Volkswagen Garagemaster! Preamp w/distortion. Gut shots :)

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars

Volkswagen Garagemaster! Preamp w/distortion. Gut shots :)

Postby flowmastergfunk » 26 Apr 2014, 22:37

Being a VW guy, I have been hoping to find a steal on one of the VW edition First Act guitars. Crappy as First Act might be, these guitars (that were included with the purchase of a new VW back in '06) were made to a different caliber. I wanted to get one just because of all the little VW emblems on it. I snagged one off craigslist that is like new, and the first thing I did was rip it apart.

I put a Seymour Duncan Screamin Demon in the bridge, and wanted to check out the onboard electronics while I was at it!

Here are a couple shots of the pre-amp/distortion circuit. I am not sure what kind of mods can be done to it yet, but I may end up taking it out at some point to experiment with on the work bench.

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and a picture of the guitar for good measure. I added the Rabbit placard ;)

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I found no schematics for the circuit, so I will likely draw one up eventually. It would be cool to find out if this was a clone of a legitimate distortion circuit and see what kind of mods could be applied to it! Maybe it has a built in ts808!

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darthoverdrive (29 Apr 2014, 13:40), Greenmachine (19 May 2014, 21:54), pinkjimiphoton (28 Mar 2015, 04:21)
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby flowmastergfunk » 19 May 2014, 20:49

I took my guitar back apart to polish the frets, sand and repaint the headstock plain white (For me, the important part is the assorted VW regalia, I can do without the First Act logo ;) ) and install a set of Sperzel tuners. While I had it back apart, I decided to remove the preamp as well. I am hoping to get another Duncan for the neck position within the week, and I will likely put the electronics back together with a passive 2-pot tone and volume setup, with the addition of a phase switch. That will give me a chance to setup the preamp on the bench and run it as a "pedal" so I can experiment with modifications and while keeping the guitar functional.

It will be a bit of a backburner project, so I probably won't have many updates too soon. However, I wrote down markings and measures from the components on the board. I will work on drawing an actual schematic. I had already contacted First Act, and they have no such information.

I am surprised how many views this thread already has, so I hope this helps and/or captivates further interest.


Tone and volume pot's are A500k. Master pot it A10k

u1 XR65RC LM6132 BIN

r1 47ohm 5% 5-watt
r2 1k
r3 10k
r4 10k
r5 10k
r6 10k
r7 10k
r8 100k
r9 120k
r10 470k
r11 100k
r12 3.3k
r13 10k
r14 2.2k
r15 4.7k
r16 1k
r17 100k
r18 1.5k
r19 15k
r20 10k

c1 f66 153j
c2 f66 103j
c3 f63 332j
c4 6f2 332j
c5 56
c6 L63 334
c7 L67 104
c8 f66 223j
c9 10uf 25v
c10 1uf 50v
c11 100uf 16v
c12 100uf 16v
c13 10uf 25v
c14 10uf 25v
c15 f65

I can't really read any markings that are on the diodes, but there are 5 in the circuit plus the LED(blue)

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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby flowmastergfunk » 26 May 2014, 23:13

I finished setting up the guitar as a passive instrument, and it is much more usable for my style. I replaced the neck pickup with a Seymour Duncan Whole Lotta Humbucker, added a more traditional Gibson style twin-volume twin-tone setup, used push/pull phase switching for the bridge volume, wired the neck pickup through a parallel/split/series mini toggle, and added a normally closed momentary kill switch.

The neck pickup was installed "upside down" as a tribute to one of my absolute favorite guitarists, and likewise it is wired so that when you pull the volume knob OUT, it puts it "back into phase". Out of phase is the new norm :)

I sincerely doubt this guitar will ever have the stock preamp installed again, but that won't stop me from messing with it ;)

I just added holes to the existing pickguard, so I ended up just putting electrical tape over where the slider switches were. Still looks bad, but better than them being wide open.

Also, changing to Sperzel tuners was a CRUCIAL improvement to this guitar! Of course, they make changing strings a breeze...but it is also much easier to get in tune, stays in tune, and you can instantly feel the difference in resonance through your fingers. Basswood body or not, this guitar sings!

Moar pictures :)

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No, I didn't bother to fill the screwholes from the old tuners. I will do that if I ever decide to reshape the neck.

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Note: I made the custom truss rod cover out of a real Wolfsburg fender emblem. The Rabbit badge was the one that came on my '79 mk1 :)

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Another simple modification that I made was the re positioning of the upper strap button. Made the balance of the guitar much more comfortable, and it does not get in the way at all.

Image
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby loosecanon » 27 May 2014, 10:39

That is a cool looking guitar, I like it. Nice job on the mods. I think it's good you ripped out all that crap that was in there. It might be worth tracing out, but then again it might be worth tossing it in the rubbish bin. We wont know until it's done i guess.
There is nothing wrong with basswood, it has great sustain.
Keep on rockin
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby vulture » 24 Jun 2014, 01:48

Excellent. Thanks for the gut shots. I just picked up one of these at a local goodwill yesterday for the unheard price of $7, gig bag included :shock:

Anymore attempts with fiddling around with circuitry? Not a whole lot out there regarding mods for this. I'm banking that switching out one of those diodes that are in series may play around with the clipping?

I'll dig around in it myself and see what comes of this shockingly surprising nice guitar.
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby vulture » 26 Jun 2014, 05:47

Update. Diodes D1-D4 are 1N1418's
I played around with them and discovered that by pulling D4 it will increase the bass, but also you'll get some funky clipping when vol. is max. Try messing around with some other diodes here. To cut down on the excessive gain levels I pulled the (R10) 470k resistor and swapped in a 500k pot to fudge with ratings. I dialed in a nice 270-300k rating. Now the vol. max's out nicely.
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby flowmastergfunk » 07 Jul 2014, 20:52

I am stoked this thread has come in handy to others and there is tinkering being had! I have been majorly distracted with a plethora of pedal projects and this has been on the far back burner for me. I play this guitar every day, but never looked back after pulling the preamp!

Edit* wait....seven dollars????? I thought I scored! Congrats!
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby astrobass » 09 Jul 2014, 13:33

vulture wrote:Update. Diodes D1-D4 are 1N1418's
I played around with them and discovered that by pulling D4 it will increase the bass, but also you'll get some funky clipping when vol. is max. Try messing around with some other diodes here. To cut down on the excessive gain levels I pulled the (R10) 470k resistor and swapped in a 500k pot to fudge with ratings. I dialed in a nice 270-300k rating. Now the vol. max's out nicely.

Well yeah, D3+D4 and D1+D2 form the clipping diodes in the negative feedback loop of that op amp. They're in series. If you remove any of the four without inserting a jumper, you're disabling the clipping on one side of the wave or the other, or both if you remove one or more from both pairs.

When you do this, the peak on that side of the wave is going to go from approx 1.4V to whatever the max swing is in that circuit. I don't see a charge pump so that's probably centre biased with a 9V Vcc rail, so potentially 4.5V peak though I doubt it's a rail-to-rail op amp so probably closer to 4V. That's still like 3 times higher though, and any time you have a large volume increase you're going to perceive an increase in bass more than anything.

And yeah, the clipping you get from allowing the op amp to go to rails is going to be a bit odd vs the relatively smooth clipping those diodes provide. Instead of removing D4 entirely, why not replace it with just a larger threshold diode? LEDs sound great in negative feedback loops, and that'll give you a typically threshold of 1.6V vs the 0.7V those silicon diodes are giving, for a total of 2.3V on one side and 1.4V on the other and none of that funky op amp rail clipping. You could even go to just four LEDs for a threshold of 3.2V or so on each side, which will make this thing extremely loud with much milder clipping (more of a clean tone) and still no weird rail clipping.

You could also socket that op amp and try others, though I'd stick to low current op amps if I were you. Not a good place to use a 5532.
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby astrobass » 09 Jul 2014, 14:14

Oh and going the other way, you could remove and jumper D2&D4, and replace D1 with a 1N34a diode. You'll get less overall volume out of the distortion but way more compression. You could even do both D3 and D1 with 1N34a diodes and your output will be limited to around the level of an undistorted guitar, but it will be very heavily clipped and with the softer activation of the germanium diodes, should sound like a bit like a fuzz.

Or you could do any of the other fun diode tricks. D1 could be a 4k7 resistor, D2 a 1N914, D3 a 1N34a, D4 a jumper. Or an LED. Or anything. There's lots of space there, it's all through-hole, and you could even swap the pot for one with a DPDT pull switch. Then you connect leads from the outer pads of D1 and D3 to the middle switch lugs and run diodes from the outer switch lugs to the outer pads of D2 and D4. Pulling the pot out then allows you to switch clipping diodes. You could go from super loud low compression to super quiet heavy compression.

Or if you're sneaky, connect the leads from D1&D3 to the same middle lug and use the other switch pole to change the voltage divider resistor to compensate so that when you go to the higher threshold diodes, you increase the resistor's value and accordingly decrease the gain factor simultaneously so that the leap in volume is less pronounced.

But there are like a million things you could do with that board. You could use pull switches to add filtering to the negative feedback loop so that you get a bass boost or treble boost on pull.
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Re: Volkswagen Garagemaster! Preamp w/distortion. Gut shots

Postby McConnor » 29 Feb 2016, 22:24

Hi There,
thanks for posting your thread about the VW Garagemaster Guitar.
I´m looking for schematics of the circuits built in this guitar. You wrote in your thread that you might draw those.
My son won one of those guitars in a VW sponsored raffle in his school in Washington DC in 2011.
Now the pickup selector switch starts acting up (so I think). I had I guitar tech at a shop in Bonn/Germany have a look at it.
Since this guitar was never sold outside the US he cleaned the switch but the circuitry of this guitar he hadn´t seen before.
I´d like to get hands on the schematics and got in contact with First Act but they said "We sadly do not have the schematics for that guitar available to send out."
So I´m at a loss here. :?

Best Regards

McConnor
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