# Amp Power Switch - What Kind of Rating Required?



## keto (May 23, 2006)

So, I have some switches that are 125VAC 25amp I was gonna use, but I buggered the holes. The ones I want to use instead are 125VAC 6amp. I have no clue what I need as a sort of minimum spec for a power switch on an amplifier. I think the replacements are fine but looking at the construction they are definitely lighter than the originals.


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## greco (Jul 15, 2007)

I am going to guess that the switch rated for 6 amps should be fine. 

6 amps x 125 VAC = 750 watts power. 
To me, it is difficult to imagine the amp needing that much power.

I await the responses from those that actually know what they are talking about.

Meanwhile, here is some interesting reading:

http://www.tdpri.com/forum/amp-central-station/239717-how-much-ac-power-does-amp-draw.html

Cheers

Dave


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## Wild Bill (May 3, 2006)

keto said:


> So, I have some switches that are 125VAC 25amp I was gonna use, but I buggered the holes. The ones I want to use instead are 125VAC 6amp. I have no clue what I need as a sort of minimum spec for a power switch on an amplifier. I think the replacements are fine but looking at the construction they are definitely lighter than the originals.


Here's how you figure it out!

First, look up the current drawn by each tube to light up the filament/heaters. Each 12AX7 draws 300 ma, or .3 amp. Output tubes draw various amounts. Look them up and add them to arrive at a total.

P(watts) = Volts x Amps

So 6.3 volts to the filaments x the total current drawn by all the tubes will give you a wattage total for just lighting up the tubes. Let's assume 3 12AX7s and an amp or so for a pair of output tubes. This let's us end up with a figure pulled out of our butt, just for discussion purposes. Let's say it's 12 watts.

Now, a typical tube amp output stage will run maybe 33% efficiency. Which means if it delivers 40 watts then it probably draws 3 times that to do the job, or 120 watts.

So 120 watts for the audio power and 12 watts to light up the tubes adds up to 132 watts.

Now, the power coming from the wall that goes through the power switch and into the primary power transformer is at about 120 volts. Going back to our equation, P = V x A means we must solve for A to get the amps involved.

Do the math and we get 1.1 amps! So your switches are comfortably over rated!

There are a couple of other factors to remember that might be important in other situations. Switching contacts are rated for breaking current. That means your switches will switch up to a steady 6 amps. An amp draws an initial startup rush of current that is quite high but only for a very short time. When you first turn it on the tubes are cold and draw a big spike of current. The filter caps may be totally empty of energy and also draw a big startup pulse. However, we're talking split seconds here! Things steady down VERY quickly!

Go back to that 120 watt guesstimate for the power tubes. That's only true for full power peaks. Music is not like that. It has parts much lower in power. It's what we call dynamics. Also, we usually don't play on 11! The AVERAGE power even when playing full out will be much lower than 120 watts!

So your switches are actually very much over-rated for your amp! It's always nice to have a big safety factor!

Wild Bill/Busen Amps


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## keto (May 23, 2006)

Well, Bill, that is an excellent, complete, and easy to understand answer. Thanks very much! 

BTW, I'm running 2 12AX7's and a single octal (KT88 is what the project calls for).


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## Rski (Dec 28, 2013)

Us electrical designers look at the VA rating of the transformer overall, which in most cases not clearly specified unless you sum together the transformers total secondary. (6.3 vac x 5 amps) =31.5va (380 vac x 1000 mA) = 380 va total would be 412 va secondary primary would significantly yield 412 ÷ 115vac rounds to 4 amps for that example. A glass time delay 4 amp fuse will easily protect your build.

With kt 88 watch the heater current with the filament draw on the 6.3 portion of your transformer.


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## Wild Bill (May 3, 2006)

Rski said:


> Us electrical designers look at the VA rating of the transformer overall, which in most cases not clearly specified unless you sum together the transformers total secondary. (6.3 vac x 5 amps) =31.5va (380 vac x 1000 mA) = 380 va total would be 412 va secondary primary would significantly yield 412 ÷ 115vac rounds to 4 amps for that example. A glass time delay 4 amp fuse will easily protect your build.
> 
> With kt 88 watch the heater current with the filament draw on the 6.3 portion of your transformer.


Quite true, just expressed another way.

I have often wondered why engineers and electricians always talk in terms of VA, yet that was never true with electronic transformers in radio, tv or whatever, all these many years! In fact, I came up through the electronic world when tubes were still common and it was not until 2003, when I worked for a short stint for an electrical parts counter, that I discovered the VA method of rating.

Maybe it has something to do with different secondaries, each with its own voltage and current factors, complicates things. An electrical world transformer usually is a straight step up/step down single ratio unit. When designing a transformer for an amp or a radio, when you need one secondary at 6 volts and a certain amount of current, 5 volts for a rectifier filament at its own amount of current and of course, the HV winding, again at yet another voltage and current rating - the idea of a simple VA rating becomes not just irrelevant but about as useful as someone with only one arm trying to show you how big a fish he had caught! :smile-new:

Wild Bill/Busen Amps


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## jb welder (Sep 14, 2010)

Bill, I think the reason for the 2 types of ratings (VA or Watts) has to do with power factor.
Watts are considered "real" power, VA is "apparent" power. For DC circuits they are always the same. For AC circuits they may be different due to current and voltage not necessarily being in phase (reactive loads). So VA can be higher than watts due to peaks.
Watts are what we pay the utility company for, VA is used for rating fuses, breakers, etc.
VA times power factor = watts
"Industry standard rule-of-thumb is that you plan for a power factor of 60%, which somebody came up with as a kind of average conservative power factor."
http://www.powerstream.com/VA-Watts.htm


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## dtsaudio (Apr 15, 2009)

> I think the reason for the 2 types of ratings (VA or Watts) has to do with power factor.


Exactly the reason. It's a more modern spec for transformers, but is more accurate. When hydro was cheap nobody worried about power factor and wattage and VA were interchangeable.


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## Wild Bill (May 3, 2006)

jb welder said:


> Bill, I think the reason for the 2 types of ratings (VA or Watts) has to do with power factor.
> Watts are considered "real" power, VA is "apparent" power. For DC circuits they are always the same. For AC circuits they may be different due to current and voltage not necessarily being in phase (reactive loads). So VA can be higher than watts due to peaks.
> Watts are what we pay the utility company for, VA is used for rating fuses, breakers, etc.
> VA times power factor = watts
> ...


Well, perhaps that is true but still, I have been at this since I was 11 years old. I'm 61 now and spent most of my working years selling electronic parts. In that world, I never once heard someone refer to the VA rating of a power transformer.

Mind you, I led a sheltered life!:Smiley-fart:

If you go to the Hammond Manufacturing website and look up their series of classic tube amp power transformers you will see the VA rating of each model in the data listings but it is almost an afterthought.

I still say, when you are choosing a transformer in the electrical world all you need to know is if it will step from one typical voltage to another, and the VA rating. So in that world, focusing on the VA rating makes sense.

In the world of tube amps, VA is almost useless! It does speed the calculation of a suitable primary fuse but experience tends to make that redundant anyway. Instead of VA you have to know what windings are available, what voltage does EACH supply and the current rating of EACH winding!

You could easily have two power transformers of the same VA rating, yet the current each winding delivers is different. One transformer may be designed to supply enough current at 6 volts to light up only a few tubes. Another might be capable of supplying 3 times as many tube filaments, but since the other windings will be supplying less current the TOTAL power with each transformer is the same!

So while both transformers have the same VA rating you have no idea which is suitable for your application unless you look up ALL the winding specs!

For that reason, no one designing or repairing a tube amp would even bother with the VA rating. Why? As I said, you have to look at the ratings of each winding to know if the transformer will be suitable. Those windings may have different specs yet still total the same VA rating. Once you've checked the specs for each secondary winding the VA rating will be automatically derived but there is no use in knowing it! Knowing the VA rating yet not knowing if the 6 volt winding has enough current to light up all your tubes means the VA rating is of no help.

Different applications, different pertinent data is necessary.

Wild Bill/Busen Amps


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## jb welder (Sep 14, 2010)

Agreed Bill (you're preachin' to the choir :smile-new. But your argument is valid for VA or watts when dealing with multiple secondaries, in either case we still need to figure out the current available from each winding. But I thought specs were usually given for all the windings for transformers?


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## dtsaudio (Apr 15, 2009)

A fellow who makes custom transformers for me gives ratings for each individual secondary in amps (or milliamps) and the VA of each winding. He also gives the total VA on the primary side.









That's the way it should be done.


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