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Dukane Intercom Tube Amp Conversion

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

  1. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    Didn't you say in another thread you had a bunch old Hammond amps? or was that someone else? If so, you could snatch the OT out of a Hammond that had two 6V6's push/pull and use it which would probably have 4/8/16 Ohm taps.

    Regarding cosmetics, I would really try to build my new amp in that existing Chassis. I think those old Dukane chassis with the perforated covers are so cool. Just me!! I would leave all those push button controls abandoned in place and install the new amp input jack, volumes and tones controls in the panel above where you already have holes for pots. I would also remove that rear panel with all the speaker run connectors/terms and replace it with a solid metal plate where you could install a 1/4" speaker jack to connect to your OT speaker taps,
    hopefully 8 Ohm most common.

    That's just my thoughts but I'm big on preserving the old vintage
    amp appearance---so that's just my thinking and take it with a grain of salt:>) Platefire

    BTW-Here is a couple of pics of my Dukane Project
    DSC00412.JPG
    DSC00414.JPG
     
    Last edited: Jul 11, 2018
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  2. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    Oh, I am definitely going to use the existing Metal Cabinet. Just going to clean it up and polish it. These things are cool as hell.
    I like the idea of putting a bias meter in the old 4 "speaker panel too.
    I plan to test the original OT because I think you're right about it having an 8 ohm tap. It has several extra wires going to ground for no good reason.
    I'm in the middle of it right now!
     
  3. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    20180712_003554.jpg This seems like an awful lot of stuff on the EF86 in V1.
    A 20 mfd electrolytic from the g3 pin to ground? With a domino type capacitor in parallel, also to ground.
     
  4. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    I like the idea of a Bias meter in the old speaker opening. I've installed external bias test points along with external bias pot, but never a bias meter? I'd like to hear more about how that works.

    On the EF86. I have never had any experience with those. From what I've seen the hook up for a Guitar preamp is quite different or more complex than a normal 12AX7 but everybody seems to agree that produce great tone. Also I know in the old PA and intercom amps they tend to put a lot of extra components and circuitry not needed on a guitar amp. IMHO I would follow the example hookup of a well known EF86 guitar amp and use that as a model. Everything in the EF86 hookup that didn't resemble the model, I would tear out and get rid of. Seems ever EF86 Pre example I've looked at has had a Triode/Pentode swictch or control including a Vox AC15CC I looked at.

    Here is an interesting example schematic I looked at that has a EF86 in V1 and very similar tube configuration as yours but the two 6V6's are running in parallel instead of Push/Pull. I never have done that but always wondered what it sounds and operates like. But this might give you a good idea of EF86 hookup for guitar including the type input jack to tube you might want to install. Platefire
    EF86 Pre Amp.jpg
     
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  5. RoscoeElegante

    RoscoeElegante Friend of Leo's

    Feb 19, 2015
    TooFarFromCanada
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  6. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    ArcticWhite

    Take a look at the old AC15 schematic(attached pdf). I think this is the simplest Input/pre-amp set up for your EF86 that also has plate voltages that gives you a good idea of DCV to shoot for. I personally would just have one input jack and use a 33K grid resistor. I would never use that second input, but that's just me. To me the only advantage of having the extra input is if you want to hook a second amp in parallel, all you have to do is run a cord from the unused input to the other amp---that's kinda nice to have.

    Again this is just my thinking. Trying to be helpful. I realize not everybody approaches this kind of stuff like me:>) So whatever you decide, it will be good. Platefire
     

    Attached Files:

  7. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    That's extremely helpful Platefire. I really appreciate your interest and support.
    I'm really excited about using the EF86 in this Amp. It's a weird tube, supposed to be very nice, but highly microphonics and sensitive. More work planned tonight.!
     
  8. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    Hey Platefire, do you have a better copy of that Schematic, or maybe a link? It gets blurry when I zoom it.
    Thanks AW
     
  9. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    The best thing to do is right click on it on the thread schematic and do a "Save As" to your computer.
    Then I was able to enlarge it to a lot bigger size. It's still fuzzy but you can read it if you look close.

    I did some searches and turned up that schematic from where I originally found it. Here is the Link. Maybe you can save and manipulate it to what your want to see. Platefire
    http://www.tdpri.com/threads/any-schematic-for-ef86-pre-and-6v6-output.339799/

    See if this copy is any better!!
    EF86 Parallel 6V6 Single-Ended Amp.jpg
     
    Last edited: Jul 13, 2018
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  10. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Part of the reason it is known for having issues with microphonics is that it was setup for very high gain in the classic vox amps it was used in. If you read that thread Platefire linked to you'll see that this is discussed along with the Valvewizard circuit values for a more "appropriate" level of gain that reduces these problems.

    Check out my "AC 4 inspired" build thread, and take a look at my schematic, it uses a 5879 pentode input in a very similar way to the EF86 in that Tubeswell schematic. The 5879 is a comparable tube to the EF86 (a good bit cheaper though) and I've used it, and liked it a lot, in one other build so far. I think you'll be happy you decided to try a pentode preamp.
     
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  11. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    ArcticWhite must be decided what he's going to do with it. Everybody throws all our information and ideas at a fellow then has to wade through it all and decided what's best for himself. I'm sure he will come up with a good direction. Platefire
     
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  12. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    I'm not sure which of the secondaries I should place the jumper across. This OT has a center tap which is grounded, and two wires which run to a very complex switch that I can not decipher.
    See the picture and schematic (slightly different model, but the OT is the same. Screenshot_20180725-233026_Dropbox.jpg 20180725_221555.jpg 20180725_221602.jpg
     
  13. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    I performed the test as Platefire directed above, and measured 6.8 VAC across the heaters.
    Tested the Green, Yellow, and G/Y striped center tap as follows:
    Green to Yellow is 82 VAC.
    Green to G/Y center tap is 167 VAC.
    Yellow to G/Y center tap is 171 VAC.
     
  14. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    666
    Apr 8, 2016
    Oakland, CA
    Nice! Could you confirm that I understood your earlier post and photo against this drawing, please? Both the photo of the chassis wiring and the schematic are very busy, and I'm not sure if this info fits with what's already known about the intercom amp.
    Screenshot_20180725-233026_Dropbox.jpg
    If this does accurately reflect the test you performed (known voltage across several different pairs of secondaries, measuring the transformed voltage across the primaries each time), then the calculation is as follows:

    Primary:secondary (GRN-YEL)
    82 : 6.8 VAC
    12 : 1 VAC (turns ratio) -- divide both sides by the secondary voltage to get 1 on the right side of the ratio
    144 : 1 ohms (impedance ratio) -- square both sides of the ratio

    Primary:secondary (GRN-GRN/YEL)
    167 : 6.8 VAC
    25 : 1 VAC

    625 : 1 ohms

    YEL-GRN/YEL is almost exactly the same. 625:1 means that if you attach an 8 ohm load the primary impedance would be 5000 ohms. If you attach a 16 ohm load the primary impedance would be 10k.
     
  15. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    Yes, thanks, your diagram shows my readings.
    I'm not sure (have no idea really) what to with your results though.
    I want to hook up an 8 ohm 25 watt vintage ALnico speaker.
    The OT has five secondary wires. I will leave the bottom two alone.
    Where should the upper three go?
    Here's the full schematic. V1 and its surroundings are different, but everything else is the same.
     

    Attached Files:

  16. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    666
    Apr 8, 2016
    Oakland, CA
    Well, here's a decision point where a couple more sets of eyes on this might be especially helpful, but FWIW if the map of the OT above is correct, then one option is run an 8 ohm load on the GRN and GRN/YEL secondaries and have a reflected primary impedance of 5k. Push-pull 6V6 power amps that come to mind have been built around a primary impedance of 6k6 (Deluxe Reverb) and 8k (tweed Deluxe? Princeton Reverb) and probably a number of impedances around that range. The 6V6 tube data sheet gives a few examples, one with medium plate voltages and a high primary impedance (315V, 8k5), one with a low plate voltage and low primary impedance (250V, 5k). None of the examples on the data sheet is made with 'modern' (ca. 1963) voltages of 400+VDC on the plates. How much difference in performance between 5k and 6k6, especially given that impedance is also frequency-dependent? I really don't have the background to say.

    Another option is to construct a 16 ohm load. 2x8R in series, or a single 16R speaker. Again w/ GRN and GRN/YEL. 10K primary impedance. Can't say if that's any better than the first option. Possibly worse.

    Another option of course is use another OT that takes your desired 8R speaker and reflects exactly 8k or 7k or 6k6, depending on the power amp style you are aiming to follow.
     
  17. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    I really appreciate this, but to me it sounds a bit like the grown-ups talking when I was eight and sitting at the kids table. I understand most of the words, but no idea what you're talking about..
    For phase one, I had hoped to use the amp in nearly in its original state, but with the OT connected to a single 8ohm speaker. Is that possible?
    Could you draw a diagram of where the secondary leads go in that scenario?
    Thanks!
     
  18. ArcticWhite

    ArcticWhite Tele-Meister

    291
    Apr 14, 2009
    Portland Oregon
    Can anyone sketch a schematic of how to connect the secondaries of the OT to an 8ohm speaker, and to ground?
     
  19. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    Sorry Deleted
     
    Last edited: Jul 27, 2018
  20. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    ArcticWhite

    I dug out my old Dukane Project file and looked at my Schematic. I scanned the part of the schematic
    that has the taps for the Output Transformer. It has all the taps labeled as where on your schematic I didn't see where any of your taps were labeled with any speaker Ohm, only numbered outlets. Your schematic also has no color coding of what wires are what?

    However if my OT schematic could be wired similar to yours as a guide, my 8 Ohm tap is the one to the extreme right labeled "Slate" color and does a dog leg down to the screw tap labeled "8" for 8 Ohms. Also on my schematic, see the tap with a ground symbol labeled "GRN"=Green that goes down to the tap screw labeled "Com" for Common or Ground. Using mine as a guide, if you could locate these two taps I think you could use them as your main speaker connections. You could just temporary cover the other taps with electrical tape and try it. Take a look as my scanned schematic and see what you think??? Platefire
    Dukane Scan.jpg
     
    Last edited: Jul 27, 2018
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