Ge to silicon osc replacement AF117
 

Can anyone recommend a silicon replacement for the AF117, that has a good chance to work as the "osc" transistor in a lw/mw Stella radio.

Regards Poppydog

Re: Ge to silicon osc replacement AF117
 

More difficult to sub the mixer/osc transistor because of its function. Why not use an AF127 which is a direct replacement germanium? No faffing about with bias trying to make something work.

Re: Ge to silicon osc replacement AF117
 

Cost is now a factor of mine. I would usually put in a Russian GT322B but am now getting very low on these, AF127's and other Ge transistors seem to have rocketed in price as has postage etc, silicon transistors are still fairly cheap. I would still like to have a go at getting one to work in there.

Regards, poppydog.

Re: Ge to silicon osc replacement AF117
 

I'd chuck a BC557 in it and see if it works myself.

Re: Ge to silicon osc replacement AF117
 

Recently done the same with BC557 in a Roberts R200 as a test and it works fine.

Re: Ge to silicon osc replacement AF117
 

Yes, try whatever you have available. There's nothing to lose - the worst that will happen is that it won't work.

Re: Ge to silicon osc replacement AF117
 

I've just looked on eBay, and there are 3 sellers (one in the U.K.) offering GT322B Transistors for around 30-50p each in packs of 10 or 12, so, for anyone who doesn't wish to go the silicon substitute route, these Germanium types are still available at pocket money prices.

Re: Ge to silicon osc replacement AF117
 

The postage is the deal killer I'm afraid Livewire on the ebay listings, there is a bloke that is selling some about 5mins from where I live , they always go for silly money.
I did do a little research on here but have yet to read about anyone using a silicon one for the osc. I will try a BC577 and see.

Regards, Poppydog.

Re: Ge to silicon osc replacement AF117
 

Are you looking at the right listing?...postage is only £1.20 and you're getting 10 transistors for £1.99.....

Re: Ge to silicon osc replacement AF117
 

It is actually more tricky than it looks, or than most suggest, if the transistor is used for a mixer function, or mixer oscillator transistor, rather than say just for an oscillator...that is if you want the mixer to work well.

Generally dropping in a silicon for a germanium can work, provided the base bias voltage with the existing resistor values and supply voltage is enough to establish a reasonable collector current, but it will still be lower (the collector current) unless the base to power supply resistor is lowered a little, because the B-E voltage drop is higher, about double for silicons vs germaniums.

However, for a typical single transistor mixer the situation is more involved. For a mixer to work well, ideally the transistor bias must be set in a low region where the relationship of base-emitter voltage results in a square law collector current.

For a germanium transistor mixer-oscillator application, using an an AF117 or similar, this voltage is surprising low at only about 200mV pp (varying between about zero and -200mV oscillator signal). Both the DC axis of the signal (its average value) and the signal amplitude of the oscillator is important to keep the transistor operating in the square law region. Otherwise there is less efficient multiplication of the oscillator signal with the received signal.

Figure 3 in this article shows the square law region of the B-E junction of a germanium transistor (taken from the Philips manual), and a photo of the B-E voltage in a mixer oscillator at the end of the article explaining transistor radio mixer-oscillators:

http://worldphaco.com/uploads/THE_PA..._FROM_1957.pdf

If you drop in a silicon transistor, for a germanium, in a mixer-oscillator , or mixer application, it will therefore definitely require adjustment (increasing the bias by lowering the bias resistor value) to the correct point for good mixing, if you want the mixer to work as well as it did with the germanium device.

While some folks might have observed dropping a silicon in place of a germanium into this location in a radio, or other stages in the radio, the radio "worked", this does not mean its working properly or as well as it could. The mixer stage is the one with a more critical bias than other stages in the RF or IF section of the set.

So I would go with the advice of using an AF127 or similar to replace the AF117, unless you don't mind altering the bias resistor.

Re: Ge to silicon osc replacement AF117
 

It's worth buying some cheap general purpose silicon transistors in both NPN and PNP variants. This will allow you to experiment and the results will usually be perfectly acceptable, even if not optimal. You could buy 100 2N3904s or 2N3906s from Chinese eBay sellers for 99p the last time I looked.

Re: Ge to silicon osc replacement AF117
 

Concerning the GT322B's on eBay, as I noted, one of the sellers is based in the U.K., so can supply them fairly quickly at modest cost. With the postage quoted by Sideband in post #9 added to the £1.99, each transistor is only going to cost 32p - a lot less that AF127's!

Re: Ge to silicon osc replacement AF117
 

As a mixer-oscillator, the thing has got to oscillate. And it's got to mix, introducing non-linearities so that the sum and difference frequencies are formed.

If it oscillates, the stage will be driven to non-linearity (else the amplitude would rise indefinitely). So it's guaranteed to work as a mixer. Whether it does so well enough is another matter.

Ge and Si have different base-emitter bias voltages, as Hugo Argus25 observes. It's likely that the bias conditions will not be optimum for silicon. No doubt the original designers spent some time determining the optimum conditions, over a reasonable production spread of AF117's.

However, I'd stick in a BC557 (as advised) or similar PNP transistor and play with the bias resistor. You may well find, that the best you can get with the Si device is as good as, or even better, than could have originally been had from the AF117. And you can have fun!

Re: Ge to silicon osc replacement AF117
 

Thanks for all the replies, I will experiment and let you know how I get on, by the way the Pacemaker Portable in Argus25s link is stunning:)

Regards Poppydog

Re: Ge to silicon osc replacement AF117
 

If you have no joy with Si Transistor in mixer position and you want an upgrade to the AF117,then take previous advice in OC171 replacement threads by Argus25 and fit AF178 which has no whisker problems it is similar size & appearance.
Maybe best to whisper this only once (as they will vanish otherwise) you can buy TWO from Langrex for £2.85 posted. As close as I can calculate this is £1.42 each and they are branded Mullard.

Re: Ge to silicon osc replacement AF117
 

Puppydog,
I have some AF239s. (I think I have subbed them for OC17x and AF11x before?)
You are welcome to one FOC to try if you PM me your address etc.
Alan

Re: Ge to silicon osc replacement AF117
 

AF11xs are actually much easier to sub than is often assumed. Most small signal Ge types will work perfectly well if the transition frequency is high enough. Domestic radio RF and IF stages aren't the most demanding of circuits.

Re: Ge to silicon osc replacement AF117
 

I would agree with Paul totally regarding RF / IF stages,taking his earlier advice to sub in BC557 and confirm they do work without issue.
Some circuits are as we know,less accepting of device type and do need bias changes around the Oscillator/Mixer stages.
The AF178 as mentioned may be worth stocking up on,my recent purchases of USSR Ge Transistors from varying sellers and countries show evidence of having been sorted and re-sorted with leads evidencing insertion repeatedly into testers,the result has been that I now have dozens of very low hfe devices compared to 5 years ago-probably as a result of the hungry Guitar effects customer base now existing.
Just a thought for those contemplating purchases from former Soviet bloc countries.

Re: Ge to silicon osc replacement AF117
 

PM sent
Regards Poppydog

Re: Ge to silicon osc replacement AF117
 

The trouble here, for the IF stages, is that there is a "no man's land" transistor, which is what a lot of the cheaper germanium ones appearing for sale on ebay are:

This is because, in early IF designs, with transistors that are typical of early types like the OC45, all the IF stages required neutralization to counter the high C-B feedback capacitance. On the other hand, when the OC171 and its later cousins got invented (AF117, AF127 etc), the C-B capacitance was so low that no neutralization for the IF was required, the R-C networks from base to base or similar, vanished from the IF circuits.

So if you are replacing a transistor in an IF circuit with neutralization, you have to pick a similar transistor with a high C-B capacitance and low transition frequency, or the IF will oscillate unless you change the feedback values. If you insert an OC171 (or equivalent AF127, AF178, etc) then you have to disconnect the R-C neutralization network, which is easier as you do not have to find the values for correct neutralization.

If you have a mid range transistor with an intermediate ft between say the OC45 and the AF127 and an intermediate value feedback capacitance, you are in "no man's land" and the IF will oscillate with an existing neutralization R-C network and also oscillate with no neutralization network.

Therefore the safest bet, at least for replacing AF117's etc in IF stages (that normally do not have any neutralization network) is to use the AF178, or others with similar specs and have no neutralization. And if you replace an OC45 in an IF with an AF127 or similar, disconnect the neutralization network.