D.W. FEARN VT-15 Recording Channel
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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-15 output, with 50 dB gain, any microphonic response above -55
dBm is excessive. Replacement is indicated unless the VT-15 always operates in a quiet and
vibration-free environment.
Remember that vacuum tubes may be quite hot during operation. Protect your fingers dur-
ing tube replacement. The VT-15 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. There are only a limited number of NOS (new old-stock) 6072A tubes in the world.
Treat each one carefully.
Electrolytic Capacitors
The VT-15 is designed and built to last for a long, long time, and it is possible that some com-
ponents (e.g. electrolytic capacitors) may reach the end of their life long before the equip-
ment becomes obsolete. The electrolytic capacitors used in the VT-15 typically will last at
least twenty years. If there is a measurable and/or audible increase in 120 cps noise, the fil-
ter capacitors should be suspected. They should be replaced with new capacitors of equiva-
lent capacitance and voltage rating, and the replacements should be specified for a minimum
ten-year service life.
Electrolytic capacitors are also used as plate decouplers. In choosing a replacement, the
same considerations as with the filter capacitors should be followed.
VU Meter Calibration
The meter amplifier circuit is stable and re-adjustment of the meter calibration is not normal-
ly required. If the output reference level needs to be changed, here is the procedure:
1. Remove the VT-15 top cover (see above).
2. Terminate the output in the same impedance as the VT-15 will normally be feeding.
This will almost always be a high-impedance bridging termination of 20k ohms or
more.
3. Apply AC power. Allow at least an hour warm-up. Feed a 1 kc tone at about -45 dBm
into the VT-15 input.
4. Measure the output and set the Preamp Gain control for +4 dBm (or whatever refer-
ence is desired).
5. Set the Meter Calibration trimmer potentiometer for 0 VU on the VT-15 meter. Be
sure the meter is in the VU position. This control is a 25-turn pot so the adjustment
is easy. A small screwdriver or alignment tool is necessary. The control is located on
the relay printed circuit board on the top of the chassis, on the far left side and it is
the only trim pot on this board.
6. Replace the top cover. Be sure the ventilation slots are over the tubes (towards the
back).