What is a 5 Tube System?
Look for the Honeywell Primus 1000 System.  

Early systems were 5 tube - an EADI and EHSI for each crew plus an MFD (Nav/Wx display) as shown below:  


source: swaviation.com

Later ones were 3 tube - 1 PFD on each side plus the MFD as shown below:   


source: airliners.net

The symbol generators are shown in the MMEL as they have reversion capability. The 'tubes' or the actual displays would be listed under section 31 Indicating/Recording Systems but aren't included in the MMEL as they are always required.  

Section 34 Navigation does allow for functional portions of the displays to be inop.