D.W. Fearn VT-F, Руководство по эксплуатации

Страница: 21 / 23

D.W. FEARN
VT-I/F Vacuum Tube Instrument Interface
23
M A I N T E N A N C E
The VT-I/F 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 periodically replaced.
TOP COVER REMOVAL
Removing the top cover allows access to the vacuum tubes. Twelve 6-32 allen-
head bolts must be removed.
Vacuum Tubes
A single 12AY7 tube is used in each channel of the VT-I/F. (A 6072 may be
used. This is the low-noise industrial version of the 12AY7.) There can be as much
as a 15 dB difference in noise level among an assortment of tubes, and the tube
used in the VT-I/F should be carefully chosen to maintain low noise. Selected low-
noise tubes are available from D. W. Fearn.
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 or distortion, or a reduction in headroom, should indicate the need for
replacement. It is recommended that you periodically perform a quick noise and
distortion check on the VT-I/F 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 indicate 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-I/F output, any
microphonic response above -55 dBm is excessive. Replacement is indicated
unless the VT-I/F always operates in a quiet and vibration-free environment.