Page 58 - Triwatt Custom Lead Amp Builder's Guide

If you are hearing scratch sounds on a pot when you rotate it, measure the DC voltage from the
terminals to ground. A leaky coupling capacitor or tone stack will cause this to happen.

Amp Buzz or Rattle When Installed in Cabinet

If you get a buzz in an amp when it’s installed in a cabinet, it could be due to any one or a
combination of the following things. Start with the easy things and work your way through the
tests.

First, is it a metallic buzz? Is it a tube (ringing) buzz? Is it a softer buzz (wooden/plastic sound?)

Try using an external speaker, isolated from the amp to see if it goes away. This should tell you it's
related to the cabinet mechanics or not.

Testing Cabinet Mechanics
 Are the Speaker mountings tight?
 Are the cabinet construction screws tight?
 Are the Vents loose? Use more fasteners; Rubber gasket between vent and chassis; hard
rubber washers to hold vent assembly on
 Does the power transformer touch the mounting boards ? Check for a gap and then
separate the power transformer from the mounting board.
 Is the Speaker cable rattling against back of chassis? Hold it & listen. Tie it down if
necessary.

Loosen the chassis from the cabinet and see if the buzz goes away. This will isolate the chassis as
the problem. If it does go away, Test the chassis mechanics.

Testing Chassis Mechanics
 Are all the nuts fastening parts to the chassis tight? (sockets, transformers, tag strips etc.).
 Are there Shields on pre-amp tubes? Remove & listen.
 Are there Spring retainers on power and rectifier tubes? Remove them or temporarily tie
them down somehow & listen. Cover in heat resistant tubing if necessary to isolate them
from the tubes; or remove them; or you can retain tubes with a small amount of silicone.
 Are the tubes mechanically rattling? Hold them and see if the rattle goes away. Replace if
necessary.
 Are the Controls loose? (toggles/mounting rings etc.)
 Is the Chassis loose? - tighten & listen
 Is the Chassis loose against backboard? - Remove backboard & listen
 Is the Chassis pushed up hard against cabinet? Tighten; Use Rubber gasket (neoprene 3/8’
X 1/8’ window/door sealer) around where the chassis touches the cabinet
 Is there a gap between panels/chassis & cabinet? Tighten it up, use rubber gasket where the
faceplate meets cabinet. Rubber gasket (neoprene 3/8’ X 1/8’ window/door sealer)
between panel and cabinet

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