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Which transformer to use in the UK with a US powered 400+...

Discussion in 'Amps and Cabs [BG]' started by Charling, May 7, 2010.

  1. Hi there,

    I've picked up a US voltage Mesa 400+ for a bargain price. At work we use US amps like these on big transformers but I was wondering what the optimal transformer would be. The Mesa is rated at 500w output (according to their website) but does that relate directly the input wattage needed?

    If I bought a 220v to 110v transformer rated at, say, 600w would that be ok to allow the amp to run at optimal performance? Or would it require some other rating for the AC input?

    Thanks in advance for any help offered!

  2. bassmanbiff


    Apr 3, 2008
    a 600w transformer makes sense when accounting for potential surge currents, but it might work just fine with something smaller since you'll rarely (if ever) reach that demand.

    The nature of the power supply in the amp will have a lot to do with the real demand, but I have no idea what that amp is like.
  3. I'd check the fuse and get something which can supply a bit more current than that.

    I don't think 600 watts is going to do it.
  4. bassmanbiff


    Apr 3, 2008
    re: malamute

    I wonder if you have a specific reason for your perspective?
  5. That amp should have a Slo-blow fuse to allow high inrush current from popping it. The 400+ is 300 watts (RMS)? 600 may survive the inrush (turn on) current, may not. More importantly it may starve the amp. Are you sure it does not have a universal transformer?
    The schematics I have show a universal primary!
  6. I'm not certain, I have a good tech who may be able to switch it though if that is the case!

    I've heard elsewhere that the 400+ is 300w, but the mesa website says it is 500w, what gives? Is it just an RMS/peak thing?

    By not survive do you mean a 600w mains transformer may be burned out by the initial current draw when you turn the amp on, but will cover the needs when up and running? The amp does have a slo-blow fuse, along with spares!

    To play it safe should I get a high wattage tranny (say 1200w) and just go with that? I have read there are further problems when running a transformer at a lot less than its rated wattage, is this true and does it have a particular effect on things like amps?

    Thanks for all the replies so far, very helpful! I know I could get a useable result with a very heavy duty transformer but I'd like to get in-the-know about the risks/effects caused as I'd like this to be my main gigging amp, it sounds out of this world!
  7. Take out the fuse.
    Multiply by 120.
    Give yourself some extra because that's a slow blow fuse and demands can peak above that figure.

    This is assuming you have already opened it up and found no tap for a 220V supply on the transformer. The 400 had that, so does most Mesa stuff.
  8. You are probably reading the average power draw from the mains on the amp. Double that to be safe with a step down tranny. If your tech needs a schematic PM me with a email addy and I'll email them to you. :) I shouldn't have to say it but make sure he changes the mains fuse when/if you rewire the trans primary. Rewire is a much better solution than a step down trans.
  9. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    the 4-500w is the amps output. It will draw much more than this. The 300w SVT my housemate had drew 1000w from the mains. You need to have a transformer rated for more than that. That will be a beefy device.
  10. Make sure you just don't need to move taps on the mains transformer first. Much better option than the step down trans.
  11. Yeah, I'd prefer to do that, but I'm unsure whether thats how the 400+ is made. I've had a look inside a 400 (not a 400+) and didn't see anything that looked like it could be switched on in the I/P tranny but I'm no electrical buff. Does anyone know if its as simple as this on a 400+?

  12. A few things, most of which are covered already.

    - Don't worry about a step down transformer til you get the amp. The "draw" will be rated on the chassis, or in a manual online if you can find one.

    - They are 300 watts on a good day, generally just a bit under. I have no idea where Mesa got the 500 watts statement from.

    - When you get your amp, have it checked out. It might have a tappable transformer. My SVT-II has markings on the chassis for a 110v line, the previous owner had bought it in the US, brought it over here and had a tech change the taps on the transformer (literally just a case of unplugging one part and plugging it into another (well, a few cables). But, not every amp will be like this. Someone with more mesa knowledge will be able to chime in on that one!
  13. bassmanbiff


    Apr 3, 2008
    this discussion of startup surge current can be ignored, it won't hurt a transformer near enough to the amp rating..

    Proposing a 300W amp consumes 1kw on the mains can also be ignored. It makes no sense unless it's again a startup surge (which it doubtful), or an instantaneous peak from an amp with a switching power suppy, which this is probably not.

    If this has a linear power supply (which is likely) you can think of something around 80% efficiency between the output power and the input power (assuming perhaps a class A/B power amp).

    If linear power supply...
    In use power surges will be handled by the internal power supply with some slower recharge surges passing through its power transformer and seen by your external transformer. These surges will result in a slight depression of the voltage presented by the external transformer with the extent of that resulting from the specific characteristics of the transformer.

    This is the major issue if your current rating is close, since any heating associated with current peaks will be transient and the merely be integrated by the mass of the transformer core and the efficiency of heat loss to the surroundings.

    This won't hurt anything, and since you'll only ever draw anything near the rated power during peaks, it will likely rarely be seen.

    If a switching supply
    Since there's little internal power reserve, there will be large instantaneous peak demands of input current seen by the external transformer. As switching supplies inherently accommodate a relatively large range of input voltages, there will be little consequence to resulting instantaneous voltage sags and you'll still be ok.

    The suggestion to check the lines fuse rating and multiply be 120 is reasonable. This will give you the max power (with some margin, often a factor of 1.5-2, as considered by the designer) ever to be seen on the input, even taking into account surges. If they're trying here to account for the integrative effects of heating in the fuse element to provide a margin, they're not doing a good design.

    I'd be surprised (as many have already suggested) if this does not have a multi-mains transformer inside, but it may be a North America only model for some reason.

    IMHO, 600w transformer would work fine. On a 240v system, this is only about a 3 amp transformer so it's not such a big deal. If you want the most conservative approach, get the biggest transformer you can afford, but there is, of course, a point beyond which you're merely wasting money and effort.

    The running temperature of the transformer will give a gross indication of whether it's too small. If it's only slightly warm when you run the amp at full power continuously (which wll probably never occur if actually used as an instrument amplifier), then all is fine.

    If fear and uncertainty rule the day, then perhaps a Monster transformer might be considered. They're good at taking advantage of uncertainty, doubt, and limited specific insight into the problems they assert to deal with.
  14. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    You are suggesting a valve amp is 100% efficient?

    This is the amp, check the back panel pic, the Mesa will be comparable.

    5A x 230v = 1150 w. It will draw more when it starts up, hence the slow blow fuse. The additional power is consumed by the heat and light put out by the valves and such.
  15. Just for info the fuse for the 400+ at 120volt is 8amp slo-blo, at 240 volt is 4amp slow blow. 80% convertion with class A/B and a linear power supply? Not likely.
    If a 600watt transformer is 3amp on the input, it misses the 240 volt fuse rating of the converted amp by a full amp.
  16. bassmanbiff


    Apr 3, 2008
    Truly does, but was being conservative.
  17. bassmanbiff


    Apr 3, 2008
    btw ... was thinking in terms of the efficiency of a solid state amp. A tube amp certainly has other power demands to heat the tubes and inefficiencies in the output transformer.
  18. bassmanbiff


    Apr 3, 2008
    has been further pondering the suggestion that I somehow said or implied a tube amp might be 100% efficient .. clearly this is unlikely to be the case -- though it would be nice. One could effectively argue that when all the consumption is accounted for it could be nearly so, and this value is indicated by the selection of lines fuse (with the attendant design margin).

    In the case of a slow blow fuse, this doesn't obviate the analysis, but recognizes that a short-term, higher current, startup consumption is accounted for and that this doesn't change the steady state (or better, operating state) demand.

    Going to the good recommendation someone made about using the fuse value to determine an effective lines power demand, which would need to be satisfied by an external lines transformer, we still arrive at a reasonable, gross, analysis.

    Beyond that, recognizing that the startup surge is of a short duration, and that a transformer's primary failure mode (ignoring voltage related failures from exceeding the standoff voltage tolerance between windings and/or the core) is the heating resulting from overcurrent, and that this must be of a long enough duration to actually achieve a temperature which might result in loss of winding insulation or worse the winding itself, one can arrive at a reasonable, prudent suggestion of the required characteristics of the transformer itself.

    So, we come back to the presumption that the designer appropriately selected the fuse. Taking into account the nature of, in this case a tube amp(?), we have a continuous demand for the base-level losses (primarily heating with some smaller amounts for biasing), increased by the variable demand for output current, we find that ... voila! The earlier analysis still holds.
  19. Surely, if using a Step down transformer which can adequetly supply the power to the amp, you don't nee to worry about changing the fuses in the amp, as it should be equivilant to what it would be plugged into if it was on a US mains?

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