The Use of an unknown Transformer??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jr_vw2, Sep 7, 2008.

  1. jr_vw2

    jr_vw2 Member

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    Here is my project as well as my delema. I picked up an old Magnavox record player/stereo console. Everything worked on the unit so I know its in decent shape. I have pulled the power amp out and I am wanting to build a buitar amp out of it. However the console had two chassis in it the power amp had a large bulk connector that ran to the second chassis which had all the controls on it as well as some other tubes. Basically I just want to totally strip the power amp and build from scratch. (maybe copy an older gibson or something) However I dont know how to ID all the wires on the PT The wires are as follows. One one side you have Green/black and Green/white I know these wires are for the filimets there is also Red/black The is the power into the trans. and there is a black wire that runs to the bulk connector and I dont know what the black wire is. On the other side of the trans you have two Red wires that go to the plates on the Rectifier tube (6CA4) there is a red/yellow wire that goes to ground there is also a brown wire and a brown/yellow wire that go to the bulk connector and I dont know what they are either. The numbers on the Transformer are 300206-1 and 606331 this was a stereo amp (two OT's) the OT's are 320082-2 any info or guidance on this would be great. I basically want to see what my limitations are going to be with these trannies so I can decide what exactlly I want to build. Thanks alot

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  2. brad347

    brad347 Member

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    Plug the thing in, and (CAREFULLY!) measure the voltages on the various secondaries with it plugged into the wall.

    You can die if you mess this up, so exercise all caution. You will want to clip your meter probes to the wires with the power OFF. You will have as much as 400-500 volts on some of the wires, and that could kill you dead. Make sure you avoid shorting the wires, as well.

    Maybe you should seek the help of an experienced person.

    Of course, you would always be safe building your power amp around the tube types already present in the stock amp.

    For the output transformers, there are ways to calculate and test what the speaker impedance should be for various tube types. You'll have to look that info up, much of it is published.
     
  3. jr_vw2

    jr_vw2 Member

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    Thanks and yes I am well aware of the safety involved in working around high voltages.
     
  4. jh45gun

    jh45gun Member

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    With some searching of the radio/stereo forums ect you should be able to find a schematic for it and then go from there. I stripped a Zenith Chassis out of a old Mono Phonograph and made a 5e3 out of it which works great. However I also made a 5f4 out of a Hammond Tone Cab that the PT was set up for less voltage on the primarys (110 instead of 120 ) which means it put out more on the secondarys. I am not sure exactly on how that works but it was explained to me that way so the tech had to do some work with some larger resistors to get the voltage up on the primary so it regulated the secondary. That may or may not be an issue depending on the PT. With my zenith it was not an issue and probably not with your magnavox either but it evidently can happen depending on the PT and its usage. Forgot to add in both cases I was able to find a schematic on line. In the case of the last amp I got and am working on restoring instead of altering a Bogen MX60 I found a library that had a Sams Photofact Collection and got the schematic there as I could not find it online at least not for free. If your local library does not have a Sams collection they maybe can find a library that does and get you a copy of the schematic interlibrary loan.
     
  5. Trout

    Trout Member

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    Hey, those old Maggies are magic & make outstanding low power HiFi amps.

    I used that exact model on my old Laptop in my garage for nearly 2 years. The OT's make excellent choices for AX84 SE amps, though IMO it would be a waste.

    Check out this thread over at Audiokarma. A lad did a really nice job of dressing one up.

    http://www.audiokarma.org/forums/showthread.php?t=134055

    Also, If you sign up over at the DIYaudio site, and do a search for Magnavox in the tube section, several guys have threads with great pictures of their conversion/updates.

    Picture below is not my amp, but it shows what could be done.

    When I polished my chassis, I used Mothers Mag and Aluminum Polish. It took about 4 hours to get a gloss like the one below


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    Edit, Check this out
    http://www.youtube.com/watch?v=WOXjFm9e92E&feature=related
     
    Last edited: Sep 7, 2008
  6. brad347

    brad347 Member

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    trout brings up a good point.

    Stereo hi fi amps are useful to someone and it would be a shame to hack it up.

    Old mono PA amps, on the other hand, are great candidates for conversions.
     
  7. Trout

    Trout Member

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    I just checked my amp archives, I found the schematic for it.

    If you decide to restore it and need a copy PM Me.
     
  8. SatelliteAmps

    SatelliteAmps Member

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    It's not an expensive piece of hifi gear. I would turn it into whatever you wanted. You can spend a lot of time trying to get that to be a cool piece of stereo gear, and it will sound good, but it won't ever sound amazing. You may not get as much enjoyment or use out of it as you would by making it into a guitar amp, or even as a learning experience.

    There are a few things about transformers that will allow you to figure out what al the wires go to. One is to just look up the tube sockets and reverse figure what each pin should have on it. The other is to see how much is under the universal transformer wiring code. Usually black is going to be your primary input, red is your high voltage, yellow is your 5 volt filament, green (or green with stripes) is usually 6 volt filament. If it has multiple similar taps, some will have additional stripes. Sometimes there will also be a center tap that will have a similar color wiring (two green and one green w/ yellow stripe for instance. Striped one is a center tap). Red w/yellow is usually high voltage center tap (perfect for a standby switch).

    Wires that go to bulk connectors are usually just to be jumped to a different chassis or a record player.

    The main issue with using that amp is that the output transformer won't sound as good as a dedicated guitar output, and trying to run it as a stereo might be more effort than it is worth, from a sound quality point of view.
     
  9. Groovey Records

    Groovey Records Member

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    There is a guy here at TGP who specializes in just what you want to do

    I have an old Motorola similar to your nice chassis and great wooden cab with stero 8" motorola speakers I wanted to make a stealth amp out of it for the living room. I didn't but he has done a few anybody remember him his sig said 1 watt of pure sound
     
  10. Trout

    Trout Member

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    No 5V taps in that unit, 95% of those had dual 6,3V taps, 1 set for the amp, and 1 set for filaments on the other chassis. No 6.3V CT in there, they used (2) 100 ohm resistors instead

    To power up the amp you will need a jumper in the gang plug. Be certain speakers are attached.

    Agree to disagree?

    If you use salavaged speakers & horn that were originally designed for the unit in a pair of DIY boxes, you will be very surprised at the actual quality of sound. These are very simple amps to service, and swapping out a couple caps really brings them to life.

    Agreed, not a "High End" unit, But Cheap tube HiFi playback? Heck Yes worthy of its intended application. You would spend way more time trying to get a decent guitar amp out of it than you would just fixin her up a bit.

    Here is a gut shot of one that a guy on a hifi forum tweeked up, pretty simple
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    Last edited: Sep 7, 2008
  11. Groovey Records

    Groovey Records Member

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    The guys name I was thinking about was PRNDL he knows his stuff he is in the Tech forum often

    Although much of what these folks is saying is true, it would be easier to make what you have into a Hi-Fi Amp then Guitar Amp. Still it can be done and PRNDL has done it. Look at his pictures on his web site

    PRNDL

    TGP Name PRNDL
     
  12. SatelliteAmps

    SatelliteAmps Member

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    I have no problem about agreeing to disagree. I do prefer to address the actual issue of what can be done with this amp as far as making it into a guitar amp, rather than trying to talk him out of building it. I'd rather he tried something and made something interesting.

    These hi-fi's are very common in my area, and they are not expensive. I don't believe that doing either project would be a disservice to it, but the idea was to build a guitar amp out of it.

    I should also add, I love old tube radios, and hi-fi's. I've got quite a few of them. A lot of them are wonderful to listen to. If someone came to me and asked about restoring this hi-fi to listen to audio with, I would have no problem helping them out with that either.
     
  13. Trout

    Trout Member

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    Yep,, Thats one of the guys over on the DIYaudio forum also.

    Great guy.


    One of the very first Champs I ever built used one of those maggie OT's I used it with a 6AQ5 which sounded really sweet. I eventually gave it to a kid up the street as his first tube amp.

    He and his Mom were going through tough times when his dad died and couldnt afford an amp. I hope he still has it, they later lost their home and moved away.
     
  14. booj

    booj Member

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    If you know where the input (120 volt) wires are, and have a variac, you could put 12 volts in and read the various voltages, and multiply by 10. If you don't have a variac, you could use a small voltage transformer to feed the input. It'll take more calculating to figure out the output voltages, however.
     
  15. PRNDL

    PRNDL Member

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    Thanks for the great comments!!

    The first step is to figure out what you have, which many have helped with.

    This page has a lot of great info. The first link shows transformer wiring diagrams (color codes).
    http://www.el34world.com/schematics.htm

    The next step is to figure out what you can build it into.

    You could keep it as a stereo amp. Someone posted a photo of a fine example. Motorola amps were known to be of high quality. One preamp tube would mean that it needs line level input, which would be from a guitar or stereo preamp.

    For guitar amps, I'd recommend choosing an amp with EL84's, which are in the same category as the 6BQ5's in your amp.

    Although your amp has two single-ended output transformers, your transformer has enough juice to make a push-pull amp, although you'd have to buy a new output transformer. Push-pull amps include the Matchless Spitfire, Vox AC15, 15 watt Marshall, and Fender Pro Jr.

    There are web sites that show how to build each of these amps in detail.

    Take some time to check out what's available from different web stores in terms of chassis boxes, turret boards, and kits. Kits are the easiest (and often least expensive due to shipping) route.

    Make certain the chassis box does not have the transformer hole punched because most likely yours won't fit.

    Good luck!
     
  16. donnyjaguar

    donnyjaguar Member

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    I did't disagree with that Trout says about converting to hi-fi use at first but.... I generally don't consider single-ended 6BQ5 based amplifiers to have a lot of hope of reaching hi-fi karma. They'll sound musical, but the lack of output power & subsequent dynamic range, teamed with the high even-order distortion products would, IMO, make them better suited to a musical instrument amplifier.
     
  17. SatelliteAmps

    SatelliteAmps Member

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    Most of the transformers on that page show what is considered a universal color coding system for power transformers as I explained above. None of the examples on the website correspond exactly to this one though.

    If you want to build a kit, it's not worth trying to salvage just the power transformer in this amp to use with someone's kit.

    If you want to use this as a platform to build an amp, that will work. I've seen a previous build of the original poster, and he is up to the task of building an amp on this chassis.
     
  18. Trout

    Trout Member

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    There is a world of truth in that Statement.

    You only get about 2.5 -3W per channel of really good HiFi sound out of that particular chassis. When combined with efficent speakers, it can fill a room pretty well.
    I plug into the speaker out on an old IBM thinkpad to drive it in my garage.

    It is plenty loud as long as I do not have a saw or router going.

    A lot of guys find a simple 5W champ to loud for living room use.(not me though)

    You know, it would be kind of cool to make a stereo V front amp out of it though. Mount that chassis in the bottom, Add a few tweaks, and maybe a small pre-amp chassis up at the top for jacks and controls and extra 12AX7.

    The PT in there will supply enough verb for extra pre-amp tubes and put the B+ into a AX84 P1 circuit range.
    Then he could save the chassis intact should he decide to build something else out of it.
     
  19. jr_vw2

    jr_vw2 Member

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    Thanks for all the input and help guys. I really think that I will be building and EL84 based amp out of this one. I will come up with a new OT and probably build form scratch. Although I agree that a Hi-Fi stereo amp would be cool its just not my cup of tea. Thanks again for all the great advice.
     

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