Tonebender 3 knob temp stablising diode

[chromesphere]

Hey guys,
Made up a few fabbed boards of the tonebender 3 knob, sounds really nice and easy to get going!  I had a question about that temp stabilising diode.  Does it specifically need to be a 1n270?  What is the parameter that we are trying to aim for here?  Leakage (of the diode)?  I subbed it on my 2nd prototype and of course works fine but I haven't taken to q3 with a blow torch so not sure if replacing that 1n270 with an oa1160 is actually going to function as it should or not!
Cheers
Paul

[dondougie]

You can use any germanium diode, even a germanium transistor (B/E). It doesn't make a difference in this case. I've used 1N34A and some unspecified Smallbear germanium before, the results are the same.

[chromesphere]

Thanks Don!  So I guess the way to check its doing its job is to apply heat to q3 and check q3 vb / vc?

[dondougie]

I think that should work, but I wouldn't take it that far. The MKIII/IV circuit is the most forgiving and uncomplicated Tonebender variation, and sounds the same with a wide range of transistor gains/leakages. So as long as it works and sounds good, it should be ok.

Edit: One important thing that I forgot: In my experience, the MKIII sounds infinitely better with a battery (Zinc-carbon) when compared to a power supply.

[chromesphere]

Edit: that's interesting Don.  Why would you say the battery makes a difference?  Does a zinc carbon have a lower voltage (8.4v)?

That's the first thing I noticed too Don.  Messed around with different gains / leakages made very little difference to the sound of this circuit.  Its a perfect beginner tonebender imo!

[twin1965]

The zinc carbon battery debate has been going on for years! Some pedal makers recommend that you use these (on fuzz face type circuits) for better tone. I haven't experimented with this so I don't know whether it makes a difference or not. There are several theories around that try to explain why it makes a difference.

Sent from my GT-I9100 using Tapatalk 2

[madbean]

There is an A/B comparison b/w zinc and alkaline in a fuzz on the Analogman site. They do indeed sound different in the clips but it isn't exactly scientific, FWIW.

[midwayfair]

You have to apply heat to both the transistor and diode simultaneously. The diode has leakage, too, and you're protecting against environmental temperature changes, not just in the transistor.

Edit: that's interesting Don.  Why would you say the battery makes a difference?  Does a zinc carbon have a lower voltage (8.4v)?

Different batteries have different internal impedances.

Personally, I think just putting a resistor in series with a power supply makes it sound similar to running on a battery ...

[dondougie]

Edit: that's interesting Don.  Why would you say the battery makes a difference?  Does a zinc carbon have a lower voltage (8.4v)?

I don't know, I just read it somewhere, probably on Analogman, and they are 1€ for 2, so why not
What I do know is that they are pretty inconsistent, most of those 9V blocks have in fact 10,5V or more when they are fresh.

Compared to the ~10V battery, the tone with a regulated 9V supply and now three different MKIII types I built is significantly harsher and stiffer, and I can't dial in a warm sound at all. It also has some nasal mids at extreme gain settings, the battery is perfect with none of those problems.

[chromesphere]

Thanks for the info! So I guess you wouldn't want a really leaky ge diode, if there even is such a thing.  I don't think I'm going to worry about it too much though and just recommend "any ge diode" in the build doc for this one. 
Cheers
Paul