Page 55 - Triwatt Custom Lead Amp Builder's Guide

Faulty bias supply in fixed bias amplifiers

A bias supply with excessive ripple injects hum directly into the grids of the output tubes. Check
that the bias supply diode is not shorted or leaky, and then bridge the bias capacitor with another
one of equal value to see if the hum goes away.

Unbalanced or not-ground-referenced filament winding

The filament power must be referenced to the DC in the tubes in some way, otherwise you may
get a lot of hum. The filaments are usually a center tapped 6.3VAC winding, with the Center Tap
(CT) grounded for the necessary reference.

If the winding is not grounded and balanced around ground, it will cause hum. Measure the
voltage from each side of the 6.3V to ground; it should be pretty much exactly half the AC voltage
at either end. If it is unbalanced to ground, tweak the pot or change the resistors to get it to be.
Note: If you have grounded center tap style supply that is not centered on ground, this indicates a
faulty power transformer.

TIP: If your heater wires did not have a center tap to connect to ground, then put a 100 Ω anti-
hum resistor to ground from each side of the heater wires to the common ground point. This will
add a ground reference to the heater voltages and help to reduce hum.

Other methods are low value pot (200-500 Ω) across the whole 6.3V with the wiper grounded.

Defective input jack
If the input jack is not making good contact to the guitar cord shield, it will hum. Likewise, if the
jack has a broken or poorly soldered ground wire, or not-very-good connection to the grounded
chassis, it will cause hum. If messing with the jack changes the hum, suspect this.

TIP: If hum or noise exists when the input plug is removed, try re-soldering the connections to
the Input jacks.

Poor AC grounding

In amps with two wire cords, defects of the ‘ground reverse’ switch and/or capacitor can cause
hum. A leaky power transformer can also cause this.

Induced hum
Placement of the amplifier near other equipment can sometimes cause it to pick up radiated hum
from other equipment. Suspect this if the hum changes loudness or tone when you move or turn
the amp. There is usually nothing you can do about this except move the amp to where the hum is
less.

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