Dropping Tube Filament Voltages

Occasionally it's necessary to lower the filament voltage in an amplifier to maintain a reasonable constant 6.3vac. Some transformers are either over-wound or wound for primary voltages of 115vac instead of the more common 120vac rating used in most parts of the USA. Of course, the most common solution is to install some very low value resistors in the feed lines of the filament circuit. However, this can be a real pain if you only need to drop an amount less than 1 volt at typical total currents in the order of 4-5 amps as seen in many guitar amplifiers. This would require a dropping resistor of about .2 to .25 ohms @ 4-5 watts. Although low values resistors are available, they aren't always easy to get.

A simple alternative is to use a back to back diode pair in series with the filament feed. The diodes will drop a constant, predictable voltage (.7V for silicon, .3V for Shottky types, etc.) regardless of current draw (obviously making sure to stay below the maximum rating of the devices). The wiring arrangement looks like this;

The dropped voltage can be adjusted by using series strings of diodes and even combinations of standard silicon and Shottky diodes.

Finally, if your transformer does not use a center tap, this technique still works will with conventional artificial center tap resistors (typical 100 ohm resistors from each 6.3v winding lead to ground). The 100 ohm resistors would be installed from the side of the diodes closest to the windings to ground.

Bob Dettorre, VP Engineering, DST Engineering
bob@dst-engineering.com

Copyright (c) 2003