How to build a 100 volt tap

Discussion in 'Amps and Cabs' started by mitch236, Aug 20, 2008.

  1. mitch236

    mitch236 Member

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    I want to build a 100v tap to install in a Furman power distribution unit. I have searched all over the web but can't find a simple, DIY with parts and diagrams. I am handy with a soldering iron and understand how to follow instructions.

    Can anyone help me? It should be fairly easy, hopefully!
     
  2. Steve Dallas

    Steve Dallas Supporting Member

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    I have no idea what you are talking about, but I'm sure I could help if you could elaborate on what it is you are trying to do.
     
  3. teleamp

    teleamp Senior Member

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    You should contact Furman, they should be able to tell you if it is doable or not. Without seeing the schematic for your furman, I can not offer any help.
     
  4. Blue Strat

    Blue Strat Member

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    I think he's looking for a regulated 100VAC output so he can "brown out" his amp.
     
  5. mitch236

    mitch236 Member

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    Yes, this is exactly what I am trying to do. I have a Ho attenuator that has the 100v plug on the back. I don't need the attenuation when I play out but I love the sound of the 100v. When I said add it to my Furman, I meant to build it in. I can figure that out, all I really need is an instructional about what parts and how to connect them to take the wall voltage and reduce it to 100 volts. Hopefully these parts are small!
     
  6. Steve Dallas

    Steve Dallas Supporting Member

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    Ah.

    There are a couple of methods for doing this. Probably the easiset is to use a pair of 20V zeners in antiparallel. Another method would be to install a power resistor on one leg of the socket in series with the supply. I would have to know how much current you intend to draw to be able to tell you what value to use.

    OTOH, why not just plug a variac into one of the Furman's sockets? You can get them for peanuts from Harbor Freight.
     
    Last edited: Aug 20, 2008
  7. mitch236

    mitch236 Member

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    The problem with the variac is size and weight. I am trying to keep my "touring" rig as small and managable as possible.

    I would be plugging a Budda SD 45 into it. 45 watts.
     
  8. oldhousescott

    oldhousescott Silver Supporting Member

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    Check out this page under the "bucking" configuration.

    http://www.allaboutcircuits.com/vol_2/chpt_9/5.html

    You'll need to find a 120/20VAC transformer that will fit inside your Furman case. I would suggest at least 3A capability for the secondary. You may have to find a toroid to get the specs you need in a low height unit.
     
  9. hasserl

    hasserl Member

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    I believe that in order to have a consistent 100 volt supply you'll need a variable transformer, a "variac". You can design circuits to drop a set amount of voltage, but unless the mains are always at the same voltage the resulting dropped voltage will be whatever the mains are minus the voltage your circuit drops. I doubt the Ho 100v tap is always at 100 volts. It probably drops 20 volts from the mains, so if the mains are at 120 then it'll be 100. If the mains are only at 118, then the voltage will be 98, etc. Where I live the mains vary from as low as 112 to maybe 120 under the best conditions. If you want 100 volts under varying conditions you'll need a variable transformer that will let you adjust the secondary voltage.
     
  10. mitch236

    mitch236 Member

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    Funny, I read that exact article this morning. While it explains how it works, it isn't quite what I'm looking for. What I would need is something like this:

    "In order to build the step down circuit first get a ??? transformer, take the ??? leg of the main power from the wall and solder it to the ??? post, then take the ??? post of the transformer and solder it to the ??? leg of the 100v receptacle.... etc
     
  11. mitch236

    mitch236 Member

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    I don't need it to be regulated. Luckily the voltage doesn't really vary enough to affect my amp. I've been using the 100v tap on the back of my Ho for long enough to know it's good enough.
     
  12. Steve Dallas

    Steve Dallas Supporting Member

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    Another option would be to have power scaling installed. That way you could vary your operating voltage without affecting the tube filaments.

    Dropping the primary 20VAC will drop the filament voltage to about 5.9VAC, which is just above the lower limits of their 10% tolerance of 5.7VAC. If you happen to play a place with low wall voltage (like 110VAC), you will be below tolerance and run the risk of cathode stripping.

    There is no hard and fast rule to how tolerant different tubes are to low filament voltages, but be prepared to replace your tubes more often. I once read that EVH had to retube after every session when he ran his Marshall at the reported 90VAC.
     
  13. vibroverbus

    vibroverbus Member

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    I say oldhousescott laid it all out for you pretty exactly. If that isn't enough, then I'd respectfully suggest that you might be at the 'if you have to ask (more), maybe you shouldn't try it' stage...

    I had also just clicked my stopwatch to see how long before somebody brings up filament voltage, but I see S2 nailed it...
     
  14. mitch236

    mitch236 Member

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    Why is that? Part of being a tinkerer is taking on new projects and learning along the way. If I wanted the easy way out, I would just call Mr. Ho, who is a very gratious man, and have him build it for me. I want to do it myself.

    BTW, I'm only dropping the voltage by 10v so I shouldn't have any problems with stripping.
     
  15. Structo

    Structo Member

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    Yes but do you understand that if you have a constant voltage drop of 10v but then plug into a wall that is 125v then you are at 115v or if you plug into a 105v wall you are at 95v.
    What S2 said is valid about the cathode stripping at low voltages.

    So you either need to use a regulator that holds the voltage at a constant 100v or a power scaling unit in the amp that drops everything except the heater voltage.
     
  16. vibroverbus

    vibroverbus Member

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    I'm the last one to discourage tinkering (as long as someone accepts the very real dangers and has the proper respect and caution for like, zapping somebody and/or burning the house down.).

    I'm just sayin' oldhousescott gave you the exact recipe. If you can't get it from that, well... in some people's opinion maybe you should think about paying somebody else to do it. Or burn down your house, either way. <joke, joke, I kid because I love, really...>

    How do you get that you are only dropping 10V, by the way? You say you want 100VAC, and North America is 120VAC. S2 has the math correct unless what you are suggesting is that you are in a country that has 110VAC... And if that WAS the case, then your current Ho' ttenuator '100V labeled' tap would actually be giving something like 91-92V which would be a much worse filament situation. Unless the Ho' ttenuator is regulated. In which case it's a regulated AC supply and we're back to ground zero...
     
  17. mitch236

    mitch236 Member

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    I was of the belief that the wall outlet here in Florida gave between 110-120 volts with the usual voltages closer to 110. Maybe I'm mistaken. Maybe I should call Mr. Ho and ask him what he is using although I know he doesn't have regulated power. I looked inside his attenuator and he has a transformer connected on multiple leads. I was hoping that I could use a physically smaller transformer so it would fit inside the 1u box that the Furman is in. I am fully aware that I am not an EE, or even an electrician (no disrespect intended). I read the article but fail to see that it gives a no-brainer approach to build a step down autotransformer. It doesn't say how to go about figuring out which autotransformer I would even need. That's the real question I guess as once I get the transformer I could use a voltimeter to check all the leads until I find the ones I need. But I think this could be a very useful piece of information since so many players would find a 100v tap very useful in finding their tone.

    As an aside, I did trash a set of tubes when I ran my variac too low!
     
  18. vibroverbus

    vibroverbus Member

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    Weeelllll....

    ...and...

    Pretty much says it all: Find a tx with the above specs. Wire it up per the 'bucking configuration'. If you have polarity wrong on one side it will be the boost config (aka 140VAC) but you can easily test the voltage to find out, then switch 2 leads and test again.

    Electricity in US is SUPPOSED to be 120VAC, so that's what you design for, obviously the reason people have power conditioners is to correct for the reality which as you say can be different...

    Really the power-scaling option is a much better idea. I just didn't want to crap on the stupid 'brown' variac hack for the 100th time, but basically I think it's the idiot's way to adjust B+...
     
  19. mitch236

    mitch236 Member

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    Doh!!! For some reason, I didn't see that part of the thread.

    Trouble is now I can't find the transformer.
     
  20. mitch236

    mitch236 Member

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    Ok, I called Mr. Ho and he is sending me a 100v tap.

    He said something interesting to me and I wanted to put it out here and see what you all think:

    He says that if you use his attenuator, you can lower the voltage to your amp considerably without stripping the cathodes. He claims that the attenuator presents a constant 30ohm load to the amp which lowers the current the tubes draw (I hope I got this right). He says that I should use the attenuator (which I already have) even if I don't attenuate the SPL's but to protect my amp.
     

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