Amplifier Biasing Information - Rivera Knucklehead Reverb KR 55T Owner's Manual

Amplifier Biasing information.

All Rivera amplifiers except for our 25
biased) are class AB. These amps have a bias adjustment potentiometer located on the
motherboard in the chassis. This requires removal of the chassis from the cabinet. This
must be done by a qualified technician or authorized repairman.
WARNING: We do not advise any owner of our amplifiers to
remove the chassis from the cabinet as dangerous and possibly
lethal voltages exist in this chassis. The information presented here
is for qualified personnel only and is for reference purposes only.
The most accurate way to measure the bias of our amplifiers is by observing the
cathode idle current of the output tube, and observe a Sine Wave from the output stage
on an oscilloscope, with the waveform just before output stage clip. The Cathodes of
the output tubes are all tied together and in parallel. They are connected to the chassis
ground through an internal switch on the Speaker Jack #1. When there is no plug
inserted into the speaker Jack #1, the switch is open, and the output tubes do not
conduct as the cathodes are lifted from chassis ground. Conversely, when a plug is
inserted into speaker Jack #1, the cathodes conduct through this switch to chassis
ground, thereby enabling the output stage to operate. This feature allows easy idle
current measurement as well as protecting the amp for operating without a speaker
load.
Use the DC Current measuring capability of a preferably digital multi-meter to observe
this current.
th
Anniversary model (this model is cathode
Knucklehead Reverb 1.0 p 38