Tube theory

Most tube amplifiers have the following sections:

• A power transformer, responsible for change the AC input voltage (230V in Europe) to a voltage required in the amp (6.3 VAC for the heaters, and 200 VAC and higher for the tubes themselves).
• A rectifier, changing the AC voltage into crude rippled DC voltage (traditional this was done with a rectifier tube, but nowadays mostly with silicon diodes).
• Filter caps, to even out the ripples from the rectifier, creating a “smooth” DC voltage.
• The PreAmp section, one or more tubes for increasing the gain from the guitar. This part is most sensitive for noise, as it works with very low gain as received from the guitar.
• The Power tubes section, to power up the gain. Most used configuration are single ended (using one tube) and push-pull (using two tubes, or one dual triode tube). Parallel configurations can also be used (for example using four tubes for push-pull mode, two paralel as in the VOX AC30).
• The Output transformer, to convert the high voltage - low current output from the power tube section to a lower voltage - higher current output to drive the speakers.

Speaker and guitar coils are not part of the amplifier itself, but are off course necessary and have an impact on the sound. Using an amplifier without a speaker (loud) will probably damage the output transformer, so that’s not a very good idea.

Uncle Doug is a very popular Youtuber (in the tube amp community ;-)). He made a very good series explaining tube theory:

• How tube amps work - Part 1: The Power Supply
• How tube amps work - Part 2: The Pre-Amp and Power Amp