Line Trans as an OT?

[drewl]

Got a few line transformers from work, you know the type used in factories/buildings that run on 70v dc and connect to the speakers, used in long distance wiring.

Provided the trans could handle the 300v or so, would they work as an output transformer?

They sound good as an attenuator, the biggest one I have is only a 35w Bogen one.

I'm thinking of building a little 5w amp or so with stuff I have lying around, could always use one as a built in attenuator.

I have one of those cheap little Vox AC4 amps that needs an OT, I could try one in there as an OT.

Right now it has an old Ampeg 15 w Gemini OT in it that one half was bad, using the good half single ended.

[Enzo]

You could try it, and it works or doesn't. It might be "wrong" from a technical point of view, but this is a guitar amp, not a hifi, and "wrong" might just sound right to you.

[drewl]

That is how many things were invented, by trying something "wrong"

Outputs are 8ohm, primary about the right dc resistance.

[Helmholtz]

Got a few line transformers from work, you know the type used in factories/buildings that run on 70v dc and connect to the speakers, used in long distance wiring.

How DC?

If you mean 70VAC sound distribution transformers, those have a rather low primary impedance around 500 Ohm @ 10W.
Single ended outputs typically want something like 10k primary impedance.
So used as an OT, output will be very low, probably only mWs.

Those transformers are rated for 70V, so don't expect them to handle much more.

[loudthud]

The primary is meant to be connected to an amp the produces 70VRMS at full power, whatever power that happens to be. The taps on the primary are rated in Watts so you can select how much power will be supplied to the speaker on the secondary side of the transformer. So, do the math P=E²/R. A 1W tap will be 4900 Ohms. A 2W tap will be 2450 Ohms. Note that the primary may not be able to tolerate much in the way of DC current. A push-pull arrangement will cancel the DC. If you want much power, consider a pair of MOSFETs.

[mozz]

These are often used on home builds, low power. I think there's a Australian guitar website where i read about it.

[Helmholtz]

If you want much power, consider a pair of MOSFETs.

Can you elaborate?

[loudthud]

As was mentioned above, the primary impedance is low on the higher power taps. Also, the wire gauge is heavier on those higher power taps. A 15W tap would be something around 325 Ohms. I don't have my books nearby, but driving that with a pair of tubes looks like a lot of current and not much Voltage. I think it works out that you would use the 15W tap as the center tap, the "0" tap for one side and the 3.75W tap as the other side. Maybe there is a tube out there that would work, good luck finding it.

[Mick Bailey]

I think there's a Australian guitar website where i read about it.

Some info here, though they refer to 100v transformers, which give a more useable primary impedance for tube amps; https://www.ozvalveamps.org/optrans.htm.

[J M Fahey]

In my book, a TERRIBLE idea, sadly popular because itīs cheap/easy.

Actually "working" does not seem to be part of the equation.

Main problems are two (and those are gross ones)

1) even if turns ratio in theory gives you useful primary impedance ,as in: putting a 16 ohm speaker into a 1 ohm reflect 16 times higher primary impedance, inductance will NOT be 16X higher but same, so chopping off all bass, overloading tube in a bad way at low frequencies, etc. A mess.

Of course tyros who build those amps are inexperienced, itīs their first or second build, and are full of builder pride and expectations.
Which does not mean those amps are actually any good, they are delighted it produces any sound at all.

2) they are designed for 70V RMS,some for 100V lines ... tube amps have 300 to 600V RMS at the output.
What could go wrong?

3) they are NOT repeat NOT designed for any DC at the primary, which will saturate core and murder any little inductance they started with.

4) in any case a plain 120V to 240V primary to 6-9-12V secondary will provide a much better impedance matching and are definitely better rated for those voltages.

Canīt stand DC either but at least a 120+120VAC primary transformer can be wired as a PP one with somewhat usable specs.

[Mick Bailey]

I've sometimes substituted a mains transformer for use as a P-P output where I've wanted to be sure of a transformer fault before ordering a replacement OT and not had a suitable replacement to hand. They work OK for testing - sometimes pretty good at low-to-medium levels.

[drewl]

So I actually got around to trying this, as this is the the first week in a long while I haven't been swamped with work.

I wired up an EL84 socket to a Marshall 2204 style preamp I had built years ago to drive tube power amps I have, which sounded great.

The preamp is only around 250v DC, tacked wires to the socket, used the .25W tap of the line trans, output Z is 8 ohms and the damn thing actually sounds okay!
It's putting out about 4-5 watts

I'll probably order a few little Weber small OT's for this and a couple other mini amps

But I just wanted to report that it actually works and sounds pretty good.