Using foreign 100v mains transformers for builds in the USA?

[olddawg]

So... I'm thinking of using a used tube amp transformer designed for countries with 100v mains in the USA. I'm thinking the major issue is the filament voltage will be about 7.2vac. Sticking a couple of diodes in the filament string will bring it down to 6.6 vac. Anything wrong with that? Should I even bother?

[R.G.]

A lot depends on how closely the original transformer iron/copper designer ran the iron to saturation. If he got too close, you could be saturating the primary and see overheating and transformer death.

If it were me, I'd use a bucking transformer ala the Vintage Voltage Adapter to knock 20-24V off the incoming 120-124V and feed the transformer the 100Vac it was designed for.

It's perfectly OK to feed a transformer winding less voltage than it was designed for. Feeding it more than it was designed for brings the possibility of saturation and overheating.

[nosaj]

So... I'm thinking of using a used tube amp transformer designed for countries with 100v mains in the USA. I'm thinking the major issue is the filament voltage will be about 7.2vac. Sticking a couple of diodes in the filament string will bring it down to 6.6 vac. Anything wrong with that? Should I even bother?

What is your reasoning for it? What do you hope to accomplish with it? just curious.

nosaj

[olddawg]

What is your reasoning for it? What do you hope to accomplish with it? just curious.

nosaj

I just have several available that drop right in to chassis I have for nothing. Years ago I used to run into grey market Japanese (100v/50 cycle) solid state stuff all the time that worked fine. But that gear had voltage regulators and such designed into them. In this case it's a PT for a very basic 36 watt 4 x EL 84 amps. I was just curious if anyone had done it? It's a 20% increase. Yeah... I didn't consider it might get warm and blow a thermistor in the primary. I'lol probably try it anyway. Cost me nothing but time.

[R.G.]

Since saturation can be done only from a primary, try it with just the transformer hooked up to your local flavor of AC, leaving the secondaries open. Measure the no-load primary current and see if it's excessive. You might be able to tell quickly whether to go to the trouble of installing it or not.

[Mick Bailey]

Most newer (90s onwards) Fender amps here in the UK run way over on their heater supplies. I regularly measure around 7.5v and they've been running like that for years, so I wouldn't bother with diodes.

[g1]

Most newer (90s onwards) Fender amps here in the UK run way over on their heater supplies. I regularly measure around 7.5v and they've been running like that for years

I don't think it will bother the amp, but what about tube life? Noise?

[mozwell]

If the TX is made for 100VAC 50Hz, the core flux will be the same at 120VAC 60Hz, so you should have no problems using the transformer, but at 60Hz only, and it the transformers you have are made for 50Hz.
Your secondary voltages will be 20% high