Bottlehead Filament Choke FC-1

The relatively large leakage inductance which is due to the two separate bays acts as a common-mode choke to reduce common-mode noise on the filament winding.

Each bay is bifilar wound and can be connected in series or parallel. In series it has 30mH inductance, 0.5 ohms DC resistance, and a rating of 1.25 amps as a choke-input filter producing 5vDC. In parallel it has 7.5mH inductance, 0.125 ohms DC resistance, and a rating of 2.5 amps as a choke-input filter producing 2.5vDC.

With these specs one can use PSU Designer II from the Duncan Amps web site to determine the required power transformer filament secondary voltage and current for your application. Be sure to allow for rectifier voltage drops, etc. It will usually be best to err on the high side in terms of the filament secondary current rating. For example, a 6.3V secondary rated for around 5A might help hit the target of 5VDC with a 300B. But please model your circuit rather than assuming these values are universal.

Dimensions:

• Mounting hole centers are 2-13/16″
• Actual length of channel frame 3-1/4″
• Height 1-15/16″
• Width of bobbin only is 1-7/8″, solder terminals protrude an additional 1/4″ on one side of bobbin.

Wiring the FC-1 —
Connections common to both tubes:

Connect FC-1 terminal 1 to one filament pin terminal on the tube socket. Connect FC-1 terminal 6 to the other filament pin terminal.

Connect the positive filament supply voltage to FC-1 terminal 10. Connect the negative filament supply voltage to FC-1 terminal 5.

Additional connections for 300B:

Jumper FC-1 terminals 7 and 9. Also jumper FC-1 terminals 2 and 4.

Additional connections for 2A3, 45:

Jumper FC-1 terminals 10 and 7. Jumper FC-1 terminals 9 and 6. Jumper FC-1 terminals 5 and 2. Jumper FC-1 terminals 4 and 1.