Page 56 - Triwatt Custom Lead Amp Builder's Guide

Poor internal wire routing

If the signal leads inside the amp are routed too near the AC power wires or transformer, or
alongside the high-current filament supply wires, they can hum. Sometimes using shielded cable
for signal runs inside the cabinet can help. It is hazardous to do, but you can open the amp up and
use a wooden chopstick (NOT A PENCIL) to move the wires around inside to see if the hum
changes. This is hard to do well and conclusively, since the amp may well hum more just because it
is open. BE VERY CAREFUL NOT TO SHORT THINGS INSIDE THE AMP.

Poor AC Chassis Ground at Power Transformer

A common problem is the main ground point to the chassis. The green wire ground to the chassis,
the ‘line reverse’ cap, the CT on the filament windings, the CT on the high voltage windings, and
other things associated with power or RF shield grounding are often tied to lugs held under one of
the power transformer mounting bolts. If this bolt becomes loose, or if there is corrosion or dirt
under the lugs, you can get an assortment of hum problems.

Defective internal grounding

There are potentially lots of places that must be tied to ground in the internal wiring. This varies a
lot from amp to amp. If one is broken loose or has a poor solder joint or poor mechanical
connection, it can show up as hum. Note that modified amplifiers are particularly susceptible to
this problem, as the grounding scheme that the manufacturer came up with may well have been
modified, sometimes unintentionally. With the amp unplugged, open and the filter capacitors
drained, carefully examine the wires for signs of breakage.

Hiss

Some noise or hiss is normal if you have used carbon composition resistors. TRIWATT kits are
supplied with Carbon Film and Metal film resistors similar to the original. This style of resistor has
inherent noise. If this amount of hiss is bothersome, you will need to replace the resistors in the
signal chain with Metal Film resistors.

Metal Film Resistor Substitutions
If you really want to eliminate hiss, use additional metal film resistors where the signal level is small
and the following amplification is high - a classical description of an input stage. The input to an
amp should probably have a metal film plate resistor to minimize noise.

Substitute them on the grid resistors in all but output stages because the signal level is typically too
low.

Substitute them on the Cathode resistors. They typically only have a few volts across them, and
they're often decoupled with a capacitor, both of which would minimize the carbon composition
resistor distortion (carbon comp “Mojo”).

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