Presence control for guitar amplifier

A guitar amplifier which comprises at least one channel, each channel having:

• (a) a differential amplifier having first and second amplifying members, each of said amplifying members having a control element, an electron emitting element and an electron collecting element, the electron emitting elements of said amplifying members being commonly coupled and an output amplifier coupled to said differential amplifier and having an output terminal selectively coupled to said control element of said second amplifying member of said differential amplifier as a feedback path;
• (b) an input terminal coupled to the control element of said first amplifying member;
• and
• (c) a user-adjustable presence control circuit selectively coupled between said control element of said first amplifying member and ground to, when selected, increase the perceived brightness by increasing the resistance from ground and lessening the roll off of high signal frequencies applied to the control element of the first amplifying member and boosting high frequencies when said feedback path is completed.