Artemis Labs DP-2 Operating Manual
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TUBES
Page 6
Occasionally a KT88 will be found that is too micro-
phonic, i.e. is very sensitive to mechanical vibrations.
All tubes are microphonic to a degree, but if sound is
heard from the speakers if the chassis is lightly tapped,
then a different KT88 should be used.
Since the KT88 is run very lightly in the DP-2, it should
last a long time, and fail by eventual cathode wear-out.
There is no need to match the two KT88s, since each
one is used for each channel and is individually biased.
2A3 - This type is used as the power output tube, and
is crucial to the sound quality and power output of the
amplifier. The 2A3s on each chassis should be matched
pairs to insure the best low frequency response and
lowest distortion. Do not use other triodes, such as the
300B or PX-4, since their filament voltage is different
than the 2A3.
Depending on the tolerances of the tube sockets and
pins, it may be possible to incorrectly insert the 2A3s,
causing possible damage to the amplifier. Make sure the
two large pins are inserted facing the outside edge of the
chassis, as shown in the diagram below.
The 2A3 was designed by RCA and announced in 1932.
The very early RCA 2A3s had a single plate, but all later
American-built 2A3s have the “double-plate” construc
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tion, essentially two triodes in parallel. Modern 2A3s are
made with either single or double plates.
The normal failure mode for these tubes is wear-out of
the cathode coating, resulting in low current and higher
distortion. However, if the tube goes gassy, it is possible
for the tube to arc over, which in the worst case will cause
the fuse to blow.
5U4GB - This is the high-voltage power supply rectifier.
Although any rectifier with the same pin-connections
and power ratings as the 5U4GB or the Western Electric
type 274B could be used, other types may produce an
incorrect high voltage. Only exact equivalents of the
5U4GB (such as the 5U4G, 5AS4, Russian
5Ц3C, etc.) or
the 274B are recommended.
The 5U4GB was brought out by all the major Ameri
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can manufacturers in 1954, and was an updated version
of the 5U4G (1939) which was an octal version of the
5Z3, designed in 1934 by RCA. It was very widely used
in televisions, audio amps, and industrial equipment in
America.
These rectifier tubes are exposed to high voltages and
currents, and may fail by arcing. This will typically blow
the fuse. Otherwise, failure is indicated by cathode wear-
out, which will reduce the high voltage and hence cause
higher distortion.
Do not use plug-in solid-state rectifiers, since their fast
turn-on and lower voltage drop will unduly stress the
amplifier.
Artemis Labs
DP-2
Push-Pull Amplifier
2A3 2A3
274B / 5U4G
6550 / KT88
12AU7 / ECC8
2
Artemis Labs
DP
-2
Push-Pull Amplifier
2A3 2A3
274B / 5U4G
6550 / KT88
12AU7 / ECC82
The tube locations are marked on the black plates on the
top of the chassis. Note that on both the left and right
amplifiers, the two large pins of the 2A3s face right when
viewing the front of the chassis, as shown above.