FiveFish VT-73, Assembly Manual

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Copyright © 2017 FiveFish Audio VT-73 Hybrid Tube Mic Preamp Assembly Guide
9. WHAT THE FAQ? (FREQUENTLY ASKED QUESTIONS)
I imagine these will be some of your questions... so I’m answering them now.
When I turn the gain knob clockwise, the sound decreases instead of increasing. At minimum gain setting, I have a very loud output signal.
It’s backwards!
Please review Step 3.15 and look closely at how the wires are connected to the potentiometer. You need to swap the location of your 2 wires if your
gain is backwards.
There is no audio output from the mic preamp.
Please review Step 4, Errata and manual changes. Also, don’t forget to solder the 100K resistor at bottom of PCB in the location shown in Step 3.20
The vacuum tube is not getting warm.
Measure voltage at ouput of Voltage Regulator. You should have approximately 12Volts DC. If you’re getting 12Volts, check solder connections
around the vacuum tube area. Also check for proper solder on the tube socket. If the vacuum tube is still not getting warm, you have have a defective
vacuum tube. Contact us for a replacement.
The bottom potentiometer (TRIM Knob) sounds scratchy when I first turn ON the unit.
This is normal. The vacuum tube needs to warm up before all DC voltages become stable and settle down to their optimum values. Please allow 60-
90 seconds for the tube to sufficiently warmp up before using the preamp. After this alloted time, the sratchiness on the bottom potentiometer will
disappear.
There is no sound coming out, but only during the first few seconds when I first turn ON the unit. After that, it operates normally.
Because this is real vacuum tube preamp. :) The vacuum tube uses a glowing heater element to emit electrons needed for it’s proper operation. A
cold tube does not have sufficient electrons emitted yet to work properly, thus you’re not able to hear any audio output. The tube needs to warm up
before all DC voltages become stable and settle down to their correct values. Please allow 60-90 seconds for the tube to sufficiently warmp up before
using the preamp.
I hear “springy” sound when I bump into the unit.
These are “microphonic vibrations” caused by mechanical vibrations being picked up by the vacuum tube. To eliminate or reduce these vibrations,
use a vacuum tube damper and/or mechanically secure/tie the vacuum tube to the PCB using a short piece of string or cut piece of rubber band. The
amount of these microphonic vibrations can vary depending on the tube.