Hi, i have a A 15 mk2 for about 8 years, recently the channel ++ started to whistle. The whistle appears from the volume to two and increases with the increase of the volume, at half gain. I think it isn't a problem of pre microphonic tube, have been tested, and anyway replaced all the pre tubes, included the phase inverter, could be the power tube (never change but they sound good...)?
Which components can I control?
Many thanks