Elementary heater/filament ground question (JMI Vox AC10)

[seveneves]

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

[sluckey]

Heaters need to be connected to a ground reference in order to reduce heater induced hum. It doesn't matter which heater pin is connected to ground.

BUT...

That's 1950's technology. Move ahead 20 years and a better method of providing a ground reference for the heaters has replaced that old '50s ground reference. Simply connect a 100Ω resistor from one of the heater wires to ground. Do the same for the other heater wire.

[LOUDthud]

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

When a transformer is wound, usually the heater winding is applied over the high Voltage secondary. This creates a seemingly small capacitance (perhaps 100pF) between one end of the secondary and the heater winding. If the heater winding is not grounded somehow, a large AC Voltage will appear as a common mode Voltage on the heater winding that can be hundreds of Volts AC to ground depending on how much capacitance all the heater winding has to ground. You can measure this Voltage with a DVM, but I recommend that you remove all the tubes to avoid insulation breakdown. This Voltage will cause enough hum to clip the output of the amp. Grounding can be in the form of a simple ground to one side of the heater winding, grounding the heater center tap if the winding has one, or a virtual center tap composed of two resistors.

[seveneves]

Thanks @sluckey and @LOUDthud for the explanation and also to the former for the 100 ohm resistors tip!

[sluckey]

Thanks @sluckey and @LOUDthud for the explanation and also to the former for the 100 ohm resistors tip!

Just to be clear about the 100Ω resistors. You can't just add them to the existing amp. You would have to first remove the existing ground connections from the filament circuit and then add the two 100Ω resistors. May not be worth the effort.

[seveneves]

Just to be clear about the 100Ω resistors. You can't just add them to the existing amp. You would have to first remove the existing ground connections from the filament circuit and then add the two 100Ω resistors. May not be worth the effort.

Got it. Thanks again!