Operating instructions

D.W. FEARN
VT-1 &
VT-2 Microphone Preamplifers
35
6.
MAINTENANCE
The VT-2 is built with only the highest quality parts and will prove to be extremely reliable.
V
acuum tubes and electrolytic capacitors, however, have a finite useful life and must be
r
eplaced eventually.
T
op Co
ver Removal
Removing the top cover allows access to the vacuum tubes, the VU meter calibration, and to
the rotary switches. 6-32 allen-head bolts must be removed (phillips-head machine screws are
used on t
he VT-1). When replacing the cover, position it so that the slotted ventillation holes
are over the tubes (towards the back on the VT-2, to the left when facing the unit on the VT-
1).
V
acuum Tubes
F
our 6072A tubes are used in the VT-2. V101 and V201 are the input stage and V102 and
V202 are the output stages. There can be as much as a 15 dB difference in noise level among
an assortment of tubes, and the tubes used in the V101 and V201 positions should be care-
fully chosen to maint
ain lo
w noise. Selected low-noise tubes are available from D. W. Fearn.
V102 and V202 are far less critical and almost any quality off-the-shelf 6072A will perform
satisf
actorily. 12AY7 tubes may be substituted for the 6072A tubes, but with slightly reduced
performance.
Tube lif
e is difficult to pr
edict, but it will probably be measured in years. Catastrophic tube
failure is rare with this type of device, but a gradual increase in noise, microphonics, distor-
tion, or a reduction in headroom, should indicate the need for replacement. It is recommend-
ed that you periodically perform a quick noise and distortion check on the VT-2 and compare
t
he results to previous measurements.
Tubes also sometimes develop a microphonic response — they will respond to ambient noise
and vibration. This can be an insidious problem since measurements in a quiet room will indi-
cate perfect performance. Gently tapping the tube shields while listening to the output at a
nor
mal monitor le
vel should reveal nothing more than a slight “clank.” On a peak-reading
meter connected to t
he VT-2 output, with 50 dB gain, any microphonic response above -55
dBm is excessive. Replacement is indicated unless the VT-2 always operates in a quiet and
vibration-free environment.
Alt
hough y
ou could purchase a batch of 6072A tubes and select the quietest one(s) for V101
or V20
1, it may be cost effective to buy a low-noise tube from the us. Current prices are
$56.00 for a selected low-noise 6072A for V101 or V201, and $24.00 for a tested but less rig-
orous noise-spec 6072A for V102 or V202. We test the tubes in a VT-2 after a burn-in period