Looking to replace hard to get Germanium Transistors with Silicon ones, is this possible please?

Thread Starter

specmaster

Joined Nov 14, 2017
14
I have these old Hacker Radios that use hard to locate decent Germanium transistors in the audio amplifiers and so I was wondering if it would be possible to replace with their Silicon equivalents. The ones of interest are T3, T4, T5 and T6, that's OC71, AC128, AC176 and AC128.

The supply rails are 18V.

I attach 2 schematics to show where these are used etc.
 

Attachments

MrChips

Joined Oct 2, 2009
19,914
You cannot replace germanium transistors with silicon transistors.
All voltages, operating conditions and components have to be redesigned.
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
You cannot replace germanium transistors with silicon transistors.
All voltages, operating conditions and components have to be redesigned.
I understand that this would be the case, but isn't it just a case of selecting the right transistor parameters to closely match the hfe & power rating of the old transistor and then altering the resistor network to provide the right voltages to it?
 

MrChips

Joined Oct 2, 2009
19,914
No. The operating bias point of a germanium transistor is different from that of a silicon transistor. There is not matching cross reference.
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
You could, however, replace the audio amplifier.

Bob
This is true, but I wanted to retain as much of the old authentic Hacker as is possible and also of course the space is very tight and I already have the amplifier and was thinking it might be just a case of altering the resistors etc to provide the right conditions and biasing for the new transistors that I thought would fit in the same locations on the PCB? Have I got this totally wrong then?
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
No. The operating bias point of a germanium transistor is different from that of a silicon transistor. There is not matching cross reference.
So what you are telling me is then that when suitable germanium transistors can no longer be located the radio and it is a British classic radio is useless?
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
You can still get germanium transistors on ebay.
Why do you want to replace the transistors?
I want to replace them because some of them only currently according to my transistor tester are only providing an hfe in single digits, OC71 hfe of 6, and the push-pull pair have massively different hfe figures which I always thought in the interests of decent sound need to be as near matched to each other as possible?
 

MrAl

Joined Jun 17, 2014
6,948
Hi,

There are some differences between the two types Ge and Si.

One of the significant differences is the characteristic base emitter voltage.
For Ge it is around 0.3v and for Si it is around 0.7v.
This means the bias point changes, but because the current gain of the Si is higher than Ge there is a chance that the bias might be ok. The best bet though is still to MEASURE the base voltage, emitter voltage, and collector voltage, then replace the transistor, then try to get at least the base voltage and collector voltage the same or maybe the emitter voltage and collector voltage. This of course requires some resistor changes.

Now as to the gain, the Si will be maybe 10 times higher than the Ge, so you may have to kill some gain with an extra load resistor so the voltage swing is about the same as with the Ge.

Obviously this will take a lot of work and if one of the transistors is already blown it will be impossible to measure the right voltages so you may have to refer to something like Sams Photofact if you can find one for that amplfiier/radio or whatever.

Another way is to do a simulation of the circuit with the Ge transistors, then replace one and try to get it to work the same way in simulation, noting the voltages again and the overall gain of the amplifier.
You have to pay attention to bias point and transistor gain, adjusting as needed.
Still some work but not impossible if you really are intent on doing this mod.

Because of the base emitter voltage difference there is a slight chance you will have to increase the power supply voltage a little.

Lastly, report back here so we can all see how it worked out :)
 

recklessrog

Joined May 23, 2013
985
Back in the day when silicon transistors started taking the place of gemanium, there was a generally accepted rule that doubling the value of the emitter resistor was way to obtain the correct bias conditions, (within limits) In a great number of cases with old radios and amplifiers etc. this worked. whilst not absolutely theoretically correct, it is worth trying. as most germanium transistors where Pnp, be sure to select the correct type of silicon replacements as some of those in your schematic are npn. substitute one at a time and check operation before going on to the next stage.
The OC71 is being used to set the bias for the output transistors with temperature compensation, is it mounted on a heatsink with them? If you change the output transitors to silicon, you will need to use a silicon transistor here to and re-set the bias to reduce crossover distortion.
I worked for a famous Hi.Fi amplifier company that produced a stereo 30 watt amplifier. The noise level of the pre-amp transistors was quite high so they re-called many of them and replaced the original germanium transistors with silicon and simply doubled the emitter resistors. After this, the noise level was very much lower.
 
Last edited:

Alec_t

Joined Sep 17, 2013
10,708
I don't know about the IF section, but my LTspice simulation shows that you could happily change all the GE transistors for silicon ones in the audio amplifier circuit with no necessity to change any resistors, the only adjustment needed being RV3 (and possibly RV4) to set the output stage bias and, by virtue of the DC feedback loops, the bias of the other stages. T5 and T6 should be reasonably well matched for gain, and T3 , as Reckless says, must be silicon if T5 and T6 are, since it is crucial to setting the standby current of the output stage.
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
Does it work as it is now?
Yes, 1 of them works but lacks volume and is distorted badly, the other one does now work but very quietly and distorted. I have a third identical set and it sounds beautiful. By taking the amp out of the good one and fitting into the other 2 sets, they all work using the good amp, so the problem is with the amps. All resistors, and caps have been checked and where required replaced now it is just down to the transistors.
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
Hi,

There are some differences between the two types Ge and Si.

One of the significant differences is the characteristic base emitter voltage.
For Ge it is around 0.3v and for Si it is around 0.7v.
This means the bias point changes, but because the current gain of the Si is higher than Ge there is a chance that the bias might be ok. The best bet though is still to MEASURE the base voltage, emitter voltage, and collector voltage, then replace the transistor, then try to get at least the base voltage and collector voltage the same or maybe the emitter voltage and collector voltage. This of course requires some resistor changes.

Now as to the gain, the Si will be maybe 10 times higher than the Ge, so you may have to kill some gain with an extra load resistor so the voltage swing is about the same as with the Ge.

Obviously this will take a lot of work and if one of the transistors is already blown it will be impossible to measure the right voltages so you may have to refer to something like Sams Photofact if you can find one for that amplfiier/radio or whatever.

Another way is to do a simulation of the circuit with the Ge transistors, then replace one and try to get it to work the same way in simulation, noting the voltages again and the overall gain of the amplifier.
You have to pay attention to bias point and transistor gain, adjusting as needed.
Still some work but not impossible if you really are intent on doing this mod.

Because of the base emitter voltage difference there is a slight chance you will have to increase the power supply voltage a little.

Lastly, report back here so we can all see how it worked out :)
Thanks for the input, I was going to trace the circuit out in Kicad or LTSpice and run a simulation when time permits. I already have the manufacturers own service manual for the radio and thats where the schematics came from. I try it out and come back.
 

Thread Starter

specmaster

Joined Nov 14, 2017
14
Back in the day when silicon transistors started taking the place of gemanium, there was a generally accepted rule that doubling the value of the emitter resistor was way to obtain the correct bias conditions, (within limits) In a great number of cases with old radios and amplifiers etc. this worked. whilst not absolutely theoretically correct, it is worth trying. as most germanium transistors where Pnp, be sure to select the correct type of silicon replacements as some of those in your schematic are npn. substitute one at a time and check operation before going on to the next stage.
The OC71 is being used to set the bias for the output transistors with temperature compensation, is it mounted on a heatsink with them? If you change the output transitors to silicon, you will need to use a silicon transistor here to and re-set the bias to reduce crossover distortion.
I worked for a famous Hi.Fi amplifier company that produced a stereo 30 watt amplifier. The noise level of the pre-amp transistors was quite high so they re-called many of them and replaced the original germanium transistors with silicon and simply doubled the emitter resistors. After this, the noise level was very much lower.
I don't know about the IF section, but my LTspice simulation shows that you could happily change all the GE transistors for silicon ones in the audio amplifier circuit with no necessity to change any resistors, the only adjustment needed being RV3 (and possibly RV4) to set the output stage bias and, by virtue of the DC feedback loops, the bias of the other stages. T5 and T6 should be reasonably well matched for gain, and T3 , as Reckless says, must be silicon if T5 and T6 are, since it is crucial to setting the standby current of the output stage.
Thanks for the good inputs, I'll try out the Si mod on the spare amp I have to see if it does indeed work OK or needs the emitter load resistors increasing in value. I'll have to sort out some suitable transistors and if need be modify the heat sinks to suit and report back.
 

Ylli

Joined Nov 13, 2015
877
Felt like playing around a bit. The given circuit actually responds fairly well to Si substitution. Needed to raise RV4 outside its adjustment range to 15K. RV3 adjusts at 8K/42K.

At an output level of 1 W, distortion at 1KHz is <1%. The circuit does have a significant peak around 80 Hz, and the tone control network kills the high frequency response, but that might be to compensate for the small built in speaker.
Capture.JPG
 

Attachments

When I received stuff, it did not ship from the US although they seem to have a California office. There was all sorts of shipping options.

7 and 2's look the same. I also have cataracts.

I got burnt with a uk shipping charge too, but not from this company.
 
Last edited:
Top