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 Post subject: Replace germanium with silicon
PostPosted: Feb Sun 16, 2020 10:56 pm 
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Location: Palm Beach County, Florida
Is it possible or practical to replace germanium audio output transistors with silicon on 9 volt radios? I'm sure the bias resistors would need to be tweaked to raise the bias on the base. I have several radios and I just hate to pitch em but germanium replacements are getting more difficult to obtain at reasonable cost and current rating for output stage use. At this point I've almost gone thru all my spares.

Id like to save the radios in a workable if not original state.

Has anyone tried this?


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Sun 16, 2020 11:27 pm 
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Location: 18424 PA
Are you talking 9v am transistor radios? Japanese 2sb types? Or American 2n ?


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Sun 16, 2020 11:43 pm 
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Location: Palm Beach County, Florida
2SA 2SB types AM pocket radios


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Mon 17, 2020 12:20 am 
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Joined: Aug Wed 24, 2011 4:35 am
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Location: Sunnyvale CA
6gh8cowboy wrote:
2SA 2SB types AM pocket radios


Probably can, but why? Germanium transistors as used in radios are cheap and plentiful.

Brett


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Mon 17, 2020 1:54 am 
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I've replaced Germanium with Silicon many times on 6-8 Transistor receivers, but I'll usually have to temporarily solder in a socket with leads, and try several in order to get the best performance. Then adjusting the base bias resistors by temporarily patching in a volume pot, to peak them.

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Mon 17, 2020 2:41 am 
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Location: 18424 PA
You can get 2SB54, 2SB56 output types on ebay for about $1-2 ea. sometimes. There are also Russian germanium that would work. 2SA types would be more for RF.

Try a pair of 2n3906, cheap enough to experiment with.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Mon 17, 2020 7:01 pm 
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I've had to replace scarce NPN germanium audio outputs with silicon, but you can salvage the exact "2SB" PNP matched-pairs out of most cheap & less desirable pockets sets of the '60's & '70's. Even most of the audio amp PCB's of junked small tape-recorders including cassette types yield them. Definitely the easiest & best way to go since the transistors are already properly paired for P-P outputs...

John


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 1:45 am 
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Location: Gainesville, Florida
are there numbers on the germanium transistors?


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 3:02 am 
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You mean like this?

Image

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 5:45 am 
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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 2:28 pm 
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It appears that nobody has answered the question.

It is really an issue of understanding the differences between germanium and silicon transistors.

There are the the DC or standing bias differences, in that the B_E voltage of the germanium is about 0.25 to 0.3 V with normal bias conditions in signal stages, vs the 0.65V or thereabouts with Silicon. Therefore, if substituting a silicon for a germanium, the bias may need to be altered as pointed out already.

However, the differences at radio frequencies are very far reaching.

In the case of many vintage germanium transistors; the base to collector feedback capacitances were very high. This results in oscillation in IF stages as the base circuit and the collector circuit have the same resonant frequency and via the base collector capacitance, energy is exchanged between the two tuned circuits and the IF will oscillate. Unless there is anti phase feedback from a neutralisation capacitor.

So if you look at vintage transistor radios up to about the mid 1960's, there will always be a neutralisation capacitor and sometimes resistor too, around the IF stages to cancel the transistor's B-E (Miller) capacitance.

There are two issues related to the above.

Later generation germanium transistors, typified by the AF11x series in the UK and by the transistor like the 2N2084 in the USA, had such low B-E feedback capacitances, that they did not require the neutralization in the IF. This is also the case with Silicon transistors.

This means that if you are toying with the notion of replacing a germanium transistor in a radio's IF amplifier, with a Silicon part, (apart from adjusting the bias voltage to suit the silicon) you need to check if the IF has neutralization or not. If it does you have to disconnect it, or with the silicon part the IF will oscillate due to the feedback from the neutralization pathway itself, because is now cancelling nothing.

Also, the transition frequency (related to the transistor's high frequency response) of a silicon device is often orders of magnitude higher than a germanium device. On account of this, in various radio frequency amplifiers, silicon devices substituted in, can burst into parasitic RF oscillation when a silicon is substituted for a germanium. And that is not all;

Germanium devices because of their inherent lower transition frequencies and higher base to collector capacitance, when used in audio stages, are virtually intrinsically "RF proof". When silicon replacements are dropped into the same circuit topology, people wonder why the amplifiers burst into RF oscillation or suddenly start receiving every radio transmission in the area.

Another classic is the Royer Oscillator or DC:DC converter. Typically these use germanium power transistors like the 2N174, somewhat deaf at radio frequencies. Then somebody comes along and thinks they can make a circuit like this with the 2N3055, and then they wonder why it bursts into HF oscillation. (due to the leakage inductance of the transformer primary halves and the high transition frequency). After that somebody who knows why, suggests putting a 0.1uF capacitors from the collector to the base of the 2N3055 and the parasitic oscillations stop, hmm I wonder why.

This stuff is really very old hat.

All it takes to answer the question about if it is ok to substitute a silicon in for a germanium, is to study the properties of the transistors you plan to use, vs the ones in the original design, to anticipate and deal with the issues that the data dictates.

With regards to thermal stability, Silicon transistors are easily an order of magnitude better than germanium, so that is one issue you don't have to bother about, going in the germanium to silicon direction.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 6:21 pm 
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Location: 18424 PA
Try a pair of 2n3906, cheap enough to experiment with.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 9:30 pm 
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ACORNVALVE wrote:

All it takes to answer the question about if it is ok to substitute a silicon in for a germanium, is to study the properties of the transistors you plan to use, vs the ones in the original design, to anticipate and deal with the issues that the data dictates.

Not really.

Consider that this is a hobbyist site, and experimentation would be the name of the game. I've done numerous substitutions W/O studying any properties, and it's always worked out just peachy.

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Thu 20, 2020 10:25 pm 
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fifties wrote:
Consider that this is a hobbyist site, and experimentation would be the name of the game. I've done numerous substitutions W/O studying any properties, and it's always worked out just peachy.


Well it can a lot of the time, especially in signal stages and people find if you drop in a silicon it will work with no changes, but it does depend on the particular circuit configuration.

If you look at a receiver like the Eddystone EC10, it has no IF neutralisation, you can drop the silicons right in there, BUT the mixer for example, runs such a low base bias voltage in that set if you stick a silicon in there without altering it , the base-emitter junction is barely conducting.

Also, in the audio output stages, most of which in vintage transistor radios are push-pull, the base bias voltage is far too low in the germanium case to bias the silicon transistors out of cross over distortion, without modification.

So, a lot of this argument depends how picky you are about performance of a radio and if you fix radios for other people or restore them for yourself. Also whether you are content that something just "works" or "works properly". Some folks are more observant and critical of any defects or reduced performance in a set than others would be and this can create a wide range of opinions on what is peachy, or not.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 1:04 am 
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ACORNVALVE wrote:
fifties wrote:
Consider that this is a hobbyist site, and experimentation would be the name of the game. I've done numerous substitutions W/O studying any properties, and it's always worked out just peachy.


Well it can a lot of the time, especially in signal stages and people find if you drop in a silicon it will work with no changes, but it does depend on the particular circuit configuration.

If you look at a receiver like the Eddystone EC10, it has no IF neutralisation, you can drop the silicons right in there, BUT the mixer for example, runs such a low base bias voltage in that set if you stick a silicon in there without altering it , the base-emitter junction is barely conducting.

Also, in the audio output stages, most of which in vintage transistor radios are push-pull, the base bias voltage is far too low in the germanium case to bias the silicon transistors out of cross over distortion, without modification.

So, a lot of this argument depends how picky you are about performance of a radio and if you fix radios for other people or restore them for yourself. Also whether you are content that something just "works" or "works properly". Some folks are more observant and critical of any defects or reduced performance in a set than others would be and this can create a wide range of opinions on what is peachy, or not.

My first post above suggests peaking performance by adjusting the base bias voltage. The term, "just peachy" refers to the radio performing as should be expected by a stock model.

And BTW, it's enjoyable (at least to me) to experiment with different Transistors to see if I can improve performance, and then to adjust the bias resistance, once the best Transistor has been selected. I'm not dealing with medical or lab grade equipment, where your dogma might be relevant; these are consumer grade circuits.

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 2:21 am 
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fifties wrote:
I'm not dealing with medical or lab grade equipment, where your dogma might be relevant; these are consumer grade circuits.


Well I can see your point there about the grade of equipment. Though its not dogma, its Electronics Engineering and basic Science.

For example, if you are doing experiments it is helpful to know likely reason why the outcomes are as they are from your experiment. For example if you put a silicon transistor into a vintage set's IF stage (especially mid 1950's to mid 60's sets) where the common germanium transistors used had high feedback capacitances and the stages are neutralized, why the IF then malfunctions and bursts into oscillation, then know how to fix it by disconnecting the neutralization. Or say if a stage develops parasitic very high frequency oscillations with the silicon device, why that happened and then know what to do to fix that.

It is true there are people who have little interest in the science behind things involving the experiments or electronic work they do, but in the long run they are worse off not knowing the details and it reduces their efficiency as technicians and cramps the ability to design circuitry as well.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 4:21 am 
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ACORNVALVE wrote:

It is true there are people who have little interest in the science behind things involving the experiments or electronic work they do, but in the long run they are worse off not knowing the details and it reduces their efficiency as technicians and cramps the ability to design circuitry as well.

There are members who simply want to get their 60 year old Transistor radios working up to snuff, and have no intentions of becoming techs, or designing circuits.

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 7:51 am 
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Joined: Jan Thu 16, 2020 12:29 am
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fifties wrote:
There are members who simply want to get their 60 year old Transistor radios working up to snuff, and have no intentions of becoming techs, or designing circuits.


Yes, that might be true but "some members" is imprecise.

If you really want to get your vintage gear "up to snuff" you do in fact have to become a tech, to the extent that you are self accomplished & self trained at least (the certificate of training from any external agency like a college is unimportant, it is what you learn for yourself that is important).

You really still have to do the study and take an interest in the theory too, if you want to know what you are doing and do a good job on your work.

So there is no excuse for not taking an interest in the theory behind your vintage radio technology, that is if you profess to be a person who is a vintage radio restorer and has at least some knowledge of the art. For people who are not radio restorers, it might be an academic question and they will seek help from those who know.


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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 8:40 pm 
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ACORNVALVE wrote:
fifties wrote:
There are members who simply want to get their 60 year old Transistor radios working up to snuff, and have no intentions of becoming techs, or designing circuits.


Yes, that might be true but "some members" is imprecise.

Of course; I can't put a number on it, because members come and go all the time. But there are always people who just want to fix a radio or two, and aren't interested in technicalities. I've seen that many times over the years.

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 Post subject: Re: Replace germanium with silicon
PostPosted: Feb Fri 21, 2020 11:21 pm 
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Nice to see you posting Hugo. For those of you that do not know him Hugo is a great person and a long ways SW of us.

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