Substitution of old germanium transistors in radio circuits

[richard6798]

The original transistor would be a PNP germanium type (AEI - TR1A). I think an RF type, or at least designated that I believe (by Tri-ang). Original battery voltage was 9V. If you were to substitute that germanium PNP transistor with a modern PNP silicon transistor, what transistor would you select? Thanks.

 

[zsolt1]

hi,
any general purpose af tranzistor will do, you just have to change the value of R1 , if don't like calculating that little amp just experiment with different values ... I built this circuit with BC171 (C54x today) and BF 215 , it worked the same way ...
If your headphone has > 2 K , then try for R1 ~ 100K ....1M

 

[richard6798]

Hi. Originally, I think Phone would have been a single earpiece of LOW impedance. "Trionic" set owners might know. :c) I've got the Trionic PCB's but not the components. I'm just wondering whether to use a crystal earpiece, or get a pair of low impedance headphones.

- - - Updated - - -

I've forgotten my radio knowledge. Detection is in the PN junction between base and emitter? And the transistor does NOT have to be an RF type?

 

[kspalla]

Also add resistor in series with head phone, this is because most of modern transistors works fine near 4.5 to 6 V.
I suggest having variable resistor that help as volume control for this receiver.
Regards,

 

[zsolt1]

yes , the tranzistor does the detection + a little amplification of the AF

Ps: i saw this simple detector circuit even with a j fet tranzistor , it promised a better selectivity .. never tried :grin:

 

[richard6798]

I see now that the configuration is "Audion Configuration". I think that is a reflex design (but might not be). An NPN version sees TR1 as BC548, R1 as 100K, phone impedance about 64 Ohms, C3 10uF, C4 100nF and and Battery Volts 1.5V.

 

[zsolt1]

no it is not reflex, audion reciver means a reciver with direct detection and audio amplification .. a reflex reciver would use the same tranzistor for RF amplification & audio amplification (usualy with separate detection with a diode)

 

[betwixt]

Definitely, do NOT use a crysal earpeice! If you do want to use one, replace the 'PHONE' with a 1K resistor then connect the earpiece across C4. Crystal earpeices do not conduct DC so if you just connect one where the headphones originally went, no power would reach the transistor.

The circuit expects high impedance headphones, several K Ohms and these are hard to find these days. If you want to use common low impedace headphones, connect them through an audio output transformer. This will satisfy the requirements of the circuit and provide a better match (clearer and louder volume) to the phones.

Any small signal transistor will work, it expects a PNP type but will work equally well if you use NPN and reverse the battery connections. At the time the circuit was designed, PNP germanium transistors were easier to find the NPN ones. Don't expect great results - it will not be particularly sensitive and it may be difficult to separate stations.

Brian.

 

[pjmelect]

The circuit would work better with a geranium transistor (more sensitive) so why not use one? If you do not have any in your junk box you could always get them on eBay etc

 

[jiripolivka]

In principle all transistor circuits can use Ge or Si transistors. The differences are two- old Ge transistors often had a lower beta, so the base resistors should be adjusted for the good operation point. Ge transistors also are more sensitive to temperature, so replacing them with Si makes the circuit more stable.

 

[betwixt]

It's also worth remembering that when that kind of circuit was popular, it was rare to find Ge transistors with an fT of more than a few MHz, some were less than 1MHz. Modern Si devices work up to the GHz region although other shortcomings in that circuit would prevent it working at such high frequencies. There is no technical reason why Ge is preferable to Si, they just need a bit more voltage to turn them on but the bias resistor provides that anyway.

Brian.

 

[richard6798]

I've been looking for what type of detection is involved, and now I see: The circuit given involves Plate Detection or, I think, known also as Anode Bend Detection. Of which the Infinite Impedance Detector is a variant. Knowing this I can read up on what has been published. Although the circuit as given is not just a detector circuit, but a radio.

 

[betwixt]

Those terms are only used in vaccum tube detectors. The principle is the same though. If the transistor had no bias at all, it would not conduct until the received signal reached about 0.5V which is far more than you would pick up from anything but a radio station right next door. If you bias it into it's linear region it would work as an amplifier and boost the RF level athough C4 might make the gain useless. Biasing it so it only just starts to conduct by getting the value of R1 just right puts it into a non-linear conduction state where the RF from the radio signal make it conduct more when in one polarity but not at all in the other, in other words rectifying ("detecting") it and giving some amplification as well.

Brian.

 

[richard6798]

Actually, that TR1A designation is probably an Ediswan XA101.

Type Designator: XA101
Material of transistor: Ge
Polarity: PNP
Maximum collector power dissipation (Pc), W: 0.12
Maximum collector-base voltage |Ucb|, V: 20
Maximum collector-emitter voltage |Uce|, V: 0
Maximum emitter-base voltage |Ueb|, V: 12
Maximum collector current |Ic max|, A: 0.01
Maximum junction temperature (Tj), °C: 65
Transition frequency (ft), MHz: 2
Collector capacitance (Cc), pF: 14
Forward current transfer ratio (hFE), min: 35
Noise Figure, dB: -
Package of XA101 transistor: TO-8var

 

[betwixt]

No, it's made by Allied Electronic Industries. I think they were swallowed up by GEC in the early 60s and their replacements were the GETxxx range.

I remember some of the AEI transistors were painted light blue and were boxes about 4mm x 4mm x 10mm long with thin wire legs in-line coming out from the small side. There was a white or red dot to mark the emitter wire.

Brian.

 

[richard6798]

Well, I'm going by this:


"Here are two unusual transistor types, marked AEI TR1A and TR2A. What is really odd about them, apart from the fact that I cannot find them in any data book (and I have a *lot* of data books on germanium transistors) is that they use the black-painted 'top hat' encapsulation that is unique to early Ediswan transistors. Ediswan fell under the AEI umbrella, but why did AEI brand these two transistor types as their own?

I am grateful to John Hills of Oregon in the USA who has explained that these TR1A and TR2A transistors were supplied to Tri-ang (Lines Bros) for use in their Tri-onic kit sets. These sets were probably only produced from 1961 until 1963. John's 1961 "A" set contains two top hat transistors marked "A.E.I. TR1A" and "A.E.I. TR2A", exactly like mine. These kits contained components in plastic housings with connecting pins that plugged into a series of circuit boards, Tri-ang designated the "A" set transistors as TR4 and TR5. TR4 is the TR1A and is described as being the RF transistor, which John believes to be an Ediswan XA101 or XA102. TR5 is the TR2A and is described as being the AF transistor, which John thinks may be an Ediswan XB102, XB103 or possibly XB104. John has other Tri-onic kits and some have transistors marked "A.E.I. TR1" and "TR2" but in metal cans with plastic sleeves that are probably later Ediswan types. Even later kits contain Mullard OC series transistors. In 1962 Tri-ang introduced the kit "A/B", This added five more printed circuits and other components including another RF transistor in the TO7 package, probably a Mullard AF11n type."

Here: http://www.wylie.org.uk/technology/semics/BTHAEI/BTHAEI.htm

 

[betwixt]

That link brings back memories. I think I still have some of the pictured transistors here and also some rather ancient US types. I think I might have some 2S004 early silcon transistors from Texas Instruments and possibly some 2S012 power transistors too. I'm getting old.... :sad:

Brian.