Lower mains voltage: What to do??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ariel Pozzo, Aug 10, 2004.


  1. Ariel Pozzo

    Ariel Pozzo Member

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    My Soldano HR100+ head is rated from the factory to run at 234 volts from the mains line. Here in my country we have a mains voltage of 220 volts...so I'm running the amp at -14 volts of its intended voltage.
    I recently rebiased the amp, and when I measured the plate voltages I got 447 volts. Bill Sundt of Soldano has just emailed me telling this amp's plate voltage is 500 volts when running at the rated mains voltage.
    As you can see, I have a problem here...I need some extra 50 volts on the plates to have the amp sounding just like Soldano designed it.
    I told Mr. Sundt I had just rebiased the amp at 30ma. on the 5881 tubes, and his reply: "30 mv should be fine. We actually set the amp cold at the factory, at about 20 - 25 ma, but that is with the plate voltage at
    500 volts. The lower mains voltage you have there is reducing the current in your amp and
    increasing it should help."
    I guess I could use a variac...but that's too clumsy since I tour with this amp. Is there any other solution you guys could think of?
    Thanks!
     
  2. John Phillips

    John Phillips Member

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    Not really. Stepping up the voltage can really only be done with a Variac or some sort of power conditioner (even more expensive and heavy).

    Rebiasing to compensate for the lower supply voltage is about the best you'll achieve without one.

    What's puzzling though is that 220V is only 6% down from 234... but 447 is 11% down on 500. This doesn't quite add up, since the power transformer should reflect the same difference at output as input.

    So could your line voltage actually be even lower than 220? By calculating back from the HT voltage, it looks like it might be as low as 209V. If so, I'd maybe start to worrry about more than just the tone - if the supply voltage is more than about 10% lower than intended, you're getting into the region of cold-running the tube filaments, which isn't good for them.

    That is of course assuming the the figure of 500V is accurate - it will vary slightly from example to example, since all components in amps have tolerances too. It may be that your particular amp does have slightly lower HT as normal.
     
  3. Old Fuzzface

    Old Fuzzface Member

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    Most electrical supplies are specified at +/- 10% of nominal voltage. This means that your nominal 220V supply could vary between 198V and 242V (and that's assuming that the power company never go out of spec - which is a big assumption).

    A piece of equipment designed to run on nominal 220/230 volts should be able to cope with supply voltages between 198V (that's 220V-10%) and 253V (that's 230V+10%) without adjustment.

    It seems to me that Soldano may have set up the plate voltage to max out at 500V for an approximate 250V input. Would this make sense?

    If so, this would mean that the 500V plate voltage would not be exceeded on any 220/230V supply that is within it's nominal +/- 10% range. It follows that a 447V plate voltage would be about what you'd expect for a 220V supply close to it's nominal value.

    In any case, I think the real problems likely to be encountered with supplies are instability and noise, rather than the absolute voltage, although some form of over-voltage protection is a good idea. As John said, the only solution is a regulator and I would think that this would be an extremely good investment for anyone touring.

    I know Scott Peterson has been through a lot of power problems - hopefully he can advise on some specific products for the job.
     
  4. somedude

    somedude Member

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  5. Ariel Pozzo

    Ariel Pozzo Member

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    Thanks for the replies guys.:)
    Now, I was wondering...I have here a sturdy 220 to 110 volts step down transformer. If I could set the amp to expect 110volts like its American cousins, I could deliver that exact voltage by using this transformer and then overcome the power mismatch between the expected 234 volts and the available 220 volts on the mains...anyone knows if a Soldano can be rewired to 110 volts, just like a standard Marshall?:)
     
  6. Old Fuzzface

    Old Fuzzface Member

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    Hi Ariel,

    I don't think using a fixed ratio step down or step up transformer will help much. In fact, it will probably make things worse...

    US standard voltage is 120V, so I assume it is possible to convert the Soldano to 120V. Your transformer will give you 110V for a 220V input. If you put 110V into an amp set up to use 120V power, it will be just over 8% lower than ideal.

    At the moment, with 220V into an amp set up for 234V, it's around 6% low.
     
  7. Ariel Pozzo

    Ariel Pozzo Member

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    Ian
    I can get me a 220 to 120 volts step down tranny. Do you think it will solve this issue if I have the amp set for 120 volts? :)
    And the most important thing...do you think it's worth the hassle, tone-wise? I have the 5881 tubes biased at 30ma right now.
     
  8. hear and play

    hear and play Member

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    Isn't Bill Sundt saying that you don't have a problem? I read it to say that 30 ma is compensating for the lower mains voltage and the result is that everything is hunky-dory.
     
  9. somedude

    somedude Member

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    As far as I'm aware (and if I'm wrong someone can correct me here), but when using a normal step down transformer, you're stepdown voltage will vary just as much as your normal voltage.

    For example, if you were stepping 220v down to 110v, and you plug into an outlet putting out 218v, you'll end up with 106v outplut.

    Conversely, if you were to plug into an outlet putting out 222v, you'll end up with 114v.

    Basically, all I'm saying is, you may solve the low voltage problem when you plug into the socket in your house, but what about when you plug into the socket in the house down the road?

    Or the one in the club in the next city?

    Are you going to carry a custom wound transformer for all those possibilities?

    What you need is some sort of digitally controlled voltage regulator, such as the Furman I posted above. It won't get you 234v, but it will get around 230v as long as the mains voltage remains between 190v and 287v......
     
  10. Old Fuzzface

    Old Fuzzface Member

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    I agree. This is the real problem. And a regulator is the only comprehensive solution.

    I mentioned Scott as I remember him posting that he uses a Furman regulator in his rig as he lives in an area prone to a whole range of supply horrors.

    The advantage of a regulator is that you will get consistency from one day and venue to the next. Having regulated power means one less variable to worry about in getting your rig sounding good every day. It also takes away the worry of bad power supplies damaging your amp since a regulator unit typically also incorporates protection circuitry.
     
  11. Old Fuzzface

    Old Fuzzface Member

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    I'm not sure that there is an issue with the absolute values.

    I don't know the maximum plate voltage that can safely be applied to the 5881s in the Soldano. Hopefully, John can advise on that. If 500V is the nominal operating voltage it might be worth trying this...

    Tone is so subjective that the only way to find out if you can even hear the difference due to small supply voltage variations would be to try your amp with a Variac. Just be careful that you do this blind (with someone else tweaking the Variac so you don't know what setting is being used) and remember to check the Variac v. no-Variac sounds to ensure that what you hear isn't just the effect of the Variac's presence in the supply regardless of any voltage variation.
     
  12. Scott Peterson

    Scott Peterson Staff Member

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    I don't know what Furman offers in the 220/230 range, but what I do here is run a Furman AR-1215 in concert with a PL-Plus and run my rig off off that. It is just a two rack space SKB cab with each of the Furman units in there; easy to lug and quick to setup/teardown.

    I have done it for years and years; never one issue even at festivals running off of generators (the deepest of acid tests IMHO).
     

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