How to use 220V Marshall in U.S.??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sws1, Feb 11, 2003.


  1. sws1

    sws1 Member

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    I just picked up a used Marshall head that was originally built for use in the UK (i.e., it is not tapped for the US market).

    Since I want to keep the head stock, and not mess with rewiring the connections to the transformer, what do I need to use this amp in the US?

    I have seen some relatively cheap step up / step down transformers that convert from 110 to 220 and vice versa. But these things do not do frequency conversion. I'm also seen some VERY expensive variacs that allow you to choose the exact voltage.

    What device to I need? Does up just step up to 220? What power requirement (# of watts) do I need on any device? Do I need to conver the frequency (60Hz to 50Hz)? etc.

    I'm sure this has been done numerous times before.

    Thanks
     
  2. John Phillips

    John Phillips Member

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    Why not just move the connection on the transformer, if it has the necessary tap?

    Simplest, safest, most reliable, cheapest, best solution.


    You definitely don't need to worry about the frequency though.
     
  3. sws1

    sws1 Member

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    I'm not sure if the transformer has that tap available. (It might. I just don't know yet.) Nonetheless, I'd like to keep it stock.
     
  4. John Phillips

    John Phillips Member

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    Sorry, I just don't follow. Moving one wire on a transformer is NOT a mod.

    Replacing old filter caps is not a mod either, and it will have to be done eventually even if the amp is not well overdue already (ie if it's over about 25 years old).

    Also, replacing or hardwiring the impedance selector (if the amp has one) is essential on many old Marshalls if you value the output transformer.

    What model is it BTW? There are a (very) few that don't have multiple voltage taps, but if it does I really can't see why you would want to use a heavy, expensive, hum-inducing, heat-producing external transformer when you could move one wire inside the amp.

    Amplifiers simply cannot be run forever without maintenance, which if not carried out can lead to very major damage - and incidentally will totally destroy any notion of 'keeping it stock'. It won't matter in another 20 years or so anyway, since there will be no working examples of vintage amps that have not been serviced.

    Sorry if this sounds like a rant... I just find this whole 'originality is more important than function' thing ludicrous.
     
  5. sws1

    sws1 Member

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    John - Thanks for the input.

    And I'll certainly consider changing the tap since that may be easy to do.

    Nonetheless, help me understand the electronics, since I'm interested in learning about voltage.

    What device would one use?
     
  6. John Phillips

    John Phillips Member

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    What you need is an offboard transformer. It needs to be a 110v input, 220v output (obviously) type, with enough power capacity to supply the amp - not the output power, but the consumption which can be anything up to four or five times higher; it may be printed on the amp somewhere in watts or "VA" (volt-amps, which is nearly the same as watts; there is a difference but I won't confuse you yet!) - and a continuous ground, so the amp is properly grounded too.

    What model of amp is this?

    The common old ones all have voltage selectors (just as unreliable as the impedance ones BTW), so I'm guessing it's not one of those.

    The modern ones all AFAIK have separate windings on the transformer, even if there's no voltage selector on the outside (usually it's two separate 110V windings, which you operate in parallel for 110V or in series for 220).

    That leaves just older, smaller amps, which were mostly not designed for export (and some do indeed not have 110V taps), and if it's one of those you have no choice except to use an external transformer, since the only alternative is to replace the amp's power transformer with a US version, which even I would not really recommend!

    Definitely the first thing to do is look inside and see if there is a 110V tap - you need to check whether things like filter cap replacements have been done if it's an old one anyway.
     
  7. teleharmonium

    teleharmonium Member

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    sws1 - I would recommend going ahead and getting a 500 watt capacity stepup transformer regardless of what you find with the existing transformer. If it has a 110v tap, unless you also use a variac, which is exactly as much trouble as a stepup tranny, you would be sending around 125v into the amp, and older tube amps do not like extra voltage in my experience and in the opinion of many techs.
    I have a Vox amp that has a 115 volt option (IIRC) and a few others up to 240 - I run it on 240 with the stepup because it sounds better that way, and the already high plate voltage on the power tubes is not pushed any further.
    The only way I would run a UK amp directly off of US AC power would be if the amp had a 125 or 130 volt tap.
     
  8. sws1

    sws1 Member

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    This is a "made for UK", "not for export to US" amp. Marshall 18-watt combo.

    Don't know about the 110 or 120 tap. It's tapped at 220V right now.

    Given that the US can hit the amp with 125 volts, maybe the transformer does make sense afterall.
     
  9. John Phillips

    John Phillips Member

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    If it's an 18-watt it may not have a 110V tap (or anything other than the single tap).

    Teleharmonuim is right about not hitting old amps with too much voltage, but remember that British amps are designed for 240/250V, so a step-up transformer from 110 to 220 will still give the correct voltage when supplied with up to 125V.

    It sounds like you should definitely make sure the filter caps have been replaced though - that amp is over 30 years old, well beyond the safe lifespan of electrolytic caps.

    At least you don't have to worry about bad impedance selectors...
     
  10. sws1

    sws1 Member

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    According to the schematic, there are only taps for 200, 220, and 240 volts. Amp is currently set to 220.

    Sounds like what I have to do is change the tap to 240, AND get a step up transformer, since US voltage is 120. If I use the step up transformer (which is just a doubler), and keep it at 220, then I will be hitting the amp too hard.
     
  11. John Phillips

    John Phillips Member

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    Yes, that's right. Check the voltage that the step-up transformer puts out, given the supply voltage you have - ideally you should meter it to be certain - and set the amp to the next voltage above that.

    Interesting that it is set for 220 - that means it was not originally intended for the British market (which was always 240) - more likely for Europe, unless the wire has been moved already in the past.

    I'm also sure I've also seen at least one 18-watter with only the single 240V tap - not surprising since Marshall were quite a small operation then and parts sometimes seemed to be used at random...
     
  12. sws1

    sws1 Member

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    Thanks for the great input. Very good explanation. Just what I was looking for.
     
  13. sws1

    sws1 Member

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    I take it that since the filament heaters are driven directly from the Power Transformer, we are talking about AC current, and not DC.

    Can I still measure voltage on the pins when the current is AC?
     

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