Germanium transistors : Sonic properties

There is a “germanium sound”. It is especially suitable for guitar amplification, and can apply to harp and some keyboards. The sound of germanium has a warmth and a granulity as pleasant to the ear as the tube sound, without being the same : The sensation of warmth is about equivalent (for analog reasons, in particular the Miller effect that dynamically attenuates the high frequencies). The sonic  grain of germanium is generally a bit rougher than the tube grain, and especially less flat and plain than silicon …
In all domains of audio, germanium transistors and their “good defects” are opposed to the cold pharmaceutical perfection of integrated circuits.

After years spent in tests and development, I feel able to specify some characteristic parameters… And happy to share this knowledge that I apply in the products shown on this site ; stuffs just to please the guitars.

Germanium transistors - ampli 10W

Prototype of a 10W amp, all russian germanium transistors .

Germanium transistors
have a moderate gain, like tubes.

The gain of the germanium transistors is rarely higher than 120, and often between 30 and 90 in Russian models, while that of silicon transistors is generally between 150 and 500. It makes preamps and overdrivers pass very gradually from a clean to a slightly distorted sound when actuating the ordinary controls of volume and tone.
The circuits are simple and can work with a low ratio or an absence of negative feedback. Consequently, germanium circuits gives a soft transmission, that silicon circuits does not because of their higher gain, which requires a vigorous negative feedback, resulting in a leaner sound.

Rounded knee in the saturation curve

Coming into saturation is characterized by clipping the upper portion of the signal. The rounded fragment of the curve that binds the undistorted part of the signal  to the flat of the clipped part is called “the knee”.

Silicon transistors have a short and almost angular knee. That’s why they sound “like bread crusts in bed” when they are overdriven. Germanium transistors have a wider and rounded one. This gives a less strident tone, as well in slight overdrive as in hard clipping.

Germanium vs Silicium - gain and knee

• Output signals of two ‘LikeMyFace’ fuzzboxes, germanium version and silicon version…
The germanium curve is on top of each view, the silicium below…

The right view shows an extended time scale.
The input signal is 1kHz, 30mV. Both circuit have their “fuzz” pot set full gain.
• The Germanium model has a lower gain (signal has less steep fronts) and shows a more rounded knee, particularly visible in the detailed screen on the right.

Germanium fuzzboxes fit to modern amps

Today’s amplifiers have a more sharp and hollowed clear channel than vintage amps (Vox AC15, first AC30, Bassman 59 and other Tweeds, first 45W Marshall). The silicon fuzz used to give a brilliance to these naturally damped amps. They may now sound a bit too sharp on modern amps. Conversely, germanium fuzzboxes which are a little dull on old amps sound to their advantage on modern ones.

Limited gain × bandwith product

More this number is high, more the transistor is able to amplify much and fast.
This is not the case with vintage audio germanium transistors. When working at their nominal gain, they are just able to amplifiy high frequencies up to about 5 kHz.

This theorical weakness is a quality in guitar amplification.

The result is a living warmth, as a dynamic low pass filter would do : The harder you work the transistor, the more it softens the sound. Sound of germanium never scrapes ear !

Note that one can use radio-frequency germanium transistors when a large bandwidth is needed. Anyway, they will keep some sweetness when pushed to saturation.

Leakage currents can be “good defects”

Leakage currents are superposed on the usefull currents which carry the musical signal, and affect them more or less according to general level and dynamics. This happens just like the wind enters a narrow street and disturbs the passers’ walking…

Objectively, these leakage currents have greatly contributed to give up germanium : they can cause noise, and alter or destabilize the circuits, unlike silicon which is impressively silent and stable. These leakage currents are nevertheless an integral part of this technology … When they are restricted and well controlled, they can become “good defects” and bring positive sonic characteristics to amplification.

They cause dynamic compression effects, following guitar playing.

These effects are fluid and follow the guitarist’s playing. This had been observed at the time of first effect circuits such as fuzz, or treble boosters like the Rangemaster by Arbiter or the Screeming Bird by Elektro Harmonix. I personally reintegrated and exploited these effects in my SweetGerm preamp.

A general compression tends to blunt the strongest levels, and to gently raise the level of the weak or sustained notes. It is mostly apparent and pleasant when applied to preamplifiers and fuzzboxes…

A second dynamic effect, less marked and not systematic, approximates the comparison with the windy street : the signal level tends to slightly inflate and deflate depending the hits of picks, powerchords, as if you were using an expander, set soft and with some delay.  It is most apparent when the guitar sound is crystalline, usually on intermediate position of microphone selectors.

Technological cause seems to me the following :

The sound signal is translated by changes in “useful” currents of the transistor. These current variations cause, on a microscopic scale, continual temperature variations within the crystal. Furthermore, you know germanium is very sensitive to temperature change …
Therefore, the gain and leakage currents vary continuously, depending on the signal : it affects volume dynamics (compression / expansion), distorsion effects (grain variations, “liquid” effect) and tone (Miller effect, treble dynamic filtering). These phenomena are all the more sensitive they are not instantaneous : thermal ups and downs require within tens of milliseconds. This delay, as in compressors or envelope filters, gives an impression of sound massage, something like breathing, which is not only sensible, but can be felt vivid and enjoyable.

Materials with defects that could be the source of sensory and aesthetic qualities always fascinated me … With experience, I learned appropriating and exploiting these good defects of germanium transistors. Not all of them : some are as dull as silicon, and others remain too uncontrollable. Far from cloning to the blind, you always have to select the right parts and to calibrate the circuits : It is this experience and this patience that my stompboxes offer to you.

Beyond fashion …

I did not know all of that at fourteen, when I was disassembling radios and tried to retrieve my first Black Glas transistors. But maybe some sound sensations remain in my memory, that I rediscovered more later, in the early 2000s.

Anyway, germanium is not a snobbery at Guitar Poppa’s shop.
It is a way of making the sound live.

Bandeau fin gris moyen - L15

Go and read about Guitar Poppa’s germanium pedals

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Last edition : 2016, april, 15.

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