Many amplifiers are prone to “ghost notes.” It helps to know what kind of amp we’re talking about.
Certain capacitor brands, like F+T, LCR have high leakage current. Other capacitors for coupling have ESR problems, sometimes caused by the physical construction, and lead type. Pure copper leads reduce this, and tubular capacitors are less prone. This is because they do not have any flat sides for the signal to bounce off of.
Some amplifiers like Fender Black/Silverface models that have the ghost note issue can be reduced by cutting the phase inverter “output” coupling capacitor values in half.
Others generally are just suffering from old, or leaky filters. Vintage amplifiers like Vox AC30s have very low filtering, I believe this was not because of design, but more as an economical approach to save money. The same is true with many amplifiers designed in the 1950s. Increasing the filtering can cure the ghost note problem.
With Marshalls there are two ways to help cancel this out:
Install a luf 600-1000V non-polar plastic capacitor across the last decoupling electrolytic in the line. This is the last filter, which filters the supply for the preamp. The capacitor will be wired to “by-pass” this filter, one lead to ground, the other to the capacitor lead.
Increase the filtering. On 50 watt models, it seems to take another 50mf across the main B+, and another 50mf for the screen supply.
For 100 watt Marshalls expect to add another 100-150mf across the main B+, and at least another 100mf for the screen supply. Now, the amp will be stiffer on bass notes, if not acceptable, you can add anywhere from a 100-200 ohm 209 watt resistor in series with the output transformer center tap, this will give back a looser feel.
Source: https://mercurymagnetics.com/pages/SSN/pages/GhostNotes.htm
