W5DXS - Heart Of Texas DX Society



Buck Boost Transformer

  Line voltage, which is the voltage present at your wall outlet, has been on the increase for the last few years. Voltage can vary depending on grid and load conditions. The opposite can also be true meaning voltage can be low.

  I monitored my line voltage for several weeks and the results were surprising. It varied between 125v and 126.6v. Since I use a lot vintage tube equipment that was designed for 110v to 115v, I began to wonder what effect the increased voltage was having.

  I researched adjusting line voltage and it became apparent either a variac or a buck boost transformer was needed to lower the voltage. Variacs are expensive, so I decided to use the buck boost transformer method.

  A buck boost transformer is configured so that the transformer secondary is either in phase or out of phase with the primary. With the secondary connected out of phase the transformer will lower voltage. With the secondary connected in phase the transformer will raise the voltage.

  Filament transformers have secondary voltages more suitable for buck boost situations. Choose one that has the secondary voltage you need with proper amp rating. I chose a filament transformer with a 6.3v secondary rated at 8 amps. The 8 amp rating is adequate for my load requirements and lowering the line voltage by 6.3v would put the line voltage into the acceptable limits.

  I assembled the buck boost transformer in a discarded power supply cabinet complete with on/off switch and fuse for safety. When first tested the output voltage was 6.3v higher than my line voltage. Reversing the leads of the secondary transformer lowered the line voltage by 6.3v. Be sure and check your output voltage before using.

  Curious as to what the 6.3v difference in the line voltage would make, I did a test with my 5U4G power supply that I used with my 6L6 transmitter. The results were quite surprising. See the table below:

Comparison of Voltages

Line voltage = 125.5v

Line voltage = 119.2v

Filament voltage = 6.9v

Filament voltage = 6.4v

B - voltage = 382v

B - voltage = 361v

  I now use the buck boost transformer with all my vintage tube type equipment. Also I have noticed that my Collins R - 390A runs considerably cooler with the 119v versus 125v. The Collins R - 390A was designed for 115 volt - 230 volt operation..

  There are several important notes to consider when you are constructing a buck boost transformer. Be absolutely sure and double check your output voltage before using. The line inputs should be fused for safety. Since the primary and the secondary windings are connected in series, the transformer offers " NO " isolation protection. The transformer secondary will carry the load so it must be rated accordingly.

The buck boost transformer has been an interesting and useful project.

73, James, KC5LDO

One final note, if you fail to double check your output voltage, this may happen.

This is not a good thing!

Articles written by James Tobola - KC5LDO
and reproduction, publication, or duplication of this article, or any part thereof,
in any manner is prohibited without the express written permission of the author.

Thank you for visiting our page and please sign with your comments into the


or drop us an


These Web Pages were created by using 100% Recycled Electrons.

Flag Counter

From the HOTDXS Workbench - How To Build It Yourself

SPC Tuner

Power Generator

One Tube Receiver

Portable Vertical Antenna

1929 Hartley Transmitter

6L6 Transmitter

5U4G Power Supply

AC-1   6V6 Transmitter

6AQ5 CW Transmitter

Pi Network Tuner

6X4 Power Supply

Type 80 Tube Power Supply

6SL7 Twinplex Regenerative Receiver

L - Network Tuner

6AH6 Electronic T/R Switch

R-390A Rack Mount Speaker

Straight Key Gallery

Equipment Rack

6DQ6 Transmitter

Buck Boost Transformer

T - Network Tuner

Return to the Table of Contents