D.W. Fearn VT-2 - Manual (Page 35)

D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
35
6 .
M A I N T E N A N C E
The VT-2 is built with only the highest quality parts and will prove to be extremely reliable.
Vacuum tubes and electrolytic capacitors, however, have a finite useful life and must be
replaced eventually.
Top Cover 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 the 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).
Vacuum Tubes
Four 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 maintain low 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
satisfactorily. 12AY7 tubes may be substituted for the 6072A tubes, but with slightly reduced
performance.
Tube life is difficult to predict, 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
the 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
normal monitor level should reveal nothing more than a slight “clank.” On a peak-reading
meter connected to the 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.
Although you could purchase a batch of 6072A tubes and select the quietest one(s) for V101
or V201, 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