Vox Pathfinder 15R becomes a proper Vox AC4

Discussion in 'Amps and Cabs' started by rmconner80, Feb 8, 2012.

  1. rmconner80

    rmconner80 Cantankerous Luddite Silver Supporting Member

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    A couple weeks ago I came home, parked my car on the street, and when I got out of the car saw this on the curb next to the garbage cans.

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    I decided to wait for dark to snag it, partly because I didn't want to make a ruckus, partly because I didn't want my neighbors thinking I was into digging through their garbage, and partly because I wanted to tempt fate and see if anybody else wanted it, or it was not intended as trash, or whatever. So by 9PM fate had dictated it was to be mine.

    Vox Pathfinder 15R. It worked but being a mad scientist as well as a tube freak I already had other plans as my thoughts drifted back to the small 250-0-250 VAC power transformer with two 6.3VAC secondaries sitting on my workbench and the 4 ohm new sensor champ OT I could easily recover from a hack build I hadn't touched in years... No solid state for me please.

    I didn't take any before shots but I did steal these off the web. It gives you an idea of what's inside this solid state Korean made beast. Lots of fancy circuit boards and ribbon cables and **** like that to confuse my simple mind.

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    Over about two weeks I gutted the amp and rebuilt it into a Vox AC4. I used this schematic formerly from the old firebottle ampage site and then later archived by Steve A on blueguitar:

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    In any case my goal was to keep costs low and get something funky out of the project. I used mostly parts that I had lying around including a 4 ohm Weber 8 (the stock Vox 8" speaker was 8 ohms), the aforementioned transformers, wire, caps, resistors, jacks. I did have to buy a few things like the 16/16uF cap, a few mallory 150s, etc and did splurge on a few items but all told I don't have much $ into the outcome.

    I also wanted to do a true point-to-point layout -no turret boards. I used the real Vox AC4 as a model (tube layout, ground buss)

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    ... but ultimately diverged in several places do to the smaller size of components manufactured today (reistors and caps), due to constraints of the chassis I was already working with, and for other reasons random to any individual / unique layout. It was challenging but rewarding. The key to success on this is to have a very detailed, drawn up plan (you'll go through many iterations) before you start and then to be flexible during wiring as you'll need to adapt to exploit opportunities you didn't expect while mitigating problems that pop up.

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    For the ground buss - an idea which was critical to being able to do a proper point to point layout - I grabbed a hank of about 2 feet of 12 AWG solid core copper wire at home depot. I carefully stripped off the PVC and cut it to length once I got it home. Then I mounted it to some old school turret strips, with ground on the right end. This stuff is a serious heat sink so I ended up using a 100W iron to get the ground solder points.

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    The real vintage AC4 circuit is nothing like the current vox production. It's a single EF86 pentode into the single ended EL84 with a EZ80 rectifier. My B+ is probably a bit low as the secondary on my PT is 250-0-250 instead of the 270-0-270 referenced on the schematic, but regardless the output is nothing like a champ or the current Vox AC4 buzz machine. The single EF86 just simply can not generate the gain of two triode stages in series like a Champ or a Princeton.

    The Vox circuit also used a low gain arrangement on the EF86 which made output relatively low and the tube not sound real clean. The screen resistor is a 5M6 on the schematic. In comparison, Matchless uses a 1M here for much higher screen voltages and more gain. While the Vox 5M6 value yielded a cool dark swampy spongy sound, I ended up splitting the difference and using a 2M screen resistor. Definitely more output, cleaner sound, but still provides a bit of grit. The trem effect is slightly less deep with the new arrangement as well, but still plenty strong.

    Other circuit divergences include a panel mount toggle instead of footswitch for the Tremelo. I think i ran out of .01uFs also so in the trem circuit I subbed a .022uF - this probably just slowed down the trem speed a bit at the lowest setting. I also added a bright cap (180pF - like Matchless) and a recylced panel mount 1A fuse holder just because that's how I roll.

    I had all the tubes except the rectifier. I slapped in an RCA 12AX7, RFT EF86, and a 80s Reflektor era 6N14EP I had laying around. I had to source the rectifier(s). I say rectifiers because I ended up using an EZ81 rectifier instead of the EZ80. The 81 provided about 7 VDC more plate voltage than the EZ80. Just trying to get that B+ up a bit higher. The EL84 is dissipating about 10.5 W.

    So howsit sound?

    It sounds good.

    I ordered a 10" Weber Blue pup that I am hoping to shoehorn into the cab. Based on my measurements it's going to be real close. If it fits, I'll need to carefully enlarge the speaker mounting hole in the baffle.
     
  2. photoguy

    photoguy Member

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    Everybody's gotta be somewhere.
  3. Scooter

    Scooter Member

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    Was that Weber in it when you find it in the trash?????????????
     
  4. pepi

    pepi Member

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    Central Indiana
    Look great !!! On word though ... NEVER wait until dark. Always some other trash collector lurking :rotflmao
     
  5. ac15er

    ac15er Member

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    Football, meat pies, kangaroos and foreign cars
    We have a council clean up atm.

    You have no idea what:

    1. People through out as junk.

    2. What other people pick up as their new found treasure!
     

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