68 tele with a 1-off mystery fuzz circuit!

[ghostsauce]

Cool. The only two wires I didn't describe was the two red wires, and they just break the + connection to the battery via a switch. I guess they didn't think to use the output jack to disconnect the battery.

[mth5044]

Hang on, missed a part. Alright, fixed.

[mth5044]

The 100k is a problem, unless you are using a multiturn trimmer.

I simulated the schematic. As you go past the maximum of the trimmer that's supposed to be there, you continue to lose voltage/volume. At 5k, you get equal voltage swing compared to the input. After that, say 10k, you have already lost have the voltage swing. That's only 10% of your trimmer. With the normal trimmers, it's very hard to get the kind of accuracy that you'd need to keep in in the 2k range. If you have a pot of smaller value, 5k even, use that instead and put it on the outside of the enclosure.

[ghostsauce]

Aha, ok.. figures.  I do have a 2k here that I'll try. I just grabbed the wrong one by mistake when installing it, haha. I didn't think it was an issue... I'm gonna go try that now.

The only other tweak I made was to make the tone knob switchable by using a DPDT to simultaneously break the connection between C1 and Q1, and also C3 and R3. So when it is engaged it gives you the whole spectrum, which is pretty much just a standard boost, except it's still quiet because of the issue I'm having. I think it'll be nice to have the extra functionality when this is sorted, though.

[ghostsauce]

Nope, I tried the 2K but it is even quieter now.

The original trimpot was a Helitrim, and had the markings 77PR2K. I assumed this is a 2K trimpot.. maybe it's more? I couldn't find a designation by googling.

[mth5044]

Looking at eBay listings of Helitrim pots, the last number is indeed the value. You tried throughout the sweep of the trimmer? Got pics of your board? Or voltages?

[ghostsauce]

Ok, here's some pics of my build. I socketed C6 and used two .02uf caps in series to give me .01uf.  It also has the tone (stack?) bypass switch ike I mentioned above, which breaks connection between C1 and Q1, and also C3 and R3.

http://www.ghostsauce.net/random/pics/HC3.JPG
http://www.ghostsauce.net/random/pics/HC4.JPG

I can't post voltages tonight, but I can reserve some time for that tomorrow.

[ghostsauce]

Yeah, idk. The more I think about it the more I wonder if it has something to do with the gain of the transistor.

[mth5044]

Throw a 2N5088 in there.

[ghostsauce]

x

[ghostsauce]

Got it working. Two of the trannies legs are twisted in the original.

BTW it doesn't work with 2n5088 now that I have it working. It stays perfectly clean

[ghostsauce]

Okay! Here's the original version that was mounted off a DPDT in dad's guitar, and a standalone pedal version. May your journey into gnarly fuzz be fortuitous!

Sound clip into a marshall class5. Amp set clean first, then dirty! --> http://www.ghostsauce.net/random/Dad's%2068%20FrankenTele%202.m4a

[ghostsauce]

I still don't really understand what is going on in this circuit though. It seems like the + voltage off R3 mixes with the signal as it passes between R5 and R6. I don't think that's normal.. or am I wrong? I've never really tried to pick apart a circuit like this before, but I'd love to know.

[stringsthings]

Thanks for posting the circuit.  I about to do a Mouser order and I'll add a couple of 2N2925's. 

As for the circuit, there are others who know a whole lot more about this,
but I believe the circuit is an NPN Common-Emitter Amplifier with Feedback Biasing.
This is a well known circuit in the guitar pedal world.

The voltage from the power supply thru R3 is a DC voltage.  The signal voltage is AC.  The AC signal "rides" on top of the DC voltage.
Output Cap C5 blocks the DC voltage from the circuit output so that we're left with the AC signal.

( Fellow Forum members, please feel free to add/correct information  )

   

[slashandburn]

This thing sounds amazing. I might try to breadboard it later and see if I can get anything from other trannies. Pretty certain I don't have any 2925's but I'm expecting some 2k trimmers in a delivery today, so if they arrive I'll take that as a sign that I was meant to do this. Any excuse really!

I wouldn't hold your breath, I don't expect to be able to get anything out of this without the specified parts, I'm probably not knowledgeable or skilled enough to really have any success where you haven't, especially considering you had no luck with a 5088.

I'll report back any unexpected victories and continue to follow this thread with intruige. Thanks for sharing this!

[ghostsauce]

Cool! Try messing with R5, C4 and C5. C4 and C5 are the input and output caps, so they should change bass response in and out.. and I have no idea what R5 is doing, so changing that might have some usefulness. A 500k trimpot could find some cool things there.

I socketed the transistor in mine, so I'm gonna toss some others in myself. I have an old germanium can staring at me right now that could be amazing.

[Hatredman]

Matt, I redrew your schemo a little bit, and split it into 2 parts. Did that because I'm a moron and wanted to understand how it worked, not because there was anything wrong with yours.

Gain stage:


"Baxandall-ish" eq circuit:


It is surprising (for me at least) that the designer chose this approach instead of the more common tone stack, even placing it properly across the transistor's feedback network.  This is kind of an engineer's decision - I guess the majority of guitarists-turned-into-electronic-hobbysts would go for a passive tone stack after the transistor. I, myself, love the Baxandall way of eq'ing things.

But, also, it is not a proper, text-book Baxandall (something like this, found somewhere else):


Anyway, as presented, Dan's dad circuit is a simple enough circuit to provide hours and hours of tinkering fun.

[ghostsauce]

That's really interesting!

As the story goes, the circuit was the result of tinkering by two students living on a very small budget, without much knowledge of how then modern day guitar effects manufacturers were doing these things. More of a 'read a couple books, tinker until you get something that sounds cool' kinda deal. No google back then!

[slashandburn]

Wow thanks for that! Delivery didn't show and I'd forgot both my BBs are occupied. This gives me some reading for the weekend.

I'm guessing then there is nothing overly special about that 2n2925 and that the unconventional tone stage is more responsible for the most of that sound then?   

Amazing what you can do with EQ and gain.

[ghostsauce]

When I removed the tone stage via a DPDT like I mentioned before, it was still fuzzy but yeah it sounded like a normal guitar signal fuzzed up. I liked it like that as well, but I couldn't get it to not buzz terribly when bypassed. I think if I use shielded wires and had it been inside the enclosure it would have fared better. I will be attempting that at some point before boxing it up.

I love the fatness of the tone stage though. In dad's words, trying to control the thing "is like a mouse trying to control an elephant on a leash".