Rock face far from temperature stable

[markusw]

I've breadboarded it using a TL071 instead of the TL061. Furthermore, there are two AC128s with Q1; hfe of 70 and leakage around 300 µA and Q2: hfe 140 and about the same leakage.



The 220R resistor needed tweaking as predicted by LTSpice simulation. The simulations was also correct in that the circuit needs some time (around 1 min) to stabilise. However, it's far from temperature stable.

Since the Easy face does not suffer from this problem I added a second circuit to auto bias Q1. Again some resistor tweaking but then it was stable. At least as long as temp did'nt change. Not a real improvement though.  

One more observation: when I measure Q2 collector voltage while I play there is an increase in voltage (DC). When I quit playing the voltage remains changed for a while and then slowly turns back to 4,5 V.

Now my questions:

1. is the change in DC voltage I measure normal and is it actually DC I am measuring (I am using a true RMS DMM)? If this change is normal how could I avoid that after playing a while the bias circuit wants to rebias the transistors and when I stop playing bias voltage has drifted away?

2. Are there any issues using the TL071 (or TL072 in case both Qs have an auto biasing circuit) instead of the TL061

3. Is it possible, that the leakage current is too high?

[markusw]

bump  

[AL]

Germanium transistors are notoriously unstable. That's a common problem. If you look at Fuzz Central on the Germanium Fuzz Face page  

http://fuzzcentral.tripod.com/axisface.html

he has a fix for the temperature problem. This "fix" originally came from the Britface article located here

http://www.geocities.com/SunsetStrip/Studio/2987/britface.html  

and also linked on the FC page. Hope this helps.

AL

[markusw]

Somewhere in the archives I read that the germanium diode trick does'nt work that great either. Thanks anyway.  
I'm gonna try it as soon as I get hold the diodes.

[Mark Hammer]

You can address temperature-sensitivity issues by using a design that compensates electronically for temperature changes, but you can also address it by finding a way to keep temperature stable, whether by keeping it artificially high or by keeping it artifically low, and effectively factoring temperature out.  Heck, if I've understood tubes even just a little, I've understood that they are designed to incorporate keeping the temperature high as an aspect of the design.  

I don't understand why the temperature stability thing in this can't be addressed with suitable heatsinking.  After all, how hot can it get on the floor in a box with a battery-operated circuit?  Or is there something about Ge transistors that precludes this?

[Aharon]

I don't understand why the temperature stability thing in this can't be addressed with suitable heatsinking.  After all, how hot can it get on the floor in a box with a battery-operated circuit?  Or is there something about Ge transistors that precludes this?

I think the temperature problem is more related to room temp than circuit temp.
From my own observations:
You go to a club and set up,little sound check and everything sounds the way you want,including your FF.
Half hour later there's 100 people in the room,lights are on, you played 5 songs and stomp on your FF and sounds like shitze.
I'm not saying there's no other cause but let's say we want to eliminate THIS one.
I remember Joe Gagan years ago (and forgive me if it was someone else) was toying with the idea of surrounding the transistor in styrofoam,I guess sort of like a hat.
I tried the diode trick and it worked for me.
Also having an enclosure that isolates the circuit from the environment could help,plastic instead of metal etc.
What about adding a 10 turn pot for biasing and making it accessible so you could tweak as you go?.

Aharon

[Fret Wire]

Somewhere in the archives I read that the germanium diode trick does'nt work that great either. Thanks anyway.  
I'm gonna try it as soon as I get hold the diodes.

The diode trick to stablize temp bias problems does work pretty well to an extent. Maybe you're thinking of the other diode trick with Ge's that lowers the gain. That's also in the Brit face article. That "trick" is iffy, sometimes it works, sometimes not.

[markusw]

Thanks guys.

You go to a club and set up,little sound check and everything sounds the way you want,including your FF.
Half hour later there's 100 people in the room,lights are on, you played 5 songs and stomp on your FF and sounds like shitze.

That`s exactly the reason why I liked the idea of a self biasing circuit.

What about adding a 10 turn pot for biasing and making it accessible so you could tweak as you go?.

I'd prefer to have it in a set-it-and-forget-it manner, set the bias to whatever voltage you like (from 4,5 V to velcro fuzz) and the temp compensation circuit keeping it stable.

I tried the diode trick and it worked for me.

The diode trick to stablize temp bias problems does work pretty well to an extent.

Which diodes did you use? Has any one tried to use another Ge tranny wired as a diode, maybe matched regarding hfe and leakage (although it's hard to put in another of those rather rare beasts)?

Any ideas why the rock face and the "double rockface" do not work that great??

[Ge_Whiz]

I think you've just identified why germanium transistors went out of fashion.

The problem is really in the packaging of the transistors, which leaves the transistor itself rather thermally isolated. High ambient temperatures will heat up the transistor metal can and slowly change the performance, making the transistor leakage current increase until it swamps the gain. Asking the transistor to pass a reasonable current in use has a similar effect. Either way, it's just plain difficult to keep the temperature stable. Insulating the transistor in styrofoam is likely to make matters worse as then the transistor would not be able to dissipate its self-generated heat at all.

Advice? (a) live with it or (b) adopt new technology (silicon).

[Fret Wire]

For the temp stablizing diodes, use a 1n34a or 1n270, Ge diode for a Ge tranny. It's not a complete fix, but it does raise the temp ceiling higher than where the fuzz would normally crap out.

[Aharon]

I think you've just identified why germanium transistors went out of fashion.

The problem is really in the packaging of the transistors, which leaves the transistor itself rather thermally isolated. High ambient temperatures will heat up the transistor metal can and slowly change the performance, making the transistor leakage current increase until it swamps the gain. Asking the transistor to pass a reasonable current in use has a similar effect. Either way, it's just plain difficult to keep the temperature stable. Insulating the transistor in styrofoam is likely to make matters worse as then the transistor would not be able to dissipate its self-generated heat at all.

Advice? (a) live with it or (b) adopt new technology (silicon).

That was my first reaction when I heard about the styrofoam hat but I don't think the problem is heat or cold(maybe I'm wrong here) but the temperature chage itself, so you'd want to keep it stable,either colder or hotter.
I doubt that a transistor serving duty in an FF would reach meltdown by self generated heat if isolated like that but who knows.
Aharon

[Ge_Whiz]

The problem is the transistor characteristics getting worse (less gain, more leakage) at higher-then-ambient temperatures. The transistor won't 'meltdown', but the heat generated in normal operation will increase the temperature further if it can't escape. In principle, this is a 'positive feedback' loop  - higher temperature means more current flows, higher current generates more heat, trapped heat leads to still higher temperature - and the transistor could go into thermal runaway. In practice, long before that happens you switch the pedal off because it sounds like crap. Try a silicon FF with the 'Miss Piggy' mod instead.

[markusw]

Thanks for your help.

I will try the Ge didode trick, maybe in combination with the Smiley indicator circuit.

Maybe I will also try a Miss Piggy. I recently built an EZ face derivative and actually prefered the sound compared to an all Si fuzz, even with more ore less gain matched Qs (70 and 140).

Hmm, about the rockface...let's see...

[markusw]

Me again :?

Are there any reason why the rockface biasing circuit should'nt work with a TL071 instead of a TL061?

Any helpful hints for a newbie?

Thanks,

Markus

[petemoore]

Wasn't it the 'Nitro Burner' design of Joe Gagan that had a VU meter, like a MPH indicator, and a knob to adjust the bias? The VU metere was used to indicate graphically the bias point of the Q and the knob was used to adjust it....IIRVC...been a while since I took a look.

[bioroids]

Yes I remember the Nitro Burner, that was very cool.

I used the "diode trick" on my Easy Face (has silicon on Q1) and never had to  touch the bias trimpot again. Anyway I didn't moved to the desert nor the pole , but the normal winter-summer variation does not have an effect on my FF anymore.

Luck

Miguel