The amplifier makes a noise like a motorboat

[FAQ Team]

This, funnily enough, is called "motorboating" and can be caused by many things. It usually goes subsonic, phut...phut... but may vary in frequency. It is a low frequency oscillation.

Generally however the following causes it:

* Positive feedback instead of negative feedback i.e. usually caused by the Output Transformer secondary being wired the wrong way around and often determined by experiment/accident. With WAD kits if you follow the wiring diagrams faithfully then normally all works correctly. If you get motorboating, only change around the secondary winding(s) not the primary windings as well.

* Poor de-coupling capacitors, generally not enough capacity or have "dried out" over a long period thereby reducing capacity.

* Excessive interstage coupling capacity ie changing the OP valve grid coupling capacitors from say 0.47mF to 1mF in an effort to increase bass response. Not always a good idea.

* Incipient instability, where the amplifier goes into motorboating after a large transient like a bass drum