Page 57 - Triwatt Custom Lead Amp Builder's Guide

The best place to use Carbon Composition (CC) resistors is where there's big signal - plate
resistors, and ideally the stage just before the phase inverter. The phase inverter would otherwise
be ideal, with plate resistors carrying the highest signal voltage in the amp, but phase inverters are
often enclosed in a feedback loop. The feedback minimizes the distortion the resistor generates.

Squealing/Feedback
Squealing usually occurs when there is coupling between the input and output stages. The positive
feedback causes the amp to become an oscillator. Vary the volume and tone controls to see if it
affects the oscillation. That will tell you if the coupling is occurring before or after the control.
Sometimes the problem can be solved by minor changes to the wiring (moving output wires away
from input wires, shortening excessively long wires, etc.).

Use shielded wire on the input jack to help a hum or squealing problem.

Ensure the shielded wire goes on top of the board, not underneath it.

Reverse the output transformer leads on pin 3 on V5, V6.

Explanation: One of the primary leads is in phase with one of the secondary taps. In a high gain
amp, this phase relationship needs to be maintained.

Radio Interference

If you are picking up radio stations on your amp:

1. Try a .01 uF or 47 pf capacitor on very short leads between the ‘ground’ side of the input
jack and chassis.
2. Make sure the chassis is fully enclosed electrically. Install a piece of thin Aluminum sheet
metal or HVAC Aluminum tape sandwiched between the chassis & cabinet and make sure
it makes contact with the chassis.
3. Make sure the 68K grid blocking resistors are located at/on the V1 tube socket.
4. Use shielded wire between the input jack and the 68K grid blocking resistor.
5. Place ferrite beads over the shielded input cable.
6. Try grounding the shield of the shielded input cable to the chassis instead of the preamp
ground.

Other useful measures to take in extreme circumstances:

1. Use a filtered IEC connector for your mains power connection.
2. Put a 100pf across the V1 Plate and cathode pins 1&3

Scratchy Sounds on Potentiometer(s)

TrinityAmps TRIWATT Builders Guide Ver. 2.31.docx
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