Thermistor protect Ge transistors against thermal runaway
Take care
Yeah I know that germanium transistors are really sensitive to heat. That is probably one of the reasons that they were phased out pretty quickly.
This Grundig amp is way more modern, better designed, less fussy, and what bwas all over the place in early home use "Transistor" amplifiers.
Grundig was a well respected brand.
They was beacause Big Business Companys care about quick easy high moneys profit
Musical art reproduction subtile quality for the peoples enjoyement are not on the top of his priorities..
But it does.
Notice 18 ohm in parallel with 68 ohm exactly at the bias circuit
ONE of them is a thermistor.
That is not an original circuit but copy of copy of copy, somewhere along the way that detail was lot.
Thermistors were *the* way to bias amps in the germanium era, I vaguely remember the Math used to calculate them.
You don't go bankrupt beacause music reproduced by amplifer ( or preamplifier) sound extraordinary pleasant
Contemporary class D amplification are bankrupt anyways, what technology "solutions" are next on the list ?
Well again is all about money profits and not the music , culture and artistic peoples expressions
Is a ""progress"" by quality of degeneration..down MP3
OK found this thing...
http://www.electronicstudio.net/schematics/audio_amp3.6w.jpg
It has a thermistor, uses bc108 in the driver stage and i should be able to replace ba103 With 1n4148 And it has decent size output cap.
Now how are thermistors marked ( in diagram it says K151 40 ohm) and how do i mount it on a heatsink.
I work with electronics quite a bit but i never needed to mount a thermistor to a heatsink...
Hopefully this works...
http://www.electronicstudio.net/schematics/audio_amp3.6w.jpg
It has a thermistor, uses bc108 in the driver stage and i should be able to replace ba103 With 1n4148 And it has decent size output cap.
Now how are thermistors marked ( in diagram it says K151 40 ohm) and how do i mount it on a heatsink.
I work with electronics quite a bit but i never needed to mount a thermistor to a heatsink...
Hopefully this works...
Regarding the volume control in post #1: The potentiometer definitely is a 1.3 Mohms one. Having survived the tube aera where it had it's roots, this value wasn't uncommon for a volume control potentiometer with loudness tap(s). I'm sure I have several stereo ones in my shelves that I once butchered from old tube radios.
Best regards!
Best regards!
IIRC the subjectively "bestest" way to drive a AD161/162 pair was to use a BC140 VAS (or a 160 if driven from V+ rail). And the input (singleton) transistor was either a BC179C/109C TO18 but a BC214/BC184 were also good. I built quite some of these back in 80s, there was something about these combos that made guitars sound really good, in face but not too upfront, and gelling well with bass (punk, new-wave era so "energy projection" was an important factor as well). If you can sacrifice some power and increase the emitter resistors to cca. 1R then there is less need to strictly use thermistors, a flat-pack diode like a BY235 could be afixed to the h/sink. Today a small TO126 BJT could be substitute instead.
OK, thanks. I will try to build something as soon as i get parts. Unfortunately my local supplier doesn't have half of the needed parts (caps and some resistors )
So I will probably need to order them from somewhere else and wait longer...
Thanks everyone. Now I have better understanding of these old amps and i have fun project.
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