Jadis JA30 Owner's Manual page 11

Tubes Installation:
First of all, remove the grid which protects the tube sockets (be sure
the mains connector is unplugged).
Attention: For mounting the tubes, do not force them, this may
damage the sockets. Be careful at dressing power tubes because they have
stop pins on the central parts of the mounting base.
Mount first the ECC82 tubes, the ECC83 tube, and then mount the power tubes.
The tubes placement is shown at page before.
Replace the grid and never remove it when the amplifier is
connected to the mains voltage, whether it is running or not.