Maintenance - D.W. Fearn VLC Operating Instructions Manual

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MAINTENANCE

The VLC 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 tube. Eighteen 4-40 machine screws must
be removed.
Vacuum Tubes
Four 6072A tubes are used in the VLC. There can be as much as a 15 dB difference in noise level
among an assortment of tubes, and the tubes used in the first position should be carefully chosen
to maintain low noise. Selected low-noise tubes are available from Hazelrigg Industries.
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, distortion, or
a reduction in headroom, should indicate the need for replacement.
Tubes 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 VLC output, with 50 dB gain, any microphonic response above -55 dBm is
excessive. Replacement is indicated unless the VLC 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 the first
tube position, it may be cost effective to buy a low-noise tube from us. Current prices are $51.00
for a selected low-noise 6072A, and $25.00 for a tested but less rigorous noise-spec 6072A. We
test the tubes in a VT-2 after a burn-in period and grade them according to noise, microphonic
response, distortion, and other characteristics. A low-noise tube from us will meet the original
VLC specifications.
The base pins of vacuum tubes supplied by Hazelrigg Industries have been chemically treated for
low contact resistance and oxidation prevention. When handling these tubes, care should be taken
to avoid removing or contaminating the treatment. Use a lint-free cloth or paper towel to avoid
direct contact between any part of the tube and your fingers.
VLC Microphone Preamplifier/EQ/DI HAZELRIGG INDUSTRIES