Mini Silverface Champ with One Tube Reverb?

Discussion in 'Shock Brother's DIY Amps' started by tvas22, Aug 23, 2019.

  1. sds1

    sds1 Tele-Afflicted

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    Well if you look at the Champ Micro Schematic on his page you'll see B+2 at 250V. Not sure what accounts for the discrepancy, he had a couple of revisions and it also matters whether you are measuring B+ voltage with or without tubes installed. We'd have to ask him. But no it won't vary case by case -- Ohm's Law cannot be argued with.

    More gain, more clean headroom.

    In the output section you will have less output power, less clean headroom compared to the higher voltages. In the reverb circuit you'll not be hitting the tank as hard and will have less reverb output signal.

    Just to be clear, nothing you are doing here is out of bounds, I wouldn't worry about it so long as your expectations are reasonable. That is to say, you've not cloned any of the 3 Champ designs we're referencing, it will not sound exactly like any of those amps -- your amp will have its own unique character.

    Since you've already selected your PT there's nothing that can be done about your B+ voltage, your B+2 to the preamp you have options though as discussed above. I'd recommend having the 3.3k and 33k dropping resistors on hand (or whichever you settle on) so you can A/B for yourself.

    Keep up the good work man.
     
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  2. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Ok, that’s great. Thanks again for all your replies, it’s been really helpful. I suppose I should start building it now and going from there!
     
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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Since Champs are single-ended amps there's not a lot of voltage sag going on anyway so adding a choke shouldn't make substantial difference to tone and playability. It should help eliminate ripple and hum. Smoke 'em if you got 'em.
     
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  4. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Thanks again for all your advice so far. I started buying parts for this build finally, and decided I didn’t want my old 5f1 build anymore. So I salvaged from that a Hammond 291AEX power transformer.

    I’m going to buy in bits over the next couple of months and I’ll keep this thread updated with progress.

    In the meantime, I had another few questions. I noticed that this isn’t really that far away from a Princeton reverb without the tremolo. I have seen the 291AEX listed for use in a PR build, and I’m going to use a Princeton chassis anyway, so I was wondering how much I would need to change the circuit to include 2 6V6s? I’m guessing I would need to add in an additional 12AX7 too, looking at the Princeton schematic.

    Aside from the added volume, how different would a self split 12BH7 sound compared to two 6V6s?
     
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  5. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Ok, so I have pretty much settled on going over to building a modified Princeton reverb - I’m going to keep the solid state rectifier and one tube reverb, get rid of the tremolo, keep the negative feedback and tone stack switches. It’s actually going to be almost the same as it was, but just with two 6V6s, and an extra 12ax7. I always wanted something Princeton-y and I was kind of half way there already...

    Anyway, parted out my old 5f1 for the power transformer. Going to try and sell off some other old bits and see where I end up. I’m trying to break even with this build - no money spent I haven’t earned by getting rid of something else.

    Started building the head today from a recycled rabbit house I built a few years ago. Living in a flat at the minute, so only hand tools and I’m not a wood worker - I think I can get good enough though.
     

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  6. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Made a little progress today, not fine work by any means but I can stand on it. On my last build I rounded the edges over and polished it up a little with a surform plane. I’ll do the same here - once the leatherette is on it will look fine I think.
     

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  7. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Going with cream leatherette and a brownface style faceplate I think.
     

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  8. FenderLover

    FenderLover Poster Extraordinaire

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    People love their finger joints, and so do I, but Matchless uses butt joint plywood and no one complains about them. Looks Good!
     
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  9. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Cheers. Plus, I figure it's a head so, it's not like the construction is going to affect the sound in any way... as long as it's not rattling about!
     
  10. King Fan

    King Fan Poster Extraordinaire

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    Just catching up; what a brilliant thread. Smart OP, smart contributors. Carry on gentlemen, you guys literally rock...
     
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  11. tubelectron

    tubelectron Tele-Afflicted

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    Here's my one tube spring reverb circuit :

    [​IMG]

    It works perfectly, but need a following preamp stage or a gain reserve able to compensate the loss of gain occuring on the 1M5 resistor : 5.05/0.651 = 7.75, that is to say a gain compensation of circa 8.

    -tbln
     
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  12. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Thanks for this - I have gone ahead with the one-tube reverb from here: https://www.tdpri.com/threads/drop-in-one-tube-reverb.957523/

    I have put this on to one of RobRob's layouts, which I have attached.

    I am having an issue with my transformer - it is labelled as a 291AEX, but there's no blue wires. Which puts it at odds with Hammond's data sheet, and I can't figure out how it was supposed to be wired.

    I contacted Hammond, but they were adamant it should have blue wires. I have sent them back pictures, so I will see what info they come back with. The one I have is identical to this one: https://nextgenguitars.ca/products/hammond-291aex-power-transformer-upgrade.html

    I've attached pictures here in case any one happens to know what's going on.
     

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  13. D'tar

    D'tar Friend of Leo's

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    *Please Note: Hammond products are subject to change without notice. The product you receive may vary slightly from the product pictured, including the associated specifications.

    Nice right. So you are short a couple options on the primary windings. IMO no big deal a couple less wires to wrangle. Your options are right on the label there. perhaps yours is an older version with less options.


    upload_2019-11-20_8-42-35.png

    120v operation

    black and brown would be joined together and connected to line/hot wire likely at the switch.

    blk/wht and brn/yel would be joined together and connected to your neutral wire

     
  14. tvas22

    tvas22 Tele-Meister Silver Supporter

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    My query was passed to a technician at Hammond who insisted all 291AEXs are all identical and have never changed... ah well!


    Thanks for this. The issue is that I want to use it for 240v rather than 120v, so I want to make sure I’m doing the right thing. It was already wired up and powering my champ but, like an idiot, I didn’t keep track of how it was wired as I ripped it out, because I figured I would be able to just look it up easily enough.

    It looks like I joined the black and white to the brown and yellow, then wired the black to the neutral (blue over here) and the brown to the live (brown over here). I found an old picture I took when I first build it, which seems to confirm this.
     

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  15. D'tar

    D'tar Friend of Leo's

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    240v

    Black to line/hot

    Blk/wht joined to brn/ylw

    Brown to neutral
     
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  16. King Fan

    King Fan Poster Extraordinaire

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    Yeah, who you gonna believe, Hammond? Or Hammond?


    Sent from my iPad using Tapatalk
     
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  17. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Great, thank you for that!

    I’m pretty certain I had the hot and neutral the other way round in the champ. Will that have made any difference though?

    Also, can I double check how you knew that just so I understand? I take it the 50/60hz label on the transformer tells you which wire to connect to hot. Why do you connect the black and white to the brown and yellow for 240v but not for 120v? What is actually going on to the incoming voltage inside the transformer? Is one way wiring something in parallel or series, that sort of thing?
     
  18. D'tar

    D'tar Friend of Leo's

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    Either way is fine I just always associate black with LINE/HOT


    windings are parallel

    windings in series
     
    Last edited: Nov 20, 2019
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  19. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Thanks, that makes completely sense and it has just clicked in my head how transformers work.
     
  20. tvas22

    tvas22 Tele-Meister Silver Supporter

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    OK, I added the transformer wiring in. Next week, I'm aiming to get the leatherette on the head and get that finished. Then I'm going to start buying parts and get moving.
     

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