Tag Archives: mammoth electronics

AC128 Germanium FuzzFace – with ‘Fuller Mods’ – from DIY Effects

Over the last few weeks I’ve had the privilege to assist SLW over at DIY Effects on a new PCB offering. It’s a FuzzFace circuit that has many great features:

  • A nice quiet power supply section that offers positive and negative ground. You simply place a jumper where you need it to be. Firstly this means you can use NPN and PNP depending on your set of transistors. Secondly with the recommended transistor sockets, (and the jumper sockets I added) you can switch between silicon/germanium or NPN/PNP in about 30 seconds. (See the ‘B’, ‘V’ and ‘A’ holes on the partially assembled picture below).
  • It has options for a couple of caps that can help reduce hum and soften the distortion a little. These are part of the Fuller Mods.
  • It has options for the other Fuller Mods; namely the ‘Less ‘ and ‘More’ pots. ‘Less’ or what I decided to call ‘Load’ (even though SLW nor I can decide on what to call it) acts upon the input. The ‘More’ control plays with the voltage on the top of the second transistor, which in effect changes the bias. This will tweak the waveform distortion characteristics. In fact, SLW prefers to call this control “Bias”, because that’s exactly what it is. More on this later.
  • The PCB features a spot for an optional trimmer to aid in deciding where the ‘Bias’ pot ‘centers’. More on this later.
  • The PCB has been designed to use DIY Effects ‘stmp’ switch and ribbon combo. This makes the wiring of the off-board stomp 3PDT switch a 30 second job. We’ve all spent much longer than that cutting and stripping 9 wires to wire one up normally, so this is a godsend.
  • The other time-consuming (and error-prone) part of wiring a stomp box is the potentiometers. The DIY Effects fuzz PCB has holes setup so that you can use PCB style Alpha pots instead of the regular lug type. Again this significantly speeds up the assembly of the board. And it gives you a very sturdy board support within the enclosure, so no need for dodgy nylon supports anymore. Of course if you want to use wires, you still can.
  • The PCB is designed to fit neatly into a 125B sized enclosure.
Partial assembly showing the new pot mounting holes and the power jumpers

Partial assembly showing the new pot mounting holes and the power jumpers

Choices to make

There’s no shortage of voodoo going around about the FuzzFace circuit; mostly surrounding which transistors to use. Some say that germanium is the only way to go, but it seems that’s more an artifact of the times rather than through any real objective truth. Jimi Hendrix was using germanium FF boxes because that’s all there was at the time (probably). The likes of Eric Johnson (or any modern hipster indie guitarist playing a Mosrite) is probably using a silicon transistor based FF. The reason being, germanium transistors are unstable depending on temperature, and they vary widely in terms of gain and leakage from one to the next. The industry switched to silicon precisely to combat this problem. So this is yet another case of a manufacturer using whatever they had on had at the time, not because of some kind of mojo.

Standard silicon fuzz on the prototype board

Standard silicon fuzz on the prototype board

Despite all this superstition, which normally sends me running, I went for a pair of AC128 germanium transistors from Mammoth Electronics. A pair of germanium transistors can cost anywhere between $10 and $50 depending on how much you’re prepared to be fooled; either way they’re orders of magnitude more expensive than their silicon counterparts. So choice one: AC128 transistors. This means I must use positive ground when I wire everything up. Make a note.

I also chose to try all the Fuller Mods. This was mostly because I hadn’t ever used a Fuzz-face before, so was eager to try it in all it’s ancient and modern glory. As a nod toward what might be called the mediocre standard circuit, I did initially build a completely standard silicon fuzz. This proved encouraging, so I ploughed on. Geofex has a fantastic discussion on The Technology of the Fuzzface (including the Fuller Mods).

Decals

This is often the most stressful, but ultimately satisfying part. I was able to designing something with much greater confidence this time as I started with a drilling guide supplied by DIY Effects. It was PDF form, and layed out accurate to the millimeter. This meant not only did I have a perfect basis for my design itself, but I could design it over the spots for the holes that I would then use for a drilling guide. Adobe Illustrator was my software of choice. Inkscape wasn’t working out as you cannot zero the ruler arbitrarily, which irritated me to no end.

The design in Illustrator

The design in Illustrator

Assembly

Initially I made a huge mistake in ordering a 1590B enclosure instead of the correct 125B size. This mean I went ahead and drilled the holes and even sprayed the enclosure a lurid gold metallic. When I came to test the layout by offering up the board it didn’t fit…at all. But that’s OK. I will use the gold enclosure for an eventual silicon fuzz. 5 minutes on the Mammoth site and I had ordered the correct enclosure with a lovely Tiger Red sparkle finish.

The ill-fated gold enclosure next to the assembled Fuzz circuit

The ill-fated gold enclosure next to the assembled Fuzz circuit

NOTE: It’s hard to see from the above picture, but there are only 4 wires coming off this finished board; signal in/out, 9V in and ground. With the new ‘stmp’ board and SLW’s choice of potentiometer, this is all you have to then wire into the enclosure. It’s magical!

Biasing and Temperature

As mentioned in the preamble, the fully featured PCB has a Bias control, and a trimmer. The Bias control is able to swing the bias on Q2 across an approximate range of 3 volts.

The typical FF circuit is setup to have -4.5v (measured at point ‘T’ on the PCB). The Bias knob allows you to swing that up to as much as -5.0v. The trimmer will allow you to decide the start of this range of adjustment.

While tinkering with these two controls I happened upon a characteristic of germanium transistors. i.e. they are sensitive to temperature. Now after reading plenty of articles about FF circuits I knew this, but I didn’t really understand what it meant. I had imagined that it meant their “performance” in a circuit might be thrown off by say 50%; much like the way most reasonably priced passive components have tolerances. What I wasn’t expecting was what I observed first hand. When I was tweaking the trimmer to adjust the range of the bias I had the unit on its back with the bottom plate off, and my left hand held the red multimeter probe on lug T. My right hand held the screwdriver placed on the trimmer. My multimeter sat behind the unit so I was forced to lean in close to see it. When I started to make the adjustment the first time, the bias voltage leaped up to about -6.5v!!

This caused me to replace R2 with different values, but that made no difference. And indeed I was getting what amounted to random bias ranges with each change. At this point I became frustrated and stopped for a while. When tempers cooled I came back to it, and measured the bias again…it had dropped back to around -5v and I could move it back towards -4.5v with the trimmer. But then as I did just that it started to rise again. I checked for the seating of the transistors in their little sockets, but they seemed firm. Then it hit me…temperature! I placed a finger on top of the transistors and sure enough, the voltage went nuts. It was being affected by me breathing on it while I was making adjustments.

So at this point I returned the circuit to standard values, set the bias for -4.5v (with the ability to go up to -5.0v using the bias knob) and called it a day. A very instructive waste of time 🙂

The finished article

Once the decal had dried, and the clear coats had been sprayed, I waited 3-4 hours for the paint to dry. The last couple of pedals I finished myself I had used Krylon clear coat. It works well, but it sprays on very, very thick, and literally takes weeks to dry sufficiently. Until that point it is prone to finger prints and other such handling artifacts. For the Fuzz I tried Rustoleum clear gloss. It sprayed on about the same as the Krylon, although it was less forgiving and I had to be careful to keep it “wet” but not by too much. Also, due to my impatience I was spraying in high humidity, so it tended to go on cloudy (moisture in the air), but cleared soon after. But anyway, all turned out well, so here’s the finished pedal:

The finished pedal

The finished pedal

A quick demo MP3 file of the DIY Effects Fuzz Circuit:

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TS Overdrive – new enclosure & sound samples

Old enclosure ready for tear-down

Old enclosure ready for tear-down, next to the new one.

Why?

When I built my first Tubescreamer clone (built with a great circuit board from DIY Effects) I was very pleased but left with a few issues that at the time I could not be bothered to fix.

Firstly, the paint finish I used on the enclosure did not turn out very well. I used Rustoleum, and sprayed it on way too thick (because I was impatient…lesson learned). It looked cool, but it didn’t wear well.

Secondly, the DC power connector I originally bought for it turned out to be the wrong size. I only discovered this when I got hold of a power supply, and it didn’t fit. In fact, it was at this point that I tried to use what I thought was an adapter for a smaller power connector. Little did I know that although the adapter fit, it was actually a polarity inverter too; so I blew up 3 JC4558 chips in the process.

So I decided to try an enclosure from Mammoth Electronics, who can provide a painted an drilled enclosure for around $10. Ridiculously cheap.

Wires for the LED "off-board"

Wires for the LED "off-board"

Getting on with it

The rehousing process was very easy; mostly a case of taking the old one apart and carefully assembling it all back into the new enclosure. I did have to redo the LED as the DIY Effects PCB allows you to solder it directly to the board, and let it just stick through the enclosure. I couldn’t do that with the new enclosure and still have it line up with the hole, so I attached wires to the LED and then soldered those to the board. Nice and easy.

The shiny copper on the MOSFET clippers

The shiny copper on the MOSFET clippers

Clipping with MOSFETs

Months ago, after I built my OD2 which uses plain diodes for clipping, I decided to change this OD to use MOSFET clipping. SLW had mentioned it in the excellent PDF file that lays out instructions for building the pedal, and he rated it highly. I was able to purchase the parts easily, and after doing the necessary physical modification (i.e. cutting off most of the mounting lug) the mod was trivial. But what a result in terms of sound! You’ll hopefully hear in the MP3 file below that it has a wonderfully soft clip. It’s as if it has rounded edges. Like an overdriven Marshall, but without the harshness. It does definitely get harsh if you turn the tone all the way up, and responds very well to tone adjustments on the guitar itself. In the clip you’ll hear a variety of pickups and guitar tone control settings (and sadly a lot of repetitive playing).

The setup was:

  • My own build of an AX84 P1 Extreme, with a 6V6 for the output section.
  • Tokai Les Paul with a 1961 Gibson PAF in the neck, and a modern Seymour Duncan JB in the bridge.
  • 2×12 cab with Celestion G12T-75
  • SM57 microphone
  • Recorded in Logic on a Mac. And a touch of reverb in the master output channel.

Click here for the MP3

Almost done

Almost done

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Rehousing a Danelectro Coolcat Vibe

Why bother doing this at all?

Bad knob positioning on the Coolcat Vibe

Bad knob positioning on the Coolcat Vibe. Who thought of that?

Take a look at the picture on the right; that’s the back end of the stock unit. While this device sounds great once you get it dialed in, it’s almost impossible to interact with it while you are playing. While I’ve never thought about it before, it seems that I like to see where my controls are when I’m playing. Perhaps it is akin to driving a car in that even though you might be just cruising along, you still want to be able to glance down and see what speed you are doing. So when you’re tinkering with the vibe’s controls you just can’t see anything because it’s all hidden.

Secondly, the big switch isn’t quite perfect. I’ve become accustomed to the basic switch that every custom builder knows and loves. This one doesn’t seem to switch effectively unless the force you apply is perfectly in line with the axis of travel of the switch.

Thirdly, it’s a really ugly box.

Preparations

I knew I would have to replace a few parts here and there. Here’s what I bought:

  • Potentiometers. I saw from this post that I needed 2 x B50k pots and 1 x C 50k pot. I found 16mm Alphas at Mammoth Electronics, who I had been wanting to try since a recommendation from Shawn over at DIY Effects.
  • DC power connector. The existing one is square and I don’t have an square drill bits.
  • 1/4″ jacks. The existing ones are probably crappy PCB mounted ones.
  • An enclosure. You can get a painted and drilled enclosure from Mammoth for $10. At that price why would I ever do this myself? When I tried it for my first ever pedal build it was messy, time-consuming and ultimately I was not pleased with the results.

Coolcat Vibe - in pieces ready for rehousing

Coolcat Vibe - in pieces ready for rehousing

Disassembly

This part was easy as I could use normal tools, so it only took five minutes. It’s a shame really; this enclosure is really well made, and would probably stand up to a considerable amount of abuse. If only they had located the knobs in a useful place (and it didn’t look like a clam).

The coolest part was finding a ‘real’ 3PDT switch underneath all that cast zinc. It fit the pre-drilled hole in the new enclosure and it seemed I would not have to bother removing the small PCB that it was soldered onto.

Still functioning, sans enclosure.

Still functioning, sans enclosure.

At all stages in disassembly/assemblyI wanted to make sure I didn’t break anything, so I made the effort to plugin and power up the device.

Mounting inside new enclosure

Mounting inside new enclosure

Assembly

The first task was to isolate the power supply jack, and the input/output jacks. You can see that the jacks are PCB mounted, and there are thin shield wires going between this PCB and the switch PCB. I chose to use those wires, and simply solder them to the appropriate replacement parts. At this point I was able to mount the switch and the new signal jacks into the new enclosure, and test once more. So far, so good; I haven’t broken it yet.

One by one the pots get replaced 'off board'

One by one the pots get replaced 'off board'

The next stage was to replace the pots. I had a feeling this was going to be painful, and I was not wrong. Over recent months I’ve replaced many a component in my amplifiers, and other pedals, but I’ve never dealt with PCB mounted potentiometers. What a terrible, terrible nightmare. I fully understand that they are designed this way so that untrained monkeys can assemble the boards, and then they slap some molten solder underneath and its done. After all, I did buy this device for $40 online,so you can imagine that its out-of-factory price is probably closer to $5. But it makes repairs very hard indeed. I managed to cook one leg of the first pot, which meant the PCB trace lifted from the board. Nothing that super-glue couldn’t fix though. My technique of removal was using a solder-sucker. It wasn’t very efficient as the suckers nozzle is quite larger compared to these component legs, and it was hard to keep the sucker ‘focussed’ on the leg AND hold the soldering iron in place. I was more careful with the other 2, so they went more smoothly. And when I say ‘more careful’, I mean I desoldered as much as I could with the sucker and some wick, and then mechanically pulled the pots off the board while trying to keep their solder molten. I know, I know.

The tabs that hold the LDR/Bulb enclosure in place.

The tabs that hold the LDR/Bulb enclosure in place.

Potentially the most treacherous part was rewiring the LED. To get to the soldered legs I had to take off the little metal box that enclosed up the light bulb and LDRs (light dependent resistors). Here’s a great article all about the inner workings of a classic Univibe.

The bulb and LDRs in action!

The bulb and LDRs in action!

You can see in the picture to the left, that there are three metal tabs surrounding the blue LED. All I had to do was gently bend those perpendicular to the board, and the box came right off revealing the bulb. Of course I couldn’t resist powering it up again, just to see the bulb working. In the picture you can see that there are two solder points above the bulb; these are for the LED indicator. Once again I had to carefully de-solder these, so as not to de-laminate the traces. I then attached wires, and connected a bright blue LED in its place.

Enclosure finished

Once I’d mounted the LED in it’s little bezel, I was basically done. I had to make sure I put the metal bulb enclosure back in place, of course, but all that was left was to tighten up any mounting nuts, and close up the box. Here’s a shot of the newly rehoused Coolcat Vibe next to my also-recently-rehoused DIY Effects OD (fantastic Tubescreamer clone).

The finished item...so far

The finished item...so far

An idea

I suddenly had an idea to make this rehouse a little more interesting. In other words, I looked at my vibe and the DIY Effects OD, which I had also rehoused (the green one, duh), and thought they looked somewhat plain. I’m basically proficient with publishing/layout-tools, so I thought I would try and create a decal for the top of the units. I took some basic measurements, and setup grids and guidelines, and just kind of went for it. Sadly I cannot find my decal paper (must buy some more), but I did do a basic printout onto photo paper. This is a work in progress, so updates to follow when I do finally buy some decal paper. The real one would not have the yellow background obviously. I can turn that layer off when I print so that it’ll be transparent, but you get the idea.

A mock-up of the decal

A mock-up of the decal

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