Ge diode for a temperature stability of a Ge tranny

[yeeshkul]

Guys, I'd have one question: when we use a Ge diode between B and E of a Ge tranny to get it somehow temperature-stable -  do we pick any particular diode or just get any?

If i understand well then:

Temp goes up -> Ic(leak) goes up -> Ice goes up -> Hfe goes up
but also
Idiode(leak) goes up -> Ib goes down -> Ice goes down -> Hfe goes down

It looks like i should pick a diode with the similar leakage as the transistor's ... am i right? 

[mac]

If good Ge were easy to find and cheap, (not my problem ) instead of the diode I would put a Ge transistor of the same kind and with similar hfe and leakage as the problematic one.

[yeeshkul]

thanks Mac, thats a good one!

[mac]

One thing I never tried is a thermistor like in the fuzz face reissue...

[Meanderthal]

 Oh! Ya mean use another Ge transistor as the diode? Good idea!

[mac]

A second Ge as the stabilizing diode will behave exactly as the transistor causing the thermal runaway. But the question is if it is enough to keep thing stable. Test before soldering.

[yeeshkul]

oh yes definitely test it first - my atempt with ge diode went nowhere 

[Gus]

http://www.geocities.com/sunsetstrip/studio/2987/britface.html

[Meanderthal]

 Yes, that's a great article... but can ya just slap any old Ge diode in there, or should its leakage be close to the transistor for best results?

[zachomega]

The trusty RCA transistor manual suggests a diode over a thermsistor for stability.  They mention the tolerances of most thermsistors are way off and their ability to respond at the same rate as the transistor itself is quite limited. 

-Zach Omega

[GFR]

INTEGRATED ELECTRONICS
Analog and Digital Circuits and Systems
Jacob Millman and Christos C. Halkias
MccGraw-Hill Inc.

There's a whole chapter on this subject. The mod in the britface is the simplest one from this book (that's where I took it from).

I slapped the old Ge diode I had laying around and it worked for me, but I've only modded my own FF clone, so I may have been lucky. YMMV.

I think John Hollis posted a FF mod using a DC servo loop.

[mac]

Rock Face by John Hollis uses an IC, a led and a ldr on Q2s collector. Never tried it.