110v > 240v Q's

[Sea Chief]

Hi. Are there any pitfalls to using a step up transformer, say if I have a 110v amp & want to use it in UK? And any comparisons with the xfmr unit itslf, to a building site type? I think these are used for safety reasons here, & look like a small bucket-sized lump. Are ones I could use for amps anything similar? tbh Iver never seen one.

Thanks, SC

[Mick Bailey]

The pitfall is if you use an auto transformer - these are not isolated. The building site transformers are fine and offer isolated protection but are overkill for amp use. You have to watch the output as this is given for a loaded transformer. Some isolation transformers also have a centre-tapped secondary which is connected to earth. So you get 55v-0v-55v. This is called a CTE (centre tapped earth) The output is taken off the ends of the winding to give 110v output, So the maximum voltage from live to earth is 55v. It's important with site transformers to distinguish between the two types; The CTE system has a floating output and no conventional neutral - think of it as having two live connections. The non-CTE may have one end of the secondary connected to earth and be marked as live and neutral.

The idea behind CTE is if you get a fault the shock is less likely to be fatal and in addition may mean that if you're working at height or with machinery the shock you get may not have other consequences.

Edit: What you refer to when using a 110v amp in the UK is a step down transformer (nominally 240v to 110v), though this usually means a non-isolated auto transformer.

[Sea Chief]

Hi Mick- very useful info there thanks. I had thought the site xfmrs were likely OTT but have never seen anyone use, whatever it is they use, for a step-down xfmr for an amp. When US bands come over, what do they use then?

What I'm after really, is wondering what say if you bought an early tweed US amp & wanted to use it here.. what sort of xfmr thing you would use. As you can tell by getting step-up-down wrong to start off.. its all new to me.

[Mick Bailey]

The bands I deal with either hire something over here, or bring a range of different transformers to suit their setup - usually a portable transformer with a handle on top and sized just enough for an amp and pedal board. I sometimes get calls from artists who've picked up an IEC lead and plugged in their amp into 240v without thinking about voltage. A band arrived from the USA early this year and the bass player had left her transformer back in the USA and didn't realize until she got to the sound check - I got a call from the promoter asking me to sort something out within an hour. Mostly though people are using newer amps with voltage selectors and often the pedal boards have either switchable power supplies or are SMPS and can operate from 90v to 250v.

I use an isolation transformer for my '61 Ampeg. Mostly I see musicians with auto transformers where they're running US equipment over here. I have a large toroidal isolation transformer for my 120v bench supply. Sometimes with older amps I come across a wired-in auto transformer that's screwed onto the cabinet. Auto transformers are fairly small compared with their current output and that's why they're common.

[Sea Chief]

Ok thanks for that. I thought maybe bands might have a tech to use ideal things. Not worderd my Q right really (US bands I saw 90's often lugged their own amps about/ no UK tech in sight).

So you would recommend an 'auto transformer' if I bought an old 110v US amp & shipped it here, just for home use?

[pdf64]

Provided that:

- the 110-120V equipment has an isolating PT and a properly grounded chassis and power connector
- the step down transformer maintains a proper ground connection between its input and output

I don’t see the problem with the step down transformer being an auto (single winding), rather than an isolating type?

[Sea Chief]

Do early tweeds have such PT's? Ive got my eye on atv tweed 5e3. probably the least likely to I'd imagine.

[pdf64]

Do early tweeds have such PT's? Ive got my eye on atv tweed 5e3. probably the least likely to I'd imagine.

I think all Fenders have had isolating power transformers.

You may find you need to further reduce the voltage from any type of step down transformer, ie both isolating and auto, as it will likely be just a simple 2:1 voltage ratio. So assuming 240V in, 120V out, whereas early 50s North American amps may have been intended for 110V. Feed them 120V and all internal voltages may run about 10% too high.
It's a big issue for vintage amp users over there, as their voltages can be up to 129V.
Hence http://www.geofex.com/Article_Folder...t/vintvolt.htm and https://www.amprx.net/

[Mick Bailey]

Many autotransformers in the UK are for 230v and give an even higher output. Also, some are not fused and have very thin windings. An autotransformer will dump a lot of current and if the mains plug is fused at 13A (as some are) and has no output fuse then they can short between windings and the voltage momentarily rise to a much higher level. RS components used to sell some really nice UK-made bare units. I still have an NOS one from maybe the late 80s that I want to put into an enclosure. When shopping around be sure that the output is fused and try to establish that it's properly earthed. I had one recently in a plastic enclosure with no connection at all to the earth pin on the outlet and no fuse. Maybe with an isolation transformer they could claim it was double-insulated, but it was an autotransformer. Hence the reason I say these can be a pitfall.

The other issue in the UK is were used to a much higher mains voltage than in the USA and stepping down gives an illusion of safety - especially that 110v site transformers are a safety device over here. It doesn't kick as hard as 240v (my local voltage is actually 248v) but it's just as dangerous.

[SoulFetish]

If your planning to do this overseas in Europe or the UK, one really important point is to make sure that the transformer you want to use was designed to be used at both 50Hz & 60Hz!
A transformer that was only built to be used in N. America (for example), might only be rated for 120V/60Hz. A 120v/60Hz transformer will saturate if operated at 120V/50Hz. This will lead to the transformer overheating and eventually failing.
The specified mains power is often labeled at or near where the power inputs to the device.

[pdf64]

If your planning to do this overseas in Europe or the UK, one really important point is to make sure that the transformer you want to use was designed to be used at both 50Hz & 60Hz!
A transformer that was only built to be used in N. America (for example), might only be rated for 120V/60Hz. A 120v/60Hz transformer will saturate if operated at 120V/50Hz. This will lead to the transformer overheating and eventually failing.
The specified mains power is often labeled at or near where the power inputs to the device.

I’m a bit sceptical about about that.
It would seem risky to purposely design a 60Hz mag circuit’s operation so close to saturation that 50Hz would tip it over the edge.
Bearing in mind that saturation would cause the primary inductance to collapse, fault current to the drawn, and hence fuse to blow.

[g1]

It does sound like questionable design practice but I have heard of it being an issue as well. Seems to me what I read was more about over-heating issues when running some 60Hz xfrmrs at 50Hz, but I could be mistaken.

[SoulFetish]

I’m a bit sceptical about about that.
It would seem risky to purposely design a 60Hz mag circuit’s operation so close to saturation that 50Hz would tip it over the edge.
Bearing in mind that saturation would cause the primary inductance to collapse, fault current to the drawn, and hence fuse to blow.

From Rod Elliot Transformers - The Basics (Part 2):

"A transformer rated for 50Hz can be used anywhere in the world - it will work perfectly at 60Hz. However, the converse is not true. A transformer designed specifically for 60Hz will overheat at 50Hz, even if the voltage is correct! This is not well understood, and leads to an enormous amount of traffic on Usenet and in forum pages everywhere. The answer is quite simple - 60Hz is 20% greater than 50Hz, so the core and turns per volt can both be reduced by up to 20% compared to a 50Hz transformer of the same rating.

Therefore, a transformer that was designed for 60Hz at 220/230V (The Philippines, South Korea and a few others use this combination [Ref]) has a smaller core and fewer turns than an otherwise identically rated 50Hz transformer. As a result, it will most likely fail with 220V at 50Hz. Operating a 60Hz power transformer at 50Hz is exactly the same as operating the transformer at its rated frequency, but with a 20% voltage increase. If you absolutely must run a 60Hz transformer at 50Hz, you must reduce the mains voltage from the rated value (say 230V) by 20% (184V). This is a large drop, and exceeds the normal mains variation allowances that are provided for in properly designed circuits.

Failure to reduce the voltage will cause the transformer to be heavily into saturation, and it may easily consume half its rated VA (or more) at idle, due to excessive magnetising current caused by core saturation. Needless to say, the secondary voltage will also be reduced by the same percentage. For evidence of the current increase due to core saturation, see the next section (specifically Figure 12.1.1).

Operating a 60Hz transformer at 50Hz is effectively the same as a 20% increase in mains voltage, but note that this does not mean that the secondary voltage is increased. For a 230V transformer that's the same as running at 60Hz, but at a supply voltage of 276V. The core will be seriously saturated, and the magnetising current will be increased dramatically. "

[loudthud]

Please consider for a moment what the load on a guitar amplifier power transformer actually looks like. The previous discussion seems to assume the transformer is used at max rating all the time. In actual practice the load is very intermittent in most cases. (Percentages are of rated VA.) When the amp in on standby or not making significant power, it's idling along at 20% to 40% or so. When you are playing loud and clean, you are hopefully in the 90% range, but you have to take into account the space between notes.

If you are in an AC-DC tribute band, you are going to be hitting peaks of 150% or more, but again it's an intermittent load. Hopefully the designer of the transformer has seen enough warranty claims to design a transformer that can take the abuse. At 50Hz a transformer is going to run warmer. If it starts to sound bad because it's going into saturation, hopefully the player will back off or get an amp that can take the abuse and sound good.

[loudthud]

In one of his books KOC suggests that for a solid state amp, the transformer VA can be equal to the RMS power output. I've tried it with a 90W amp, a 48V at 2 Amp transformer. It works and doesn't get too warm playing really loud for 30 minutes or so. I think I'd like to be a little more conservative and get a 3 Amp transformer.