use a Variac as a variable output load device?

[capacitorless]

Bear with me, smart folks - this is new territory for yours truly.

I have a 400hz/2 amp (likely Navy) Variac that I'd like to use for something. It's of course not useful for standard civilian Variac duties. I'm wondering if it could be incorporated into a load device, for (in this case) a tube guitar amp.

It would see no more than 40w and typically lots less, and it's beefy enough that I'm not worried about it getting hot (other than maybe changing the resistance some). Any thoughts? Dumb idea? I'm known for those!

Edit: http://forum.metropoulos.net/viewtopic.php?t=24736 says this is doable and might sound decent. It's important to put a parallel resistor across the variac. I would like to make this a more complex, inductive load, though. I'll get it out of storage and check the resistance and inductance at a few different settings.

 

[MagnetoSound]

400Hz?

 

[Brian Roth]

400Hz?

I'm not an expert on things such as AC power used in aircraft and military applications, but 400 Hz is commonly used....even in 2024. One reason seems to be that the size of any required transformers are much smaller than those operating at 50/60 Hz, hence saving space and weight.

FWIW, I believe that even new DVM AC specs extend to 400 Hz because of the ongoing usage of the 400Hz systems in aircraft.

Bri

PS....many years ago I wondered what I was hearing on an airline flight when the flight attendant was making announcements....some sort of "test tone" in the background???? It was that 400 Hz power system bleeding into the PA system!

 

[merlin]

I have a 400hz/2 amp (likely Navy) Variac that I'd like to use for something.

400Hz 2A translates to 50Hz 250mA. Measure its inductance at 50/60Hz, then we'll have enough info that we can come up with a novel use for it!

 

[gyraf]

.such a variac is a nice continously-variable inductor, I see a fully-parametric passive filter in the making..

/Jakob E.

 

[capacitorless]

We picked up a couple of them in a pawn shop/curio store near Pensacola, home of a major naval air station in NW Florida. Lots of interesting used/old/decommissioned military electronics stuff can show up there from time to time.

I didn't know anything about 400Hz either, until my brother & nephew were trying it out in a conventional way, and having it get hot and not working like it should.

https://forums.mikeholt.com/threads/60-hz-transformer-operating-at-400-hz.81425/ :

It all has to do with the flux density of the magnetic ciruit. As the frequency goes up the amount of required core steel goes down. And so, as the frequency goes down, the more core steel is required.

 

[JohnRoberts]

We picked up a couple of them in a pawn shop/curio store near Pensacola, home of a major naval air station in NW Florida. Lots of interesting used/old/decommissioned military electronics stuff can show up there from time to time.

I didn't know anything about 400Hz either, until my brother & nephew were trying it out in a conventional way, and having it get hot and not working like it should.

https://forums.mikeholt.com/threads/60-hz-transformer-operating-at-400-hz.81425/ :

Historically aircraft power distribution used a lot of 400 cps power, so they could enjoy smaller motors and transformers.

Don't know about passing audio.

The classic variac application for guitar amps is to variac down the mains voltage to induce saturation at lower power output levels.

JR

 

[capacitorless]

Yup, as well as a bench supply to bring up mains power slowly, sometimes in series with an incandescent light bulb to look for shorts/excessive current draw. I've been using them in both roles in guitar amps and other equipment for ages. In my case, I'm looking for an alternative way to use an otherwise unusable piece of hardware.

Anyways, I'll see if any of my rather low budget meters come up with reasonable inductance values. I need to invest in a good LCR.

Very intrigued by trying to use it in an eq circuit. I'll have to see if my brother still has the one I gifted to them. A pair of passive, fully parametric mid eqs could be pretty cool.

 

[capacitorless]

Just fished out of storage. I have yet to find a meter here that does inductance. I Googled again for specifications before posting these photos, but this time around I can't find any (I used a few combinations of parts of the labeling).

It's a General Radio M2; minimum 350Hz, rated at 2.4A. I'm guessing the M series is for military/civilian aviation use.

Note the "special model" notation, but it does look like it's set up as a standard variac would be. It measures 7R4 between the outer connections. Dusty (I'll clean it up) but unused.



Edit: nice brush assembly:

 

[merlin]

Anyways, I'll see if any of my rather low budget meters come up with reasonable inductance values. I need to invest in a good LCR.

Few meters can measure transformer inductance reliably. Do it the old fashioned way: hook it up to a 60Hz voltage supply (about one tenth of its rated 400Hz voltage)*, with a 10 ohm resistor in series. Measure the RMS voltage across the resistor and calculate the current. Ohm's law then tells you the impedance, and from that the inductance can be estimated: L = Z/(2 pi f)

Another test you can do is to hook it up to a variable 60Hz supply (i.e. another variac). Set the Navy variac to somewhere in the middle of its rotation and view the output waveform on a scope. Then gradually wind up the supply voltage until you see the waveform develop a sort of crossover distortion -that tells you when it is starting to saturate. You now know what maximum voltage can be applied at 60Hz.

* I'm no military expert, but I'm guessing it was intended for 115V 400Hz originally, so it ought to be able to handle about 17V at 60Hz. Gyraf's suggestion is looking most likely as an audio application.

 

[capacitorless]

This rocks as I have a 60hz Variac stored away, a Tek 465M (another retired Navy unit) and a 10 ohm resistor. I'll get these set up and run both tests.

 

[FIXITNOW]

Interesting project Have this 400hz 3 phase one general radio company variac type m2 3 gang , this looks like a good use for it