In Japan (100v) with US (120v) SS combo amp

[CHiZZoPs]

First of all, will under-powering hurt the amp? Secondly, I opened it up and noticed that the transformer in this pre-Fender SWR Workingman's 10" is a "Topmark" brand transformer, and is labeled as "120v 100v 0v." Furthermore, there is a brown wire coming from the transformer that is uncut and has not been used. Would replacing the black lead to the power switch with the brown rig the transformer to 100v?

Answers to these questions would be appreciated! Cheers.

[xylix]

Hi,
I'm no expert in this area by any means, but I have lived in Japan for most of the past decade (and used electronics things I bought over from Canada @ 120v). My understanding is that in general you are almost always OK underpowering anything. If you go the other way - plugging in a 100v appliance to 120v mains - you could be in for some trouble. It is like having a variac and turning the juice down a little bit. Your hair dryier or toaster isn't going to mind one bit (althought you may notice the hair dryier running slightly slower). It shouldn't make a bit of difference for an amp either (as far as I can tell). Think about an amplified device that runs on batteries. As they get emptied the voltage drops, but the device will keep operating normally (until the battery dies). By the way, I know nothing at all about the insides of Fender amps so can't address that other question.

If you are REALLY concerned you can always by a transformer. I bought 4 or 5 of them when I moved back to Canada for a while a few years ago. In that case I was taking some very expensive stereo and other electronic equipment made for 100 v and plugging it into 120 v which would make them run 'hot' and burn out components (so I was told).

One last factor to consider is the fact you you never really get EXACTLY 100 or 120 volts. That is the average, but it fluxuates up and down by some amount (that I don't know). So electronic devices are made to operate on one very narrow voltage setting anyway.

Hopefully someone more knowledgeable will get back to you. By the way, where are you? I am in Osaka. If you are in the Kansai area there are 'duty free' shops in Nipponbashi (or in Akihabara in Tokyo) that have 120-100v transformers.

Craig

[teemuk]

Whether the different mains voltage affects the device operation is – naturally – a device dependent issue. Basic appliances like toasters or ovens don't mind a bit since they are basically just resistors, but things might get different with more delicate applications such as amplifiers.

You can always compare the voltage transfer ratios and estimate the behaviour if you know something about the circuitry involved. Take for example and amplifier with 60V secondary @ 120V; this drops down to 50V in 100V system. The lost 10V can be a quite significant figure when it comes to output power so it is obvious that the drop reduces both output power and headroom. In tube amplifiers one could easily loose 100V from B+. Yet, if circuitry is fairly basic, both SS and tube circuits still might work "right". Difference between 120V and 100V is not as sinificant as difference between 120V and 240V and going downward in voltage means that voltage ratings still stay within specs.

The issue comes with circuits that are picky over specific voltage. Take for example tube filaments: Tubes deteriorate quite fast if their filament voltage is not about exact (about 12V or 6.3V). You might loose 2V from a 12V filament system by switching to lower voltage mains supply. Not good. (You can refer to below diagram if you know the voltage difference in percent). Similarly, delicate bias circuits, voltage dividers, comparators that reference input to regulated voltages etc. might work erroneously or stop working altogether. (All this is naturally case specific). Same thing could happen with digital circuits, but fortunately they derive their supply from a regulator and therefore do not mind about lower mains voltage. So, in most cases, unless your amp has tubes it should be fine – but, less powerful and lacking in headroom.



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Very likely the extra wire in the transformer is a tap to convert the transformer to a 100V mains system. The transformer even states this. Of course anything isn't 100% sure until tested.

[CHiZZoPs]

Great explanation! Thanks a lot  :tu: