Hughes & Kettner Attax 100 no sound.

No, this is not normal. At 6.3 V pins 4 and 5 are connected together, probably right at the tube base. One side of the 6.3 V is than connected to these pins 4 and 5, while the other side of the 6.3 V is connected to pin 9. In this way the two filaments are connected in parallel.

In the diagram you posted, the two arrows at "4" and "5" of the filaments indicate the connections inside the tube with pin 4 and pin 5. They often depict it like this to prevent more crossing lines in the diagram, so as to make it a bit easier to see what is connected to which pin.

It is impossible that the 12AX7 is glowing with 0 V between pins 4/5 and 9, even if one of the two filaments would be interrupted.
 
1) filament voltage is 6.3VAC

2) as schematic shows on bottom left page 2, power transformer T1 has 3 secondary windings:
* main one: 34+34 VAC feeding bridge rectifier B1 and capacitors 4700uFx63V C64 and C65 getting +45/-45 VDC

* Filament one: 6.3 VAC feeding 12AX7 through Fuse F3 and connector Plug 4 pins 1 & 2 .
Said voltage continues into (ribbon cable?) Plug 3, pins 1 & 2
Heater wiring is called Hz1 and Hz2

As mentioned above, to avoid drawing a loooonnnnggg criscrossing pair of lines to complicate an already complex drawing, they indicate them by name.

Hz1 and Hz2 at the transformer and connector reach tube pins labelled Hz1 and Hz2, at the tube socket called 4+5 joined together getting Hz1 and 9 getting Hz2

So my test is checking whether both filaments get 6.3V AC

IF they reach said tube pins, no need to check *all* the path, which must be a nightmare.

If they don´t, we must check why.
Starting with F3 condition and plugs/connectors/wire ribbon?

So far since at least 1 filament lights, the other is suspect.

You may pull the tube and measure resistance between pins 4-9 and 5-9 straight at the bottle .

Use a low ohm scale (200 ohm?), I expect a low value, a few ohm, filaments have even lower resistance when cold.
 
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