ASTRA SILICON FUZZ
5
BUILD NOTES
Transistor selection
The original Astrotone used 2N3565 transistors, which varied widely in gain. Some batches were in the
180-220 range. similar to the 2N3904, and others were more like 400-500, similar to the 2N5088. The
lower-gain ones were more prevalent.
The exact gain characteristics of the 2N3565 transistors used in the original Astrotone are not known,
but it doesn’t matter too much. The important thing to know is that there is nothing special about the
original vintage transistors. The
2N3904
will sound exactly the same.
Diode selection
Photos of original Astrotone units show that it used DO-7 glass case diodes, which are usually
associated with germanium. However, the
original manual for the Astrotone
claims it uses all silicon
semiconductors (which includes diodes)—and in the 1960’s when small-signal silicon diodes were first
coming on the market, there were a lot of them in the DO-7 case, since that was the format used by most
diodes. Therefore, while DO-7 silicon diodes are rare today, this would not be uncommon to see in 1966.
The other evidence in favor of them being silicon is more practical: since they’re hard clippers with no
gain recovery afterward, they would directly impact the max volume level. At ~0.3V forward voltage
(typical for germanium), it would clamp the maximum volume at unity or lower, and the original units
would have a hard time achieving unity gain if they were in fact germanium diodes.
So, on balance, we’d conclude that despite looking like germanium didoes, the Astrotone was an early
adopter of silicon diodes. And allowing that they are in fact silicon, the standard
1N914
would be an
exact replacement.
Analogman mods
The Analogman version of the Astrotone uses
1M2
for R1 and R4,
470R
for R6, and
470n
for C1 and C4.
The vintage version is very thin-sounding, lacking in bass, and these modifications help to round it out.
Even if you stick with the original vintage circuit, you may find it worthwhile to use Analogman’s tone
control modification, which reduces R6 to
470R
. All the tones of the original unit are still present except
at the very extreme end of the range.
