joining transformers together

[glinjik]

Hi was wondering the best way to join these 2 transformers together to increase amps not voltage
it's an old amp transformer the diagram below shows the outputs so the terminals at 17.5v 14v and 18v the 5v i don't need

[Dave]

If they are identical transformers, then you can wire them in parallel. But you must ensure that the outputs are in phase, otherwise you're going to burn the windings.

You can do this by connecting the primaries to mains in parallel and then tying JUST ONE end of the secondaries of each transformer together. You then measure the voltage between the ends which are not yet connected together.

If you're measuring zero or very close to it, you can safely tie those together.
If you're measuring a voltage that is roughly double the secondary voltage of the transformers, they are out of phase and you need to flip one of the windings and try again.
There is the third option, that you end up measuring a small, but still significant voltage. This would mean that the windings between the two transformers aren't matched well enough and tying them together would cause a current to flow between them, killing efficiency at best or burning them out at worst (it depends on transformer output impedance). A remedy for this might be to unwind a turn or two from the transformer that is putting out a higher voltage and trying again.

[glinjik]

Thank you I don't think I was very clear, it's one transformer with 2 identical sets of windings or if you can say two transformers on one core

[Dave]

Identify which windings have a matching voltage on the left and right side and then follow the same procedure as I described above to wire them in parallel.

[tooki]

Thank you I don't think I was very clear, it's one transformer with 2 identical sets of windings or if you can say two transformers on one core

We’d say it’s a transformer with multiple secondaries (which is very common). The thread title definitely makes it sound like you mean two completely separate transformers.

[glinjik]

My apologies

[tooki]

No apologies needed! Just an FYI for learning!

[tunk]

No expert on this, but I think I would load each separately
at e.g. 0, 25, 50, 75 and 100% of max load and measure
the AC voltage. If they're pretty close (wouldn't know what
the definition of "pretty" is) then it might be ok.

[ledtester]

Here's a good video on how to safely detect the phasing on both the primary and secondary sides:

How to use transformer windings in series or parallel - without releasing magic smoke -- TheHWCave
https://youtu.be/MEUdun87ErU