Chassis grounding... one leg of a 6.3V AC tube filament supply?

[systemtruck]

Hi all,
I’ve noticed sometimes, i suppose mostly in old tube radio/stereo schematics, that a filament output from transformer will have one leg grounded to chassis.
This is pretty different than the standard (seeming) 6.3 AC filament setup where sometimes a ground is created in the middle by adding a pair of resistors or a pot to adjust hum.
One example has BOTH.. this is the Fisher R-20 unit from the late 1950’s, which is a radio and a mono amplifier.
One system of 6.3 filament is dedicated to the audio, and the other is dedicated to the radio section and 3 indicator lamp bulbs. The latter has one leg grounded. Most of the schematic attached below..

Can anyone explain what this does and why it’s done? And what would happen if the bottom leg was NOT grounded? Further more, is there any difference between the two transformer 6.3V output pairs, if all circuits were disconnected?

Thanks so much !

 

[ruffrecords]

Tubes have a maximum heater to cathode voltage specification. If the heaters are left floating then there is no telling how big this voltage might be, not to mention the fact that this will certainly lead to a high level of hum. So there needs to be a dc connection between the filament and 0V (because the cathodes are usually connected via a resistor to 0V) sot he heater to cathode voltage is defined. You can do this by connecting one side of the filament to 0V or by connecting a heater winding centre tap (either actual or virtual (the two resistor method)) to 0V.

Cheers

Ian

 

[Matador]

Can anyone explain what this does and why it’s done?

It's also must easier/faster/cheaper to use the chassis as a return rather than running a dedicated wire.

 

[ruffrecords]

Looking at the schematic you posted, it seems the humdinger heater supply is only for the audio tubes which makes sense because the audio stages are sensitive to hum. The separate winding and heater string feeds all the rf tubes. These are not really sensitive to hum but you do need to stop RF propagating along the heater wring which explains the additional LC circuits used.

Cheers

Ian

 

[Rob Flinn]

When I have been point to point building in the past I have experimented with filament grounding. What I found was that just connecting one side of the filament secondary to ground made a much, much bigger difference to hum than changing from one side grounding to centre tapped grounding. This was to the point that I didn't notice much of a difference.