Microwave Oven Transformers (MOTs) can KILL you because they produce about 2100 volts!  They are totally unforgiving.  Do not work with them unless you really know what you are doing.  I take no responsibility for any information about them found on or through this website and take no responsibility for your working with them.

MOTs salvaged from discarded microwave ovens can provide inexpensive high voltage transformers for your home-brew linear amplifier.  There are many videos and articles on the web showing how to knock out the current-limiting shims and removing the 3 volt winding. Removing these two items allows the 2100 volt winding to be used as efficiently as possible.

The HV secondary winding can also be removed and a new, low-voltage secondary can be wound in its place.  If rewinding, figure about one volt per turn but overshoot a bit.  It’s easier to remove an un-needed turn but harder to add an additional turn.

If, for example, you want a 13.5 volt DC supply, wind about 20 turns so that you overshoot the AC requirement, and test the finished supply under load.  If the AC input to the regulator is more than you want, just take a turn or so off the xfmr secondary.  It is to some extent a cut-and-try project.

In my homebrew low voltage supplies, I wound the replacement secondaries with #14 house wire, which is good for 15 amps.  I put a layer of electrical tape around the frame where the new winding was to be installed.  These supplies have run about fifteen years with no problems.

In my homebrew high voltage supply, I use two of them with their primary windings connected in series.  Having these two coils in series adds enough primary winding turns (i.e., AC resistance) that the house circuit breaker will not trip if the unit is energized at precisely the wrong part of the AC input cycle.

The secondaries are also connected in series, which means that the grounded end of the secondaries (grounded to the transformer frame) can remain grounded.  The two then look just like a center-tapped “regular” transformer secondary, with the center tap grounded.  Thus the transformer frames have no voltage on them.

Using two means that each primary and each secondary is run at half voltage.  This in turn means that the transformers run cool, there is no strain on the insulation and they produce the full ~2100 volts.  No fan, no heat-sinking and no soft-start needed.

Do try to use two MOTs of approximately the same wattage and resistance or else one of the secondaries will try to take more of the load than the other.  Since it is at half voltage this probably won’t matter much but it’s best to keep the load as equal as possible on each transformer, in my uninformed opinion.

Clever switching of the AC supplied to the primaries also means that the HV supply can provide half voltage to the tube plates when tuning up.  Once tuned, go to full voltage.  See a future power supply drawing for the simple 500/1000 watt Linear amplifier on this website.

Here is a technical reference that might be of some value.  It discusses the weakness of MOT secondary winding insulation.  Also discussed are ways to overcome the initial AC surge incurred when AC power is first applied.  But if two transformers are used in series, these problems won’t occur.