a 220 to 110V thought..some comments please?

[dschwartz]

Hi all:
i have an american tube amp, 110V, and here in chile we use 220/230V.
I currently power the amp through a 220/110 transformer, wich is heavy and noisy..i hate it..

I thought about rewinding the power transformer, but that can be expensive..

so i thought of this:

Putting a big diode in series with the AC line, so the AC voltage swings only half amplitude, getting aproximately 110V AC (swinging from 0.7V to 150V or so).. then put a big dc coupling cap , so the swing is +-75V (thats about 110V RMS)..

am i crazy and want to destroy my amp? or i ´ve got something here?

[earthtonesaudio]

I don't have an answer for you, but your idea sounds like it probably won't work.  Tube amps want lots of current, and that would have to be a huge capacitor to be able to supply that much current.   Maybe about as big as your amp.

And I have the general impression that converting 220 to 110 is tricky business that requires either brute force (like your transformer) or lots of thought and trickery (like universal-input switching power supplies).


Seriously I think your best bet would be shopping for a new power transformer that fits your amp.

[sean k]

The AC still swings to 220 +/- on both ends of the transformer so a diode won't work but a bunch of high power zener doides end to end straight off the AC might work like you do on the centre tap of a trans secondary but they'd be big mothers. If you've got a 30W amp that'd be 270mA at 110V, given the amps 350VAC so a 25V zener dropping 270mA is about 7 watts so 10W zeners is what you'd want. If you needed two chains of zeners to earth off the neutral and active I'd think it might be a viable option to get a tranny wound even if it was a straight forward 220/110 at the voltage wattage rating you require.

[R.G.]

You're not crazy - just unschooled in that part of transformer technology, and smart enough to ask first.

And it would likely destroy your amp unless you used a DC blocking cap that's as big, heavy, and much more costly than  the transformer. All of the power for the amp has to go through the cap. Most caps are not rated for letting a lot of power through. You'd probably need motor-run rated caps which are rated for letting through lots of AC power, and they're heavy and expensive. If the cap fails, your amp's power transformer dies in fractions of a second.

I should have added more detail. A half wave rectified 240Vac is half-sines of  346V peak separated by flat spaces of equal width. The DC voltage level is the average of that, or 0.318 times the peak. The series cap blocks out that peak, but now the two unequal "halves" of the waveform go into your amp's transformer. The rectifiers on the secondary can only conduct on the half-sine peak because it's bigger, and now the unequal DC current pulses "walk" the voltage on the cap until equal current-time integrals flow on each half cycle, and this messes up the volt-time equality the primary needs. The transformer saturates a little bit to one side. That may or may not be fatal to the transformer over a longer period of time.

The 220-110 transformer you're using - is it an isolating transformer or an autotransformer? An autotransformer for a few hundred watts isnt' all that heavy. I suspect you're using an isolating 220-110. And if the transformer is really rated for 220 and you feed it 240, you may be getting the extra hum from saturating the primary of the autotransformer.

[dschwartz]

ohh thanks for your responses..

i have seen the diode trick used with soldering irons for half power, but with xformers is another story..
I simulatd the idea in LTC, and saw all of what R.G. said..the AC output of the diode is a 150Volts peak with valleys at 0 volts, but the wave is fairly assimetrical. of course, the RMS voltage is about 110V in amplidude, but with a DC component that would saturate the xformer´s core permanently (as RG said, it may kill the xfrmr)..

i tested a coupling capacitor, and with the equivalent load of a transformer, the cap would have to be humongus (like 50.000uF)..
so i´d better stick with my isolation transformer and brake my back everytime i move it (it´s a peavey triumph pag120, a really heavy amp)

[Jered]

  Here is another possibility.
  Here in the states, 110 vac is your standard household voltage. But there are appliances that need 220 vac to work properly. What they do is run two separate 110 lines to the electric panel (breaker box) and where needed run a 220 vac circuit by combining the two 110 lines (kind of). There is no ground on a household 220 circuit, although neutral and ground often share the same bussbar, you have 110, 110, and a neutral. But it is supplied as two separate 110 lines. More modern houses do have a ground with 220 circuits.
  I have no idea, but maybe it is done in the same manner where you live. Check the incoming wires at your breaker panel and see. If so, its as simple as rinning a dedicated line where you want to plug in your amp.
  Or you could even find out by checking your wall receptacles. Are they three prong? If so, with your dmm, put a test lead in the bottom receptacle hole and the other in either of the top holes, then the other top hole. If each reads 110 then just run off one of the 110 lines and neutral, and you do really need a ground for safety reasons.

[earthtonesaudio]

Note that if you're not sure about what you're doing, sticking metal things in wall sockets is a quick and painful way to die.

[Ice-9]

Its worth checking the PSU inside the amp first as lots of amp designs these days have transformers that are capable of both 110/240.  Its usually that the primary windings of the transformer are split. eg there is usually 2 primary windings that are  wired in parallel for 110 volt operation, for 220/240 they are wired in series. So some equipment actually have  wire links on the PCB to select which voltage you require. This saves manufacturing costs as the same PCB / components are used in both versions.

BUT be careful to check that this is possible as dealing with high voltages is dangerous. Also other components in the PSU circuit eg. caps might not be rated at the higher voltages.

If anyone else is able to explain this in more detail please do.

[George Giblet]

> An autotransformer for a few hundred watts isnt' all that heavy. I suspect you're using an isolating 220-110.

+1

[dschwartz]

first thing i did was check the transformer for a separate primary winding, but no, it´s only for 110V..
and jered..here we have 220V from the wall, with 3 wires, live, neutral and ground..

[Jered]

  Alrighty then, scratch that idea. Good luck. Have you tried posting on an amp forum?

[richon]

Daniel,

why is it noisy?  ground loop? does it even has EARTH/GROUND conection for the AMP cable?

[dschwartz]

is not noisy..it just doesn´t have true ground

[Zben3129]

Note that if you're not sure about what you're doing, sticking metal things in wall sockets is a quick and painful way to die.

After months of tube amp building/work, I electrocuted myself for the first time three days ago.

The ironic thing is that I electrocuted myself plugging in my phone charger, I must have touched something by mistake. So after all the care I take with tube amps to not get electroed, I get shocked doing a mundane everyday activity. Great 

Lemme tell ya...If you've never gotten electrocuted before, don't do it. Its horrible 

Zach

[dschwartz]

i know, i´ve been shocked a couple of times and it sucks..thanks god never from power amp filtering caps..

[Jered]

  Daniel, you've gotten shocked by 220 before? Pretty scary uh? I've only experienced 220 once and it was strong. Been shocked by 110 more times than I'd like to admit, but its not that bad. Instinct tells you to pull away and its done. Instinct does the same with 220,...but it doesn't want to let go! Thank god for circuit breakers. Took me a couple minutes to get my head clear, and a lesson learned I'll never forget.
  Don't even want to think about filter caps, I ALWAYS discharge them.

[dschwartz]

you made a circuit breaker jump? wow.

once i touched the metal can of a cap of a running tube amp, and just felt a big púnch on the chest..it was a negative voltage filter, so the metal can was carrying the voltage..i was jumpy for like 5 minutes!!..it gets your adrenaline a boost..

from that day i allways put tape over the caps

[R.G.]

Been shocked by 110 more times than I'd like to admit, but its not that bad. Instinct tells you to pull away and its done.

Let me add a bit more caution to this. The reason it was "not that bad" was that you only made a high resistance connection to it. The amount of current needed to stop your heart is shockingly (sorry - I had to!  ) small. Twenty milliamperes through the chest and you're history. Dry skin brushing against a smooth, dry metal surface can let through only enough to be bad.

If the skin isn't dry, metal has a sharp or rough surface to get past the dead skin layer, or you happen to be where your muscles contracting pulls you INTO the metal, not making you jump away, the current gets much bigger. The "instinct" you mention is your muscles firing from the electricity going through them, not from your nerves firing them. You have zero control of that, as it's not your nerves making the signals and your nerves can't cancel that out. If you happen to have grasped your hand around the charged metal and your other hand's or foot's response to the current is to press into the return path, there is nothing you can do. The current's going to hold you there and cook you. In this case, unless someone else can stop the current, you're dead.

A truly deadly situation happens on stage when a guitarist holds the grounded strings of a guitar with one hand and grabs a "grounded" microphone with the other. He (or she) is usually hot and sweaty, so the skin is wet and its resistance is low. The hands are folded around the metal objects. And if either the amp or the PA has a ground lift and a bit of AC leakage, it's all over. Or if a guitarist has a leaky amp and is standing on a wet grounded surface. Exactly these situations have happened. Dying at a workbench is also common, it just doesn't make the news.

There isn't any shock that is "not that bad". Except for static electricity zaps where the energy is low, they are mostly all potentially (sorry again) deadly.

[earthtonesaudio]

The rhythm guitarist in my old band used a secondhand PA head for his amp, and the ground wire was cut on it.  He had a habit of walking up to a person (like me) who was using a properly grounded amp, and grabbing their arm or guitar strings.  Needless to say the band didn't go very far.   

Some things you can do to help protect yourself from shock and/or electrocution are:
-Get and use an outlet tester and check all the outlets you will be using.
-If that's not possible, use a wireless system.
-Or, use a ground fault circuit interrupter on your gear.
-If that doesn't fit your style, use the "back of the hand" trick and slap every surface you might touch while playing.  If you get shocked, your muscles will tense and pull your hand away, which is a lot better than grabbing on and not being able to let go.  Plus the resistance of the back of your hand tends to be higher than your palms so the shock isn't as bad.

[michael_krell]

Why not try a variac?