for all the info. The tubes are definitely shot, el's anyway. They are popping and arcing, not like the flash of the amperex. They are branded Matsushita on all of them. They AX7's seem fine. I think I am gonna restore it and get a turntable to see how music used to sound when I was a kid!!!:crazyguy
     
  15. Trout

    Trout Member

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    That is an excellent idea. I have a couple old console chassis here I restored for use in my shop and garage. Even using a cheesy portable CD player as a source they still sound great. If you watch on craigslist in the electronics category it is very common to see old bookshelf speakers pop up REALLY cheap. I put a pair of old Scott's in my garage and it is just right on volume. Mine is a Magnavox with a quad of EL83's. I made a cheap cover for it out of chicken wire lol
     
  16. SatelliteAmps

    SatelliteAmps Member

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    I agree. Excellent idea. My shop stereo is an old Fisher with a pair of old Seeburg jukebox speakers. One of the best stereo's I've owned.
     
  17. donnyjaguar

    donnyjaguar Member

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    I'm with Trout. Its probably worth either restoring or building a stereo hi-fi amplifier from.
     
  18. hasserl

    hasserl Member

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    Well I'll be the lone holdout then. Although I can definitely see the attraction of using it for a stereo system, there are also some advantages for a two output section guitar amp that you just can't do with a mono amp, a couple of which I mentioned earlier in the thread. And to me they would be more valuable and enjoyable than a stereo system for the shop. There are plenty of good old transistor stereo amps out there, cheap too, that do the job of music reproduction just fine.
     
  19. wavey63

    wavey63 Member

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    I am gonna just restore it. WOrse comes to worse I sell it far some ca$h to get another piece of equipment. Plus I am curious as to how much better it will sound. I have a few friends who have told me about the quality of sound from a tube amp and phono. I have hated the sound of cd's since I bought my first player at age 15...way too harsh on everything I have heard.:phones:phones
     

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