Astre Systems - Filament Supply Retrofit

Considerations For a Townsend
Filament Power Supply Retrofit

Due to variations in design of Townsend transmitters, it may be necessary to replace the vacion supply as well when replacing the filament supply transformer. Some Townsend transmitters use a filament power transformer with both 26 VAC and 8 VAC secondary outputs. The vacion supply is powered by 26 VAC from the filament transformer. Since the original filament transformer is removed, the vacion supply will need to be changed as well. Many of the original Townsend vacion supplies came with a meter with a scale that looked like the power output meter. That is, the scale went from zero to 150%. This meter is often a 200 ua meter and is connected as a voltmeter across the secondary of the vacion circuit. Since only the vacion CURRENT is of concern, not voltage, this meter is of little use. Additionally, the 200 ua meter draws 100 times the current that the vacion itself draws under normal circumstances. We suggest that this meter be removed and replaced with a meter circuit that indicates vacion current, typically 1 or 2 microamps. The vacion circuit operates at cathode potential and usually is installed on the filament power supply mounting board.

The filament transformer, rectifier bridge, and filter capacitor are removed and replaced by a voltage and current regulated power supply. This power supply is normally powered by 120 VAC, however, since the whole supply operates at cathode potential, a one to one isolation transformer with 40 kVDC secondary insulation is used to "float" this supply. Existing filament voltage and current meters are sufficient and do not need to be replaced. The circuit breaker feeding the power supply will need to be replaced. This modification can usually be done in about 3-4 hours per amplifier cabinet.