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The entire purpose of this site is to help people repair their Thomas Organ Vox amplifiers running and out of the landfills. Many amplifier repair techs will simply refuse to work on these amps, often from a lack of knowledge about them. I hope that providing some knowledge will enable more amp techs to take these on, or help electronically-capable owners take on the job themselves.
The first step is the Vox Owner's Safety Net. This is a 160 page book giving an overview of the Thomas Vox line of amps, with technical descriptions of the circuits involved and how they work. An amp tech might be able to repair your Thomas Vox amp with just this as a background reference. Thomas re-used the same circuits in a mix-and-match fashion to make their amps, so understanding a few things make you more capable of understanding them all. The book is available for printing on demand.
In addition, there is a handbook for how to repair the Thomas Vox amplifier footswitches. These were frequently lost or broken, but it's entirely possible to repair them, or build a replacement from modern parts.