Prescription Electronics COB Question

[pedalbob]

Hi everyone-

My friend let me borrow his original Prescription Electronics COB to trace and make a clone.  There is one copmponent on the PCB that I can't figure out what it is. It is a clear glass tube wrapped in some silver foil marked "50V" and has axial leads.  It is quite a bit bigger than a 1/4 watt resistor.  In the circuit still it measures about 100K ohms across it and with my diode tester it is about 1.57 volts in one direction and 0.76 volts in the other direction.  I've attached a picture.  I'll gladly post the schematic when I get it done.  I was also thinking about having a batch of boards made as the board is pretty simple .

[Mark Hammer]

It's a capacitor.  Chances are good it's the 1000pf cap between the base of Q3 and emitter of Q4.

[soggybag]

Styrene cap.

[Jarno]

"Styroflex"
Can be had in pretty tight tolerances too.

[amptramp]

This is a polystyrene capacitor.  The dielectric is the same material they make model airplanes from.  Styrene is also used for coffee cups.

Polystyrene capacitors are used where the designer wants a capacitance that will remain stable over a wide temperature range.  But the dielectric has a limited upper temp of 70°C, so you have to be careful soldering them.

[Rodgre]

By the way, the schematic has been traced many years ago, if you want to save yourself some tracing time.

Check out this thread: https://www.diystompboxes.com/smfforum/index.php?topic=129039.0

Roger

[Mark Hammer]

Such caps tend to be larger for any given capacitance value, relative to caps of other materials or construction.  That's why I figured it was the lowest/smallest cap value in the circuit - 1nf.

The COB is a decent circuit.  I've built a couple and been pleased with the result.  The name of the pedal tends to mislead, however.  It is not the octave that is clean.  Rather, the circuit allows one to blend in a boosted-but-unmodified signal, WITH the octave (which will be fuzzy).

[pedalbob]

 Thank-you so much for all of the replies.  As always I learn something from the experts when I post a question here.  Hoping I can reciprocate somehow eventually but I am still a humble neophyte at the DIY pedals ... but I became addicted as something to do during covid ...
Thanks for the schematic too !  I had found some tagboard/vero board layouts but none were exact looking at the real thing ... this will save me a bunch of time ...
Thanks again guys !!

PS. I did identify a couple of components that have slightly different values than the schematic online  ... one of the film capacitors is .047mfd not .1 mfd like the other 2.  Transistors in this one are actually MPSA05 which shouldn't matter vs. 2N3904.  If I find anything else I will post an update.

The PCB is old school single layer bare copper and almost looks hand drawn.  I definitely am going to build one and may make the PCB too.

Why would they use the styrene cap over a more conventional film or ceramic disk ?  Is it just a boutique mystique thing or will the tighter tolerance have an impact on that circuit ?   

[antonis]

If we're talking about C5 here: https://postlmg.cc/KRnkFwQd, it's just a NFB cap (it would have the same effect as if it was wired between Q3 Collector & Base) and, IMHO, it's tolerance is irelevant..

[ElectricDruid]

If we're talking about C5 here: https://postlmg.cc/KRnkFwQd, it's just a NFB cap (it would have the same effect as if it was wired between Q3 Collector & Base) and, IMHO, it's tolerance is irelevant..

+1 agree. A polystyrene cap here seems like expensive overkill. There's no need for super-tight tolerance, and from an audio point of view, any other (cheaper) film cap would sound just as good - polyester or polypropylene would be fine.

Polystyrene caps used to be the best for VCOs for the temperature stability, and good for VCFs for the tight tolerance. These days there are alternatives.

[Mark Hammer]

When it comes to any sort of fuzz, my view is that cap type is largely irrelevant.  I mean, the goal is NOT clarity.
Many "classic" fuzzes used whatever was cheap, available, and not too large.  In the case of things like the MXR Distortion+, cap choices could include tantalums, because they could more easily be placed on their side, to accommodate the limited space.

I have an original Shin-Ei FY-2, and it uses big old ugly ceramic discs.  Early EHX pedals used whatever Mike Matthews could get cheap in the NYC area, presumably surplus.

There ARE occasions/applications where cap material does matter.  For instance, where a voltage needs to be held, requiring low leakage, or where other properties of this or that type of cap can impact on phase relationships in wide bandwidth acoustic recording.  But when it comes to fuzz, nah.  Some may be noisier than others, but if one is aiming for fuzz, I'm sure there are more than enough other sources of noise.