Substitute transistors

[trashcanman]

I'm building an 1176 rev D and am having trouble finding some of the transistors that are discontinued, such as the 2n3391 used in the preamp and line amp sections. I already have some 2n4124s and was wondering if they are an ok substitute in this instance.
Comparing the specs of the two:

Type Designator: 2N3391

Material of transistor: Si

Polarity: NPN

Maximum collector power dissipation (Pc), W: 0.2

Maximum collector-base voltage |Ucb|, V: 25

Maximum collector-emitter voltage |Uce|, V: 25

Maximum emitter-base voltage |Ueb|, V: 5

Maximum collector current |Ic max|, A: 0.1

Maksimalna temperatura (Tj), °C: 125

Transition frequency (ft), MHz: 70

Collector capacitance (Cc), pF: 12

Forward current transfer ratio (hFE), min: 250

-------------------------------------------------------------

Type Designator: 2N4124

Material of transistor: Si

Polarity: NPN

Maximum collector power dissipation (Pc), W: 0.31

Maximum collector-base voltage |Ucb|, V: 30

Maximum collector-emitter voltage |Uce|, V: 25

Maximum emitter-base voltage |Ueb|, V: 5

Maximum collector current |Ic max|, A: 0.2

Maksimalna temperatura (Tj), °C: 135

Transition frequency (ft), MHz: 300

Collector capacitance (Cc), pF: 4

Forward current transfer ratio (hFE), min: 120


It looks as though the 2n4124 has very similar ratings other than the transition frequency and collector capacitance. Will these two make much of a difference?
Attached is a picture of their place in the circuit.

 

[jensenmann]

Have a look at the gyraf.dk page. Jakobs schematics contain european replacement types.

 

[trashcanman]

You mean the original UREI schematic? I just checked and it lists BC107 as a replacement which looks within the same bounds as the 2n4124 so I'm guessing it may be ok. 

 

[jensenmann]

Make sure that your substitute transistors have more or less the same Hfe.

 

[MrDude]

Make sure that your substitute transistors have more or less the same Hfe.

The BC107 and the 2N4124 have half as much current gain as the 2N3391.

Would that mean two times the load on the previous stage, therefore a possible increase in distortion?

 

[jensenmann]

BC107/8/9 are available in A,B,C versions. C being the highest Hfe type. They should be available in the same range as your original transistors. If you buy a BC107 without A/B/C rating then you have to expect widely spread Hfe between these.

 

[trashcanman]

The lower current gain did worry me, but there's no specification in the schematic but I'm going to guess you'd be expected to match the 2n3391.  Would the low current gain make the amplification of the circuit lower? Can the values of the resistors be changed to compensate?

 

[jensenmann]

I´d not go too scientific about that topic. 1176s went in production at times when transistor tolerances were a disaster. That has been adressed in the circuit, which should work fine within a wide range of Hfe. In your case the 2N3391 has a range of 250-400. Chose your BC107/8/9 that it fits the same range. IIRC BC107/8 B is fine as well as any BC109 (verify that yourself, my guess is from memory, not from looking at a datasheet).
Hfe isn´t a reliable parameter anyway. It fluctuates with temperature, voltage, weather, and the mood of your wife. It´s good practice to design a circuit in a way that it relies not too much on Hfe being constant.

 

[trashcanman]

I decided to go with the Bc547B which is actually closer to the bc107 than the 2n3391.