Center-Tapped Filament Transformer

Thread Starter

patlaw

Joined Sep 7, 2010
6
Merlin Blencowe in this Tube Preamp book says that no current flows in the center tap of a 6.3 v filament transformer. Each leg to the center tap measures 3.25 vac. Why does no current flow in the center tap? I used to know, but I've forgotten.
 

dl324

Joined Mar 30, 2015
12,437
If you're not using the center tap, then no current would flow in it. If you use it, then current has to flow.
 

crutschow

Joined Mar 14, 2008
26,340
The only current that flows is to whatever load is connected to the center-tap.
If the meter is the only thing connected, then the only center-tap current is the small current that your meter takes to do the measurement (typically a 10 megohm resistance).
 

OBW0549

Joined Mar 2, 2015
3,566
Merlin Blencowe in this Tube Preamp book says that no current flows in the center tap of a 6.3 v filament transformer. Each leg to the center tap measures 3.25 vac. Why does no current flow in the center tap? I used to know, but I've forgotten.
If I recall correctly (it's been many years since I last used "vacuum-state devices"), the center tap on filament transformers was normally connected to chassis ground to minimize mains frequency "hum" being induced in sensitive, low-level signals. Since the filament transformer only supplied current to the tube filaments, no current flowed through the center tap.
 

AlbertHall

Joined Jun 4, 2014
10,894
Alternatively, the transformer had no centre tap but the track of a pot was connected across the filament winding and its wiper connected to ground then you could adjust the pot for minimum hum.
 

MisterBill2

Joined Jan 23, 2018
7,964
Feeding the filament circuit with a small positive voltage is what we did to keep it from introducing hum. yes, it was more complex than just grounding it but it did work well. And now for some circuits there is a great deal of current tha flows in the heater center tap. Directly heated amplifier tubes in a grounded grid linear amplifier are a prime example. BUT the current flowing through the center tap is not the induced transformer current, it is the cathode current from the heater-cathode.
See the Collins "30L1" linear amplifier circuit for a good example.
 
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