Vox AC4 based build

Discussion in 'Shock Brother's DIY Amps' started by tizianod, Jun 26, 2019.

  1. tizianod

    tizianod Tele-Meister

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    Hi fellow tube amp tinkerers,

    I want to get to know English amps better from a technical point of view. I currently own a Vox AC30 handwired model and a couple of years ago I had a Vox AC4TV. I liked the amp, but it lacked some personality and the low end was too party… I know very subjective ;) I was getting into building my own amps, so I sold it to free up the budget. So what better than just building one instead! :)

    A while ago I bought "A Service Engineer's Guide to the Vox AC30 Valve Amplifier” by Stephen Grosvenor. It has listed all original schematics, resistor and capacitor values. I also did some research and found out that Andyfromdenver, a member of this forum, built one a couple of years ago. Here is the thread he started. I don’t know if he’s still active here? Anyhow, his findings and knowledge of fellow members is very useful for my build. So I want to give credit to them as well!

    Basically my initial thought is to start from the original schematic, but implement some modifications from the start such as a 10” speaker, a single input jack and a depth control. I want to keep the circuit as simple as possible, so I won’t go too wild with extra features. I also chose to remove the combined on/off tone pot and add a separate power switch.

    Besides the drawings from Andyfromdenver, I couldn’t find a layout for this amp. So I decided to draw one for myself. It’s the first time I draw a layout myself, so so keep in mind beginner's mistakes. The layout is far from perfect as well… I’ll upload a draft version of the layout with the original values for the components. Of course the values can still change, it’s just theoretical right now. Also I’m not sure if the depth pot is in the correct position like this. All insights and tips on this build are appreciated!

    At the moment I don’t know what the deadline will be for building this amp. I need to fine tune the layout and schematic, source parts for this project, I have no idea what type of chassis and cabinet I will use... So have some patience with me :)

    The hyperlink to the original schematic

    diagram.png
     
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  2. tizianod

    tizianod Tele-Meister

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    I was going through the layout yesterday and I noticed a couple of mistakes...
    C1 should be 0.1 µF and the numbering for the resistors isn't correct. Because I skipped the second input jack, I don't have R2 so I should update the numbering of all resistors in the layout as well. I'll upload an updated version soon.
     
  3. tizianod

    tizianod Tele-Meister

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    Does somebody know a good schematic and layout builder? I know DIY Layout Creator, however the latest version doesn't seem to be compatible with the latest operating version of Apple.
     
  4. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Interesting looking project, should keep you busy for a while :)

    Tubeampdoctor, Musikding and Tubetown are a good source for parts, chassis etc if you can't find anything local.

    Keep us posted.
     
  5. Paul-T

    Paul-T Tele-Meister

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    I used Mouser and tube-town for a recent project, alongside local suppliers. But beware, Mouser add VAT to their UK prices after you've hit 'order' (which is essentially illegal), so factor that in.
     
  6. tvas22

    tvas22 Tele-Meister

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    You can run it on PlayOnMac (free software that lets you run windows programs) well enough - it's a little glitchy but it's not too bad
     
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  7. tizianod

    tizianod Tele-Meister

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    thanks for the tip. I'm going to check that out. Meanwhile I had the chance to update the layout.
     

    Attached Files:

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  8. tvas22

    tvas22 Tele-Meister

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    I’m going to be watching this closely, I have been hankering after something voxy for a while and was literally searching for an AC4 layout last week. Good luck with the build!
     
  9. tizianod

    tizianod Tele-Meister

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    @tvas22 same here... I was doing some research, but couldn't find plenty of information online. Hopefully this thread will be qualitative enough for future builders! I know for sure I'm going to make lots of mistakes. But that's part of the process for me.
     
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  10. corliss1

    corliss1 Friend of Leo's

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    Andyfromdenver had a falling out with some people here and is no longer active.

    Good luck with the build!
     
  11. tizianod

    tizianod Tele-Meister

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    Oh that's sad to hear! Anyway his thread was a huge inspiration. I first need to service a Tremolux from '64 that I bought a couple of months ago. The previous tech didn't do a great job, so the amp is in need of new electrolytics and probably other components as well. I'll try to combine both project in one order.
     
  12. tizianod

    tizianod Tele-Meister

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    Hi guys, sorry for the absence of updates on this thread... I've been busy for last months. I still have lots of amps to repair/service. I'm also stepping up my side business by moving to a dedicated workshop. But I picked up this project again. I set myself a deadline... The build should be ready by the end of the year.

    A close family member, still a high school student, has a school project and will build the cabinet for me. So I've been looking for wood lately. A while ago a found a picture of a Mesa Boogie amp and I really like how the amp looks. Unfortunately, the wood used is Myrtle and very expensive in Europe. I have a couple of European alternatives which should look more or less the same, but less expensive.

    Here is the original photo I found:

    myrtle.png

    Some other alternatives, Flamed amouk:

    amouk.png

    Or flamed walnut:

    walnut.png

    Next step is to design a, hopefully, beautiful cabinet for the amp. I'm still doubting if I should go for a combo or head version. I want to pay homage to the original design, but with some original twists. We'll see what we'll come up with. I'll keep you updated!
     
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  13. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  14. tizianod

    tizianod Tele-Meister

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    @Tom Kamphuys thanks for the tip! I didn't notice that Apple made a change in the software. I've got it up and running... Thanks!
     
  15. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Glad I could help.
     
  16. tizianod

    tizianod Tele-Meister

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    I made some sketches for the cabinet design. I'm not sure what style to go for... Maybe a small cabinet with a top or front panel? Or a piggy back style of cabinet?

    IMG_0940.jpg

    IMG_0941.jpg
     
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  17. tizianod

    tizianod Tele-Meister

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    Hi! Another update in this thread... I'm still looking into the design of the cabinet. I've decided I'm going for the dimensions of an AC4 adapted for a 12" speaker. But with a front panel design and a cut-out on top. So it's easy to reach to the controls. The wood will probably be flamed walnut. I've also requested an estimate for the wood planks.

    I almost have all the parts for the turret board assembly. So as soon as everything has arrived, I can start building the board. I don't have all components for the chassis yet, but I'm working on it :) Yesterday I was trying to find some transformers and I came across two manufactures: Classic Tone and Trans Amp. Both sell PT and OT for a Vox AC4 style amp. They both have their differences, so I'm a little bit confused...

    The PT from Trans Amp matches the specs from the original schematic. The one from Classic Tone has a slightly higher B+.
    The OT from Trans Amp has a much lower primary impedance and a match for power handling. The Classic Tone OT comes close to original specs, but with a much higher power handling.

    So for the PT I'm leaning towards the Trans Amp and for the OT towards the Classic Tone. It has larger power handling, which will easily handle the 12" speaker and hopefully the overall sound will benefit from it. Another option is to custom make the transformers. I know there is a shop somewhere in Italy who makes them. I've used a couple of them on other builds. But please educate me! I've read many articles about transformers, but the more I know about them, the more confused I get .

    So my next steps are:
    • Know which transformers to select...
    • Do the lay-out for the chassis. Measure out all components and try to fit everything into a British (Marshall) style chassis top mounted.
    • Finalize the cabinet design.

    Lot's of planning... It's my first complete custom build. Then I hopefully can start building...
     
  18. tizianod

    tizianod Tele-Meister

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    Finally I'm able to post another update in this thread... I went wood shopping this weekend! It was a two hour drive to the timber trader, but it was worth it. What a nice guy! I was a little bit too generous with the dimensions and the wood needs to be cut in half. Fortunately because I forgot to take the front and back into account :)

    Currently I'm also finalizing the hunt for a power and output transformer. I'll probably go for trans-amp or hammond. Hammond has a PT with 275 – 0 – 275 for the B+. That's slightly higher than the original specs, but I wonder if it would make a big difference?

    FC3C0CED-92ED-49D7-90D0-FCF80BA33CE5.jpg IMG_1129.jpg IMG_1128.jpg
     
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