PT primary wiring question

[nsubulysses]

a while back I got a custom transformer from Heyboer and asked for it to have 120V and 240V winding. I thought I would be getting a PT with one common wire then a few different taps like how a Hammond 290HX is but instead when I got the transformer and the datasheet I was confused because it appeared to have two 120V windings. I emailed to ask and they said "if you want 240V operation, connect the black and blue/yellow together in series and use the black/yellow and blue as your mains wiring." oh duh, of course," I though. I hooked this up to a switch so I could switch between 120V and 240V for the mains.

A few months later while glancing at an Ampeg VT40 schematic I realized I might have wired the transformer wrong. I have the black/yellow and black hooked up ONLY when the amp is in 120V mode. I did not hook the black/yellow and blue/yellow together as one primary winding and the black and blue together as the other to use as my 120V mains like how it is wired on this VT40 schematic.

I called Heyboer to ask if this means the transformer might fail one day and they seemed kind of doubtful but did say, "well, I guess if it gets hot enough eventually." A real non-answer!

So I know I'm asking you to guess. But if it were you would you just re-order and replace the transformer??

Here is the transformer wiring and the V4 schematic for reference



Here is another link if the PT wiring diagram doesn't work -- https://drive.google.com/file/d/1ub7...ew?usp=sharing

[Helmholtz]

The Heyboer link is corrupted.

Seems you did not wire the 2 primary 120V windings in parallel. It may be necessary to share the primary current between the 2 windings, if wire gauge is just sufficient for the lower primary current with 240V input.

[nsubulysses]

The Heayboer link is coorupted.

Seems you did not wire the 2 primary 120V windings in parallel. It may be necessary to share the primary current between the 2 windings, if wire gauge is just sufficient for the lower primary current with 240V input.

heres a better link: https://drive.google.com/file/d/1ub7...ew?usp=sharing

[Helmholtz]

To be more precise: If the transformer is meant to be operated with both 120V primaries in parallel, using only one of them may result in critical wire temperature at full load. Primary copper losses will more than double.

[Chuck H]

And to add... No you don't have to replace the transformer if it has not failed. Just wire it correctly.

[nsubulysses]

The whole point of getting the custom PT was so it could be used here or in Europe cuz the band tours there regularly. So if the PT is made to do either 120v or 240v but not really be switchable between the two without rewiring it, then I guess it's not the right transformer for the job :/

Amp is half way across the country and is about 6 months old

[nsubulysses]

I guess what I'm mainly wondering, which is hard to know without getting more info from heyboer, is will it most likely fail or likely be fine. And can I please have a PT with primary with 1 common and a 120v and 240v tap like Hammond 290hx next time please

[Leo_Gnardo]

The whole point of getting the custom PT was so it could be used here or in Europe cuz the band tours there regularly. So if the PT is made to do either 120v or 240v but not really be switchable between the two without rewiring it, then I guess it's not the right transformer for the job :/

Amp is half way across the country and is about 6 months old

If it hasn't failed yet, looks like it's gonna survive.

There are commercial amps, PA gear, other stuff that have series/parallel switches built in. So, it can be done. Just don't have it in 120V mode when plugging into 220-240V.

FWIW I have another view on the two primaries question. In the end it amounts to the same thing, but here goes anyway. Current through each of the two primaries develops enough magnetic field within the transformer to allow it to operate at full rated current/voltage in the secondaries. Take away one of those primary windings, less magnetic field is present and secondaries' voltage will suffer as current is drawn from the secondaries.

Hope you get a second swing at this amp so you can correct the primary situation, & put in a supply voltage switch.

[Chuck H]

I understand this. But it's just not how it's been done most of the time and (therefor) it's probably not something you can automatically expect. There ARE switches that will handle the job. I've bought 3PDT switches rated for HV on a few occasions for things I was doing at the time. I'm sorry, but I'm not going to dig into my archives to look them up now. But I know I bought them at Mouser.

[eschertron]

But if it were you would you just re-order and replace the transformer??

No. Put in the right switch, no more difficult than wiring up a speaker cab for 4 or 16 Ohm operation.
Cheaper and easier than replacing the PT, regardless of how far away and inaccessible the amp is right now.

[nsubulysses]

I used this switch -- https://www.cedist.com/products/swit...crew-terminals


I wired it like this which is I guess incorrect since it only uses one half of the PT primary when in 120V mode

[eschertron]

The switch will work just fine. The poles of each switch go not to input, but to one end of each winding, so that
L1 -> start of one winding, end of that winding -> pole
(other) pole -> start of second winding, end of that winding -> N
in one position the switch connects the end of the first winding to the start of the second (240vac)
in the other position the switch connects the windings to L or N as needed to complete the circuit for parallel 120vac.

I'm going by a schem (first website I looked at) found here: http://www.colomar.com/Shavano/4x12wiring.html

hope that helps!

[pdf64]

Toggle switches aren't a good idea; too easy for it to get knocked inadvertently, with potentially disastrous consequences; unless it's mounted internally, which would defeat the purpose.
Schurter make screwdriver / coin rotary switches for this application, eg https://www.mouser.co.uk/datasheet/2...tpl_-22536.pdf but Mouser are out of stock currently.
I guess that alternatives are available.
This type of 'worldwide' PT primary is super common, I think most Marshalls are like that, so maybe try spares intended for them.

[Helmholtz]

Current through each of the two primaries develops enough magnetic field within the transformer to allow it to operate at full rated current/voltage in the secondaries. Take away one of those primary windings, less magnetic field is present and secondaries' voltage will suffer as current is drawn from the secondaries.

Wiring the 2 primaries in parallel does not increase the magnetic field, they just act as a single 120V primary with doubled copper cross section. The magnetic field depends on amperes x turns and each primary carries half the current so the result is the same as with one primary and full current.

The difference is in the copper resistance and as there are safety limits for max. current per square mil (current density), a transformer having a continuous 240V primary with a 120V tap typically needs a heavier wire gauge for the full winding (requiring more winding space).

[Jazz P Bass]

I would get the slide switch.
https://www.digikey.com/product-deta...3002-ND/181459