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Dukane Intercom Tube Amp Conversion

Discussion in 'Shock Brother's DIY Amps' started by ArcticWhite, Jul 8, 2018.

  1. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Just picked this little guy up for ten bucks at garage sale. The seller's dad worked at a Ford dealer and he grabbed it 50 years ago.
    Obviously some blown parts, but it looks pretty solid. Tube lineup is EF86 6SL7GT, pair of 6V6GT. 5Y3GT rectifier. Big iron.

    The small speaker tests at about 32 ohms. There are so many outputs, I'm a little concerned about what speaker impedance is required for this thing.
    There are a pair of cap cans.
    I'm surprised to see a strain relief grommet on the power cord, and a fuse.
    There's a death cap too.
    Anyway, I'll start pulling off stuff that isn't an amp later tonight.
    20180708_174052.jpg 20180707_123934.jpg 20180708_175332.jpg 20180708_175351.jpg 20180708_174200.jpg 20180708_174058.jpg 20180708_174112.jpg 20180708_174932.jpg
     
    Last edited: Jul 9, 2018

  2. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Nice! Thats a pretty great selection of tubes for amp building, I would never have thought an old intercom would have so much good stuff inside.
     
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  3. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    It's a pretty beefy build because it could drive up to ten 10 speakers at once. Unfortunately however, I think the OT wants a 32 ohm load. Is this a characteristic of the OT, or can the power section design be modified to reduce the impedance required?

    I have a bunch of Hammond amps from old organs, so I have a selection of OTs to choose from.
     

  4. jhundt

    jhundt Doctor of Teleocity

    Age:
    64
    Mar 23, 2003
    Netherlands
    man - you have to get a big old analog ammeter and put it in that big whole in the front, so you can display your bias!
     
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  5. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    621
    Apr 8, 2016
    Oakland, CA
    http://www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach

    Here is an example of a long-tailed pair phase inverter using the 6SL7GT that I have used.

    EF86 -> LTP -> 2 6V6GTs seems like a great early Vox/Dr. Z kind of basic topology. You should test the OT to see its impedance ratio. But you do probably have a number of 20ish watt 8K:8R OTs in your Hammond collection, too. Good luck!
     
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  6. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    This is over my head right now.
    How do I check the impedance ratio of the OT?
     

  7. Kriticaster

    Kriticaster Tele-Meister

    Age:
    41
    407
    Nov 5, 2017
    Crete, Greece
    That big hole in the front is the speaker, right?

    I guess it’s not gonna stay there thou??
     

  8. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Yes, that's the speaker hole. It's covered by a flat black felt material.
    Dunno what I will put there. Gotta get the thing running first.
     

  9. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    ArcticWhite---I been doing a little research on this because I love old Dukanes. You have the right tube configuration to build a vintage tweed Era 5A3 Fender Duluxe. Because of the single gain stage EF86, you would have to build it one channel input instead of two. Don't know if you can read schematics but I've attached a pdf file of the 5A3.

    I can't help but believe your output transformer would have a 4/8/16 Ohm taps. I tried to read the model number to find a schematic for your Dukane but it's not clear enough in your picture to see. If you could post the model number, I could possibly find a schematic that would show if it had the 4/8/16 Ohm speaker taps. So if you post that, I'll see what I can find out.

    I did a conversion to a Bassman 5F6A from a Dukane 11460 PA Head. Completely stripped down the chassis and rebuilt it using existing chassis, transformers and tube sockets. Here is a Flicker link to pictures of that project.
    https://www.flickr.com/photos/12623503@N04/albums/72157601876931407

    If you never taken on this kind of thing before it can be kind of daunting but there are plenty to help here to instruct you along the way what ever measure of conversion you want to take on. Happy Amping! Platefire
     

    Attached Files:

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  10. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Thanks Platefire.
    I can read schematics, and recently converted a Hammond AO-35 to a Carmen Ghia (Dr Z).
    This is a slightly more complex project I think .
    Model number is 4B100B.
    Here's a picture of the OT taps.
    The off white and yellow/grn stripe output taps go to ground. The black one goes to the monitor volume and pin 6 of the 6SL7. Green and Yellow go to the selector switch marked Program - Listen - Talk.

    Edit: I already own a 5e3 clone, so I'm not sure if a 5a3 will be my best option.
    20180709_130744.jpg 20180709_130744.jpg
     

    Attached Files:

    Last edited: Jul 10, 2018

  11. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon

  12. magic smoke

    magic smoke Tele-Meister Silver Supporter

    232
    May 28, 2017
    Quantum Entanglement
    Great score! Some guts look a bit crispy... I wonder what happened to that wire wound on pin 8?
     
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  13. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    You forgot to post the picture of the output tranny taps

    I've done a pretty extensive search for a 4B100B Schematic
    and I didn't turn up anything:>(
     
    Last edited: Jul 9, 2018
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  14. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Added the pictures of the taps above.
    I found a website that sells schematics but they want 15 bucks for it. Goes against my grain.
    Can I check the transformer impedance with a multi-meter? Thanks for the help!
     

  15. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Someone had been through the amp and messed with it quite a bit. Lots of extraneous wires - looks like a bit of a science project, done with little knowledge.
     

  16. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    621
    Apr 8, 2016
    Oakland, CA
    Once the tubes are removed and it powers on without blowing a fuse or lighting your current limiter bulb, then drain the capacitors and desolder all five leads to the OT: brown, red, blue (the primaries), green and black and possibly other leads (the secondaries). Your OT may not follow that common color scheme, but you can still tell the primaries and secondaries by where they go. 'Brown' to a power tube plate, 'red' to the B+, 'blue' to a power tube plate, 'green' to a speaker jack tip, 'black' to ground, often at the same speaker jack sleeve. Even if the colors don't match.

    Measure voltage across your heaters (like pin 2 and pin 7 on a 6V6 socket). Write it down. Power off.

    Attach two jumpers from those pins to the green and black OT wires (which are flying loose -- no connections to anything else). This puts your heater voltage across the secondaries. It's probably 7 VAC.

    Clip your voltmeter probes across the primaries, blue and brown (ignore the center tap, leave it unconnected and safely out of the way).

    Power on. Read voltage across the primaries--this is the transformed voltage. Write it down. Power off.

    To get the impedance ratio there is a short calculation once you have those numbers. Unfortunately this can't be done if you would rather leave all the wires in place--it's possible to leave some of them in place but it seems like a poor trade-off of safety vs convenience for someone reading a walk-through of this process.

    If you can't trust an unknown OT and are going to pull it out to replace it anyway, then you have to desolder those leads anyway. Nothing lost. Good luck!
     
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  17. RoscoeElegante

    RoscoeElegante Friend of Leo's

    Feb 19, 2015
    TooFarFromCanada

  18. ArcticWhite

    ArcticWhite Tele-Meister

    283
    Apr 14, 2009
    Portland Oregon
    Excellent, thank you. I have no problem desoldering leads, testing, etc. Will do Wednesday night!
     

  19. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Here's a link with more info on the transformer testing method Snfoilhat gave you: https://www.radioremembered.org/outimp.htm it also explains the calculations to get the impedance ratio once you have your voltage measurement.

    I would also recommend you measure the actual voltage across the leads you connect the low voltage source to (OT secondary usually) while it is connected. In my experience connecting to the OT can act as a load and cause it to sag (depending on the source) a bit from the unloaded voltage you may have measured before, from something like 7v to 6.8v or something like that. You'll want to know if this happens so it doesn't throw off your calculation.
     
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  20. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Yes. Calculate the turns ratio of the OT. Then we figure out what the resistive load will be and advise you accordingly on power tube selection.
     
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