D.W. Fearn VT-7 - Manual (Page 22)

D.W. FEARN
VT-7 Compression Amplifier
27
6 .
M A I N T E N A N C E
The VT-7 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/Bottom Cover Removal
Removing the top cover allows access to the vacuum tubes and some of the calibration con-
trols. The top cover allows access to the VU meter calibration controls. Sixteen 6-32 phillips-
head screws must be removed. When replacing the cover, position it so that the slotted ven-
tilation holes are over the tubes (towards the back on the VT-7).
Vacuum Tubes
Eight tubes are used in the VT-7. Six are 6N1P and two are 6072A. There can be as much
as a 15 dB difference in noise level among an assortment of tubes, and the 6072A tubes used
should be carefully chosen to maintain low noise. Selected low-noise tubes are available from
D. W. Fearn. The 6N1P tubes are far less critical and almost any quality off-the-shelf tube will
perform satisfactorily.
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-7 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-7 output, with unity gain, any microphonic response above -55
dBm is excessive. Replacement is indicated unless the VT-7 always operates in a quiet and
vibration-free environment.
CAUTION! With the top or bottom covers removed, voltages of up to
400 are exposed. These voltages can be very dangerous, even lethal in
some circumstances. Only persons with experience with high voltage
vacuum tube equipment adjustment and repair should operate the VT-7
with either the top or bottom covers removed!