Tone Bender MKII Bias & Temperature

[Gristlepig]

So I've recently built a couple MKII's. One with OC75 and another with CV7007. Mostly stock values, no trimmers.

With it being cold here in New England, and my basement mancave temp averaging at 63 degrees, what might I expect from these pedals as the weather warms?

I have these pedals' Q3 collector currents biased to about 6.5v assuming that the collector voltages will rise with an increase in temperature. Am I correct in this assumption? I'd ideally like Q3 to be between 7v and 8.5v at 70 degrees. Both pedals sound fantastic at 63 degrees, but as one is for a friend, I really don't want it to sound misbiased or fart out on him at higher temp.

[PRR]

> assuming that the collector voltages will rise with an increase in temperature. Am I correct in this assumption?

Move it closer to the wood-stove. (Yes, I am chilly in New England also.) How does it sound now?

If you ran out of wood (oil, propane) already, put it near an electric heater.

Remember how hot it was that one sunny Sunday last August? Don't get quite that hot, unless friend does a lot of summer noon gigs, then you might want to try "ow" (not "YOW!").

[Gristlepig]

I should be more clear. I did up each circuit on the breadboard, then built up each circuit on veroboard. I've actually not boxed them up yet.

I thought I had taken the cooler temps into account by biasing at 6.5v, but I am now unsure that the collector voltages will rise with increasing heat. And so, they remain unboxed. I suppose I could rig one up to a breadboard and bring it upstairs and see how it responds, but all of my breadboards are currently occupied with other projects.

I was just second guessing myself and wondering if I was correct in assuming that collector voltages would rise in warmer temperatures.

[Rob Strand]

I have these pedals' Q3 collector currents biased to about 6.5v assuming that the collector voltages will rise with an increase in temperature. Am I correct in this assumption? I'd ideally like Q3 to be between 7v and 8.5v at 70 degrees.

It could go up or down.  You will probably find it depends on which transistor is leaking more than the other and how much.

I thought I had taken the cooler temps into account by biasing at 6.5v, but I am now unsure that the collector voltages will rise with increasing heat. And so, they remain unboxed. I suppose I could rig one up to a breadboard and bring it upstairs and see how it responds, but all of my breadboards are currently occupied with other projects.

It's always best to trim in the middle temperature range.    Even then if you have a radical drift problem it still might be too far off when you go to the temperature extremes.   You can only keep control of things by selecting transistors with a leakage lower than some acceptable limit.

[antonis]

A hair dryer should prevent Ebers-Moll calculations & reverse leakage current measurements..

[Gristlepig]

Thank you Antonis! Hair dryer it is.

Will report back.

[mac]

In my experience, the first transistor masks the "FF" stage thermal issues, to some extent.

If it gets hotter, Q1 C goes down driving Q2 and Q3 harder.
At the same time Q2 goes a bit down. Q3 amplifies changes in Q2 leakage and hfe, so Q3 goes up fast.

With a 100k at Q2 C, chances are that Q3 C is close to the battery side.

A hair dryer should prevent Ebers-Moll calculations & reverse leakage current measurements..

Doh!
I didn't use a dryer, or mechanical statistics, just Kirchhoff.

https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=41446&g2_GALLERYSID=f8c91e4af1d11cc3d4a969872baf0023


mac

[Gristlepig]

30 seconds or so with the hair dryer on low brought q3 and q2’s collector voltages from 6.5v and .48v respectively up to 8.2v and down to .16v. Just right... exactly as Mac described.

With more heat the sound begins to gate, yet sounds great doing so. Very pleased.

Mac, thanks for posting that link. I’ll be wrapping my head around it all afternoon.