There are two type of electricity in tube amplifier circuit, AC voltage and DC voltage. AC is alternating current at a particular frequency, basically is a sine wave or a combined sine wave like audio signal. DC is just a static voltage like a 9v battery or an audio power supply 280v.
In tube amplifier, AC voltage can really be DC voltage that changes value, suppose you measure 280v multiple time in a few second, you will get 290v, 280v and 270v. This wound be considered ac signal which to have a 20v changes.
When we put an audio signal into our circuit which is in Dc voltage.We actually modulate our AC signal with DC voltage which then will be amplified to create a new AC signal which is again sitting on the DC voltage. Then we use a coupling capacitor to filter out the DC voltage. What''s left is the 20v audio signal oscillating around zero voltage.
So, the first thing to check in a tube amplifier are the DC voltages., even before we start listen to any tube amplifier.
