Fortunately, replacing germanium devices with silicon ones isn't entirely
guesswork. When the industry switched over in the 1960s and 1970s, it provided
excellent replacement parts. At the time, these parts had average performance characteristics
that would suffice in many circuits. Over time, designers have immensely improved these
characteristics. Today, with care, you can swap in the silicon parts, and make a few bias
changes. Perhaps your equipment might require a few repairs, too. Afterward, your
electronics will be as good as new.
Suggested small-signal transistors. For small-signal NPN devices, substitute a
2N2222A or 2N3904 silicon transistor. For small-signal PNP devices, substitute a 2N3906 or
2N2907 silicon transistor. These are universal replacement parts. They'll reliably
stand in for about 80 percent of the transistors in legacy preamplifiers. (Universal
replacement parts also work in most silicon preamps.) For the universal replacement
parts that I've mentioned, the package shape is TO-92. Adjust the bias voltage as
in our Conversion Method
table.
TO-92 transistor (from datasheet)
Specialized Transistor Applications
TO-3 transistor
For power devices, start with the world's most popular silicon transistor: A 2N3055
might work in some circuits. This NPN transistor comes in two package types, the large TO-3
package, plus various smaller packages. In germanium days, PNP devices were more common. So
we'll mention that the PNP complement for the 2N3055 is the MJ2955. (Also silicon.)
Another superb power transistor is the TIP31. This NPN device comes
in a TO-220 power tab case. Its PNP complement is the TIP32.
Some low-power circuits, such as transistor radios, will do fine with
universal replacement transistors (See above): Even in dual-ended power stages.
Don't forget to use heat sinks as necessary.
TO-220 transistor
TO-225 transistor
For line-powered preamps, try the MPSA42 (NPN) or MPSA92 (PNP). Both come in TO-92 packages.
If you need a little more power, use the MJE340, an NPN transistor, and its PNP complement MJE350.
These devices come in the TO-225 case. All these transistors are 300-volt (maximum)
devices.
For AM radio RF and IF amplifiers, use the same, universal replacements as above.
For FM—TV—VHF—UHF into low microwaves, try a 2N918. This is an NPN silicon
RF transistor with four leads. The 2N918 comes in a metal TO-72 case.
TO-72 transistor
For low-voltage circuits, below about 2 volts, change the power voltage to 3 volts. There are
two junctions in a silicon transistor. Each one requires 0.7 volt. Add in voltage drops across
collector and emitter resistors, and you'll need at least 2 volts, and preferably 3 volts. Germanium
transistors could operate at lower voltages.
Other Components That Affect Performance
Capacitors & Diodes
Coupling Capacitors
You used our Conversion Method, but the circuit still clips: Suspect a leaky base-coupling
capacitor. Leakage disturbs the base bias, causing distortion.
Do you have an electrolytic capacitor? Replace it with a new aluminum
electrolytic.
Do you have a wax-and-paper base capacitor? Replace it with a new polypropylene capacitor
of the same value.
Check the amplifier's DC output voltage. If the voltage is now about half of Vcc
(within reason), have a listen to your amplifier or radio. Sound better?
Electrolytic Capacitor
Germanium Diodes
For vintage radios. Sometimes the detector is a germanium diode. Nothing wrong with that!
Keep it as-is! (But if you must replace it with silicon anyway, try a Schottky diode.
These diodes are most sensitive, and turn on at a lower voltage than do most silicon PN diodes.)
Germanium diodes. Your radio might also use germanium diodes to bias its complementary output stage. If you're
replacing the output devices, you'll have to replace the germanium bias diodes with silicon
diodes. Use standard, PN diodes, not Schottky devices. In transistor radios,
type 1N4148 diodes will do the job. In a big power amp, you'll need something more robust.
Germanium Diode
Resistors
Don't neglect resistors. Resistors, particularly those in power circuits, do occasionally fail.
As they age, sometimes resistor values drift out of spec. If a transistor fails, sometimes it will
draw excessive current through its emitter and collector resistors. These will then sustain heat
damage. Check the circuit resistors. Replace them if they're more than 20 percent out of spec.
▲ WARNING. The author assumes no responsibility for your success or failure in using methods on these pages. Further, the author
neither makes nor implies any warranty or guarantee as to the accuracy or effectiveness of these methods. Proceed at your own
risk.
