Page 53 - Triwatt Custom Lead Amp Builder's Guide

Troubleshooting

When debugging a newly built amp the first things to do are check the wiring, make sure the
correct components are installed, and look for bad solder joints.
Use a voltmeter to check voltages and compare them with the voltages listed on the schematic.
Remember that you can calculate current by measuring the voltage drop across a resistor and
dividing by the resistance.
An incorrect voltage or unusual current may give you a clue to the source of the problem. A low
voltage often indicates that something is drawing more current than the power supply can handle
and dragging down the voltage.
Probing with a non-conductive object such as a chopstick while the amp is powered on is a good
way to find bad connections or problems with the way the wiring is laid out.
Remember that dangerous voltages are present when the amp is powered on. Always drain the
filter caps and disconnect the mains before working on the amp. To learn how to do this safely,
see the following ‘Faulty power supply filter caps’ discussion.
Never operate the amp without a load or you will damage the output transformer. You can use an
8 ohm 15 watt power resistor as a dummy load in place of a speaker.

Hum

Hum is the most common problem and is usually caused by AC line noise leaking into the filament
wiring or input stages and getting amplified. Here we provide a comprehensive step-by-step
troubleshooting guide.

First, measure the resistance from each parts ground connection to the chassis. All readings
should be less than 1 ohm, typically 0.5 ohms.
Make sure the Mains ground at the chassis is very tight.

Volume Test

A good way to troubleshoot is to divide and conquer by turning the volume control(s). If the hum
changes levels as you do this, then the source of the hum is something that affects the stages of the
amp before the volume control. A faulty, humming preamp tube can be isolated this way very
quickly. Conversely, if the volume control does not affect the hum, the cause is somewhere after
the volume control.

Faulty tube

Tubes sometimes develop internal hum. Do some tube swapping to locate the problem. Use the
volume control test.

TrinityAmps TRIWATT Builders Guide Ver. 2.31.docx
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