D.W. Fearn VT-3 - Manual (Page 22)

VT-I/F Vacuum Tube Instrument Interface
D.W. FEARN
24
microphonic response above -55 dBm is excessive. Replacement is indicated
unless the VT-3 always operates in a quiet and vibration-free environment.
Although you could purchase a batch of 6N1Ps and select the quietest one(s), 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-3 after a burn-in
period and grade them according to noise, microphonic response, and other char-
acteristics. A low-noise tube from us will meet the original VT-3 specifications.
Remember that vacuum tubes may be quite hot during operation. Protect your
fingers during tube replacement. The VT-3 should be turned off before removing
tubes. Allow at least one minute for the filter capacitors to discharge before tube
removal or insertion.
Tubes are made of glass and will break if dropped or even bumped in a critical
area. Handle with care.
Electrolytic Capacitors
The VT-3 is designed and built to last for a long, long time, and it is possible
that some components (e.g. electrolytic capacitors) may reach the end of their life
long before the equipment becomes obsolete. The electrolytic capacitors used in
the VT-3 typically will last at least ten years. If there is a measurable and/or audi-
ble increase in 120 cps noise, the filter capacitors should be suspected. They
should be replaced with new capacitors of equivalent capacitance and voltage rat-
ing, and the replacements should be specified for a minimum ten-year service life.
Electrolytic capacitors are also used as plate decouplers. In choosing a replace-
ment, the same considerations as with the filter capacitors should be followed.
TROUBLESHOOTING
Most problems will be traced to defective vacuum tubes. However, if normal
tests do not easily reveal the problem, feel free to call the factory for assistance. If
you lack access to a qualified service technician with vacuum tube equipment