ETA/Valjoux Caliber A05.H31

ETA/Valjoux caliber A05.H31 is an automatic cam type chronograph movement. This movement based on the ETA 7753, from the same family as the caliber 7750, but with upgrades such as a longer barrel for an increased power reserve of 60 hours.

Caliber Confusion:

The A05 series of movements is derived from the ETA/Valjoux 77XX family of movements, however, they are from the post ETA availability era of the watch industry. This means that while the A05 movements are based on the same framework as the 7750 and 7753 the community is used to finding in microbrand watches and non-Swatch Group watches, the A05 calibers are only found in timepieces produced by the Swatch Group conglomerate of watch brands.

Because of renamed movements like the A05 series not being made available outside of the Swatch Group, that also means that official technical documentation is not as readily available. Therefore, much of the information being gathered by the watch community is made by experiential observation, assumption, and hearsay by salespeople and influencers.

It doesn’t help that with the lack of available information and changing of the decades old caliber numbers, the brands using the A05 movements do not appear to be fully transparent about the movements in their watches.

What is generally understood is that various Swatch Group brands are using the A05.H31 in some capacity, albeit under different monikers. For example, Mido is using a similar caliber they call Caiber 60.

Apparently, Tissot is the brand using it as A05.31, although there are few mentions of this caliber number on their site. In most cases, Tissot simply refers to the diameter (in lignes) of the movement in the “calibre” section of their product descriptions:

Despite the A05.H31 being used in Tissot chronographs watches, it is not a “Tissot caliber A05.H31”. Tissot themselves refer to it as a “Valjoux A05.H31” in their description for the PRX Automatic Chronograph.

“Created for those with an eye for design and packed with twenty-first century features in a Tissot case shape from 1978, the PRX Chrono is a must-have on every watch fan’s wish list. This 42mm satin-finished steel watch follows the PRX’s sleek, angular form, adding to the laid-back 1970s sports watch style with a so-called ‘reverse panda’ dial. The PRX Chrono is powered by the Valjoux A05.H31 caliber, which provides the design with the classic 3, 6 and 9 chronograph dial layout and an extended 60-hour power reserve.” –Source

Additional Resources:

• Tissot A05.H31 user manual (pdf)
• ETA caliber 7753