Elementary heater/filament ground question (JMI Vox AC10)

[seven]

Hi there,

Apologies for the seemingly simple question in advance.

I have a newly acquired 1964 Vox AC10. Original electrolytics so changed them out and installed a fuse.

I was comparing the amp with the schematic and noticed something that I could use some help in trying to understand.

I see that pin 5 (heater) on the ECF82 appears to be grounded.





The schematic shows pin 4 grounded:



Is there an issue with this amp having pin 5 grounded instead of pin 4? I'm not sure if the schem has an error or the amp was wired incorrectly but I'm fairly confident it doesn't matter for the other tubes but pin 9 on the 12AX7 is confusing me here and overall I don't think it matters whichever pin is grounded but want to confirm.

More importantly, *why* do the filaments need to be grounded?

Thank you for your patience, wisdom and understanding.

Chris

[Helmholtz]

Makes no difference if pin 4 or pin 5 is grounded.
All heater circuits need to have a ground reference.
With floating heaters there would be strong heater hum/buzz as well as unwanted coupling between different stages.

Just had a deja vu at the ampgarage.

[Jon Snell]

Absolutely no difference at all.
These were built when valves has less leakage between heater and cathode meaning a centre point was not needed nor the jacking up with DC volts to attempt to stop leakage hum with the use of inferior valves.
AS long as one side is referenced to chassis, that is all that was needed.

[seven]

Thank you for the explanation Helmholtz and Jon Snell!