Replace Germanium Transistors with Silicon PNP ones!

[Teknow]

If you ever had a vintage gear where you needed to get it to work but needed a replacement for Germanium transistors especially in audio circuits and you wondered if simply replacing with  Silicon PNP transistors would work (with some tweaking and minor adjustments), the answer is yes.

I got a Siemens Club RK 231 vintage radio which had two dead audio output transistors AC141/ AC142. These are hard to get and I needed to get the radio working for somebody who didn't mind modifications, though I prefer vintage repair.

So I replaced them with BC327 for the AC142, and BC337 for the AC141. I had to add some cooling stuff and luckily there was a quiesent current adjustment trimmer.

I had another vintage russian VEF 204 radio, http://www.radiohistoria.sk/Oldradio/main.nsf/wcatalid/0002688. The two output PNP Germanium transistors (MP41)were also damaged. I just replaced them with two BC327, and had to fiddle a bit with biasing, and the audio amplifier worked just fine!

The third attemp was trying to repair the audio amplifier of a Siera vintage transistor radio. Unfortunately, the cover had lost all other information. I couldn't find a similar schematic I almost checked all semiconductors resistors capacitors, but all in vain. I gave up and the owner dindn't mind an LM386 instead! 

[Kleinstein]

It does not work in all cases, at least it may need a good understanding of the circuit so one known knows where to adjust the bias.

In RF circuit it can be even more tricky - getting fast silicon PNPs ( e.g. comparable to the old AF379 with 1.2 GHz Ft) can be tricky and than there are more parameters to match.

[Teknow]

It does not work in all cases, at least it may need a good understanding of the circuit so one known knows where to adjust the bias.

In RF circuit it can be even more tricky - getting fast silicon PNPs ( e.g. comparable to the old AF379 with 1.2 GHz Ft) can be tricky and than there are more parameters to match.

You are right in RF concern, but I pointed out and restricted the first attempts to audio circuits. Some knowledge and expertise are a must too. I remember I once replaced a hard to find PNP Russian RF transistor in a Selena B215 radio in the FM tuner with a BF324 and it worked just fine, they were of course both silicon ones.

[floobydust]

What is the tempco for germanium transistors?
I haven't seen VBE or hFE behaviour/data over temperature for germanium parts. For audio power output stages, common is something with a thermistor in the bias network.
I remember Ge parts do not show beta droop like silicon counterparts as Ic goes up.

[Teknow]

What is the tempco for germanium transistors?
I haven't seen VBE or hFE behaviour/data over temperature for germanium parts. For audio power output stages, common is something with a thermistor in the bias network.
I remember Ge parts do not show beta droop like silicon counterparts as Ic goes up.

Well, I know those differences of Vbe ~0.3 V Germanium, ~0.6 Silicon and the temperature things. But the topic is bringing a dead vintage radio to work when there are no vintage parts at hand.

Yesterday, I even found 3 shot RF transistors P422/ P423 in the radio VEF 204 itself, I just replaced them with 2N2907A (plastic TO92) and the radio worked alright! Unfortunately a lot of bands and too few stations!

though I have some European AF HF Germanium transistors I prefered to test those more available ones like 2N2907 and 2N3904 as their Ft 300MHz.  I think any npn BC556 etc would work.

[Kleinstein]

Cicuits with germanium transistors should be relatively tolerant with a variable hfe, as hfe tends to drift (often up) over time.
The thermal dift of VBE should be slightly smaller than for silicon transistors, as the same rule of thumb ( VBE extrapolates to the bandgap voltage at 0 K) should apply.

The reduced number of AM stations is more like a political thing - in Germany AM radio is essentially dead.

For the RF part more parameters, like the input impedance can become inportant - so a not so suitable replacement transistor may show poor impedance match and higher noise. The 2N2907 is relativel large for a RF input stage, at least for a new design one normally uses smaller types. The RF types usually have a smaller base to collector capacitance.

AF germanium transistor are often really slow and may not be suiteable for RF anymore.  Especially some of the germium power transitors were incredibly slow. Modern small Si transistors are also quite fast.

[joseph nicholas]

Why not just replace them with germanium.  I don't want to be a buzz kill here but they are still available new.

[Teknow]

Well, I know the differences between Germanium and Silicon transistors very well. It's not always easy to get vintage things everywhere. In some countries you have to buy through Ebay account, and consequently make a PayPal account, and finally you have to pay % tax + % customs etc. which makes things expensive! So, for this radio it's not worth it, as now I have to struggle with adjustments because someone had fiddled with all the coils before giving it up.