From The Mailbag: Marshall Valvestate VS265 Repair

GKB reader Lucas B. from Lima, Peru wrote us to ask for some help with his Marshall Valvestate VS265. If you're not familiar with it, the Valvestate VS265 is a two-channel 65-watt combo amplifier with reverb and chorus.

Here is Lucas' question:

Hello! I have a Marshall Valvestate VS265 and I've opened it to replace some potentiometers, but when I opened it I broke a black ring magnet or something. I really don't know what the ring is for. After that I closed the amp and heard a little hum that inceases when turning the overdrive reverb potentiometer fully clockwise. Does this ring have something to do with the hum? I've enclosed a picture of the ring.

Thanks Lucas B.

Valvestate VS265 Answers

The first part of the answer is that the broken "black ring magnet" is a ferrite core. Ferrite is a material with high magnetic permeability (the ability to support a magnetic field) coupled with low electrical conductivity (the ability to conduct an electrical current), making it ideal as the core for transformers and inductors. When a wire or cable is placed through the center of a ferrite ring, or wrapped for a turn or two around the ring, it greatly increases the inductance which is very effective in blocking higher frequency signals, such as from electromagnetic interference (EMI) or noise. Typical applications in an amplifier are to use ferrite on a reverb cable or AC power cord, or anywhere else that EMI is found to enter the amp.

However we weren't familiar with the specifics of the VS 265, and the schematic didn't show where ferrite was being used, so we had to ask Lucas for a couple more photos of the chassis interior.

Here's what he sent: It's clear from the photos that the reverb cable, with the red and white connectors, was coiled around the ferrite core ring until it was broken, and that's the cause of the hum that Lucas is hearing when he turns up the reverb. What's the solution? Fortunately it's easy and inexpensive. Lucas will need to purchase a new ferrite ring, and then re-wrap the reverb cable. Ferrite cores are widely available, but one source is Newark Electronics. Here's a sample from their catalog showing some possible replacements.