D.W. FEARN
VT-12 Microphone Preamplifer
35
6 .
M A I N T E N A N C E
The VT-12 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 Phillips-head bolts must be removed. When replac ing the cover,
position it so that the slotted ventillation holes are over the tubes (towards the back on the
VT-12).
Vacuum Tubes
Four 12AT7 tubes are used in the VT-12. 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 12AT7 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 periodi cally perform a quick noise and distortion check on the VT-12 and com-
pare 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-12 output, with 50 dB gain, any microphonic response above -55
dBm is exces sive. Replacement is indicated unless the VT-12 always oper ates in a quiet and
vibration-free environment.
Although you could purchase a batch of 12AT7 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. See our web site,
www.dwfearn.com for current prices. We test the tubes in a VT-12 after a burn-in period and
grade them ac cording to noise, microphonic response, distortion, and other character istics.
A low-noise tube from us will meet the original VT-12 specifications.
Summary of Contents for VT-12
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