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Scrap Scratch build - Definitely need advice

Discussion in 'Shock Brother's DIY Amps' started by rojo412, Aug 1, 2019.

  1. rojo412

    rojo412 Tele-Meister

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    For quite a while, I've been wanting to build a tube amp from scratch.
    While I know that a kit build is likely the best way to start for a total noob, part of the challenge I'd like to learn about is the ability to start with old parts and make something that works. I'd like to know what I should look for in the wild and figure out how it does what it does.

    Yesterday, I ran across this while cleaning out my grandmother's basement:

    IMG_5546.JPG
    IMG_5547.JPG
    IMG_5548.JPG
    To be perfectly honest, I don't know if this would even make sense to start with, but it was free, it has a bunch of parts to learn about, and if I can eventually use pieces of it for a small tube amp, that would be great.

    First and foremost, can anyone tell me whether this amp would even be something to consider for parts? It's an old Fisher Stereo Receiver. It claims 150w.

    The transformers are stamped:
    "T-563-204-926"
    "T-592-152-926-2B"

    The tubes are 12AX7s, EL-37s, a 6AU6, SV4GA, and a slew of "6B_6" with "A,B,C, etc".
    As far as I know the 12AX7 are useful, possibly the sockets for others, but none of those tube numbers jive with a particular guitar amp.

    I figure worst case, I learn how to take it apart and test the various pieces.

    Any help or advice to get me started is very much appreciated.
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What type of amp do you want to make?

    It seems that would likely be a 5V4GA and not an "s" there. That tube is your rectifier and is 5V @ 2A, so that would tell me the PT has enough juice to do a champ or deluxe build most likely.

    https://www.amplifiedparts.com/sites/default/files/associated_files/5v4ga.pdf

    When looking at things to use from old equipment, you want:

    -transformers
    -tubes
    -that cool vintage fuse holder there
    -sockets if they are good quality and not cracking/damaged
    -cool knobs/lights/indicators

    Resistors and caps are more or less scrap in my world as those are the cheapest parts of an amp and I'm certain the caps aren't going to be any good anyway.

    I do like to get the specific output transformer I'm going to be working with for my design since that really is what makes the sound of the amp be "correct"

    The first thing I would do is get the power transformer out and see what voltages you have on the high voltage windings. That will really determine what type of amp you can build with it.
     
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  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Also, the "150W" number on the rear of most things that isn't guitar related is how much electricity the device draws from the outlet, not the power output.
     
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  4. rojo412

    rojo412 Tele-Meister

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    Really, at first, I'd just be happy to make something that powers a speaker, that's my first goal. If I had my druthers for an eventual amp, it would have 2 channels and reverb.

    I did watch a video on testing the transformer. When I get a chance to start pulling things apart, I plan on journaling all of the connections, marking the wires, then testing them.
     
  5. RollingBender

    RollingBender Tele-Afflicted

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    Usually, I have found radios and such to be of minimal value but you have a gold mine there. I was just looking at the schematic.

    https://www.thetubestore.com/lib/thetubestore/schematics/Fisher/Fisher-500-Service-Manual.pdf

    2-KT66 in push-pull, a workable OT, PT with plenty of reserve power....

    1st thing, if it still will fire up, get it going and get some readings off the PT for voltages so you know what you are working with and to confirm the OT is working and making sound. Then, tear it apart and build a guitar amp.
     
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  6. rojo412

    rojo412 Tele-Meister

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    That's great to know that it's a good donor.

    So to confirm, if I can get the thing to power up as is, I just test the sections at where they are currently soldered?
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yes - if you can get AC volts of where the power transformer hooks up right now that will get you started.
     
  8. dan40

    dan40 Friend of Leo's

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    Be aware that if the amp has not been powered up for many years, the electrolytic capacitors are likely dried up. If they are, they can pull large amounts of current which can ruin the PT and possibly cause the capacitors to go "bang". If the amp has been used in recent years, it may start up just fine.

    Be very careful checking voltages inside the amp if you are not experienced. There will be 250-300v AC on each side of the high voltage winding and you will see 400-500v DC after the rectifier tube. Never place both hands inside of the chassis when measuring because it increases the risk of a deadly shock. Also be sure to use a meter that is rated for the expected voltages you will be measuring. Uncle Doug has a great series of videos on Youtube on how tube amps work and how to work safely on them. Our forum member Rob also has a great sight on tube amp basics...https://robrobinette.com/How_Amps_Work.htm
     
  9. rojo412

    rojo412 Tele-Meister

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    @dan40 you bring up a great point: this amp has been sitting down there for at least my entire lifetime, not being turned on. So even if I replaced the fuse, it sounds like there's a good chance I'd probably have all kinds of fireworks happen.
    Uncle Doug used a home made "current limiter" in this video:

    (made in this video)

    Screen Shot 2019-08-02 at 9.15.20 AM.png
    I was kind of figuring that this would be needed after I took it all apart. While it would be kind of nice to keep it together to test them, if there's risk of blowing them in that process, I'd much rather do something like this.
     
    Last edited: Aug 2, 2019
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  10. Nickfl

    Nickfl Friend of Leo's

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    You absolutely need a current limiter for testing old equipment and for testing new builds as well. It is as basic a piece of amp building gear as a multi meter or a soldering iron IMO and there really should be a sticky at the top of this forum titled "before you plug it in, build a damn light bulb limiter".

    If you are going to scrap that and build a guitar amp from it, go ahead and pull the transformers and other useful parts corliss indicated. There really isn't much point testing the existing circuit if you aren't going to use it.

    FWIW, you were very lucky to find this compared to a lot of the old tube gear in peoples grandparents attics. This transformer set should work perfectly for anything in the 5f6a bassman/JTM45 family. Most of the old tube radios and such I've scrapped have been single ended with weird, less than ideal preamp tubes, and sometimes field coil speakers. Those can be used to make small practice amps, but they generally can't be used to build any classic amp circuits without any major modifications. A lot of old stereos are dual single ended as well and push-pull outputs, especially with big bottles tubes like this, are comparatively rare. Add to that all the cheap transformerless table top radios and phonographs out there that are basically unusable for parts and what you have on your hands is an excellent find that I would be very happy to have gotten a hold of!
     
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  11. tubelectron

    tubelectron Tele-Afflicted

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    :eek::eek::eek:o_O

    If it was me, I would clean it, search schematic, service manual and infos on the web, and restore it to its original, in specs working condition...

    But it's me, OK ? :D
     
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  12. rojo412

    rojo412 Tele-Meister

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    @tubelectron It was a consideration, but honestly not something I'd want to undertake. Looking at all of the caps and resistors and old tubes, it would cost a lot of money and take a lot of time... to end up with a radio.
    Frankly, I hate what is broadcast on the radio around here and have no use for one. Nor do I have LPs, CDs, cassettes, etc.
    I'd much rather learn to build a guitar amp.
    (Of course, if someone told me this thing would be worth a small fortune as a restored Fisher 500, I may reconsider the previous statement. Or if you have any interest in doing that... :D)

    Today yielded a decent amount of goodness, I will say. First off, I went to get the supplies for the current limiter and that was easy and cheap:

    IMG_5552.JPG

    Then my wife and I went to her parents' old house, where we were told we can take anything we want because it's full of junk. And what did I find in the basement...

    IMG_5549.JPG
    IMG_5550.JPG
    IMG_5551.JPG
    Whether or not these two old-school tube radios yield anything of interest, we shall see. But if anything, the bigger one could house the chassis of what I'm hoping to put together as this first attempt. The little guy, well... I doubt I could make anything super interesting happen with that setup, but again... it's free and has tubes!

    Time to build a current limiter.
     
  13. tubelectron

    tubelectron Tele-Afflicted

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    Yes, you can, @rojo412 ! :eek: :D

    [​IMG]

    [​IMG]

    [​IMG]

    Here is the schematic of my little Radio-Tone. I think it would probably enter the smallest of your radios ;)... The power transformer is a Fender Reverb Unit 6G14 replacement Hammond 290W, the output transformer can be a Champ model Hammond 1750A :

    [​IMG]

    That said, I really wouldn't rip the Fisher 500 AM-FM reciever, and go for the two other old radios to make an amp project.

    But again, it's me, and it's your stuff, OK ? :D

    -tbln
     
    Last edited: Aug 2, 2019
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  14. rojo412

    rojo412 Tele-Meister

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    DUDE! That is awesome, thank you so much for sharing that! Love that little thing, you certainly gave me more thoughts for little amps.

    The Fisher would definitely donate its parts to something a bit loftier. From the sound of it, that could be something that ends up as the next build or just an evolving project, certainly a bit more than I'd need for a micro head.

    Here's my current limiter build:

    IMG_5553.JPG
    Plugged it into the wall and plugged something into it and it powered up. I didn't test it for limiting current or having the light illuminate yet, but I think I'll need to get some alligator clips for that.
     
  15. rojo412

    rojo412 Tele-Meister

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    Okay, so here's what I've got so far:

    T-592 Transformer
    IMG_5554.JPG
    LEFT -
    - Yellow (maybe white?) leads went to 5V4GA socket
    - Red also went to 5V4GA socket (and yes, I drew a diagram where)
    - Black went to ground around fuse and somewhere else deeper, I wasn't sure what
    RIGHT -
    - Green connected to the 6BA6 tube socket
    - White grounded to 5V4GA socket
    - Browns went to 6X4 tube socket
    - Red/Yellow went to a "400ohm 5W" piece mounted inside

    T-563 Transformer

    IMG_5555.JPG
    LEFT -
    - Red went to the front EL37 socket (with a 6L6 tube in it)
    - Blk also went to that socket
    - Blue/Wht went to the rear EL37 socket

    RIGHT -
    - Blk (maybe navy) went to ground
    - Green went to spkr out 8ohm
    - Yellow went to spkr out 16ohm, which then tied to a 12AX7
    - Brown went to spkr out 4 ohm

    If anyone has insight into what all of this really means, please feel free to chime in.
    And the wire sheathing is quite crispy on these, by the way. It's not shattering into pieces, but it certainly crackles when you move it.
     
  16. RollingBender

    RollingBender Tele-Afflicted

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    That jives with the schematic I posted earlier (the transformers are shown on page 6 of the document). On the schematic, T1 is your power transformer and T2 is your output transformer.
     
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  17. rojo412

    rojo412 Tele-Meister

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    Okay, so I tested T1 with the multimeter, no power into it. Here's the ohm ratings I got:

    Browns: 32.3
    Reds: 62.4
    Red to brown on all contacts: 14.6, 16.4, 46.5, 48.2
    Yellows: 0.6
    Red to Red/Yellow: 30.2, 32.6
    Blacks: 1.7

    Also, I tested the current limiter and it works. Not gonna lie, that was a little scary. But it works like it should.
     
    Last edited: Aug 3, 2019
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  18. rojo412

    rojo412 Tele-Meister

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    T2 numbers:

    Red & Blue/White - 121.9
    Red & Black - 146.7
    Black & Blue/White - Blip near 190 then nothing

    Grn Yel - .8
    Grn Blk - 1.1
    Grn Brn - .6
    Blk Brn - .9
    Blk Yel - 1.3
    Brn Yel - .9

    No reactions between left and right side leads in any pairing.
     
  19. rojo412

    rojo412 Tele-Meister

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    Okay, so please let me know if I'm doing this right or not:
    - I connected T1's black leads to a power cable with alligator clips on the end but didn't plug it in yet
    - Connected my multimeter to the red leads with alligator clips and set the meter to "V~" on "500", making sure no leads were touching elsewhere
    - Plugged the power into the voltage limiter and the multimeter read "839"
    - Unplugged it from power and moved the clips to the brown leads, plugged back into power and got "431"

    So what should I be determining from this info?
    This is all new to me, so any and all help and constructive criticism is definitely appreciated.

    And a total side note question: If I'm looking to find more parts for these types of projects in the future, what should I be looking for in the used/cheap/scrap markets?
    For example, if I have a Pioneer solid state receiver from the 90s, does that have a transformer worthy of salvage?
    Or what if I have a basic SS guitar amp that works but sounds bad; is there any sense in taking the transformer from that to make a better amp?
    Just wondering...
     
  20. RollingBender

    RollingBender Tele-Afflicted

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    Unloaded and testing on the bench, your voltages will always be higher. See what you get from the yellow secondaries and then from the green secondaries...

    Really nothing worth salvaging in solid state equipment. They don’t have that big B+ voltage or proper heater taps which tube amps need.
     
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