I have a pile of used xfmrs that I've been using to supply heater filament voltage. None have a 6v secondary, but a few of them have 240v primary and 12v secondary or something similar to that. The ratio gets me down to 6.3 volts on the button when Xfmr is unloaded.
I figured that I'd have 4 amps (or less) of heater current at 6.3 volts. So any Xfmr rated for .025kva or more should be fine. I found a .050 kva Xfmr and wired it in. But I had a voltage drop! I was getting about 5.6v. I didn't expect that.
Now I'm wondering if the current is too high. I know the power and voltage ratings of the Xfmr are okay but I don't know what gauge of wire was used to wrap the secondary.
Does this make sense? Do I need to step up to .075kva?
If that's the case then I might just give up on my salvaged parts and shell out for a heyboer.
I figured that I'd have 4 amps (or less) of heater current at 6.3 volts. So any Xfmr rated for .025kva or more should be fine. I found a .050 kva Xfmr and wired it in. But I had a voltage drop! I was getting about 5.6v. I didn't expect that.
Now I'm wondering if the current is too high. I know the power and voltage ratings of the Xfmr are okay but I don't know what gauge of wire was used to wrap the secondary.
Does this make sense? Do I need to step up to .075kva?
If that's the case then I might just give up on my salvaged parts and shell out for a heyboer.
Kva.. Are you an electrician? THAT would be a BIG tube amp... Yes there are such beasts.
Anyway, Since the transformers you are using are rated at 240 volts in and you are using half of that as input, then that will half your VA rating of the transformer. So lets say as an example, that you have a 240v in / 12 volt out transformer rated for 4 amps @ 12 volts output. If you use that transformer with an input of 120 volts, then you will get a max of 4 amps @ 6 volts output. You will not get 6 volts at 8 amps out. Because you are using the transformer with half of its rated input voltage, the VA rating gets cut in half at the get go. So, basically you have a very heavy transformer doing the job that a transformer half its size could do. If you don't mind the extra weight, then go for it, it won't harm anything.
If the transformer you are using is dipping down to 5.6 volts, then it's not rated for the current that your circuit needs. It sounds like you're using control transformers, and they are rated strangely... If it says .050 Kva on it, that means that it will consume a max of .050 Kva on the circuit that it is wired into. It does NOT mean that it will provide .050 Kva of output.
Hope this helps.
Anyway, Since the transformers you are using are rated at 240 volts in and you are using half of that as input, then that will half your VA rating of the transformer. So lets say as an example, that you have a 240v in / 12 volt out transformer rated for 4 amps @ 12 volts output. If you use that transformer with an input of 120 volts, then you will get a max of 4 amps @ 6 volts output. You will not get 6 volts at 8 amps out. Because you are using the transformer with half of its rated input voltage, the VA rating gets cut in half at the get go. So, basically you have a very heavy transformer doing the job that a transformer half its size could do. If you don't mind the extra weight, then go for it, it won't harm anything.
If the transformer you are using is dipping down to 5.6 volts, then it's not rated for the current that your circuit needs. It sounds like you're using control transformers, and they are rated strangely... If it says .050 Kva on it, that means that it will consume a max of .050 Kva on the circuit that it is wired into. It does NOT mean that it will provide .050 Kva of output.
Hope this helps.
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Actually real transformers tend to cheat a bit on the windings ratios. So it may not scale linearly as expected. But first I would check the AC line voltage. Then if you want to get silly you can wire a second transformer to boost the incoming line voltage to the filament transformer. To do that wire the primary as normal but put the secondary in series with the AC line. One phasing will drop the AC line the other boost it.
Yeah. It's as I thought.
Thank you for confirming that for me.
I am an electrician and I was trying to use a control Xfmr. I don't have a single one that'll do the job. I found a .075 kva that goes from 480 to 24, but it's hefty and only going to give me about 3 amps anyways.
I have one more idea. I think I have a dc power supply salvaged from some drive unit. I'll have to check that out. I'm not optimistic.
Thanks for the assistance.
Thank you for confirming that for me.
I am an electrician and I was trying to use a control Xfmr. I don't have a single one that'll do the job. I found a .075 kva that goes from 480 to 24, but it's hefty and only going to give me about 3 amps anyways.
I have one more idea. I think I have a dc power supply salvaged from some drive unit. I'll have to check that out. I'm not optimistic.
Thanks for the assistance.
If this is a double post please delete.
Most 240 to 24 volt control transformers will output 27 volts +/- a volt in the very popular and cheap 40 VA size
Most 240 to 24 volt control transformers will output 27 volts +/- a volt in the very popular and cheap 40 VA size
Hammond Filament Transformer 166Q6
I think this would be perfect for me. Thanks again for all the advice.
I think this would be perfect for me. Thanks again for all the advice.
The main thing to do here is to figure your total heater draw. For example, a 50W amp with SS rectifier, no tremolo or reverb has two 6L6 for 1.8A and let's say 3 12AX7s for .9A total. This gives you 2.7A. For that 50VA tranny, this shouldn't even come close to loading that one fully, so it makes me wonder why you had that much voltage drop. The VA rating is for full load conditions.
So list all your tubes, look up the heater draw and add them up. This will be what you base your transformer size on.. Then add maybe 20% in case you want to add anything else later if you think this is a possibility. Or just go to the next standard size. It shouldn't be all that much more.
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The GE tubes measure 1 ohm across the filament whereas the Mesa branded tubes measure 1.4. I think that my power tubes may be at the high end of heater current draw. (I suppose I could've measured that...)
And the 50va Xfmr is used, it was stripped out of some unknown device years ago. I think it's a combination of things.
The Xfmr that failed otoh is a 25va. It was definitely overloaded.
I came up with a 75va but it was also old and overly large and it wasn't even rated down to 12v like the others. It was 480/24. Another questionable choice.
@simon7000 - I think the point must have gone over my head. I don't get it.
I'm still struggling with some of the simple electronics. My job as an industrial electrician has kept many things fresh in my head and I know many things that are relevant to my job, but I have rarely had the opportunity to troubleshoot at a component level. I had electronics school about 15 years ago and I used to know a lot more, but most of it is gone, or maybe at best hidden deep in memory.
I don't know what I'm missing here.
I'm still struggling with some of the simple electronics. My job as an industrial electrician has kept many things fresh in my head and I know many things that are relevant to my job, but I have rarely had the opportunity to troubleshoot at a component level. I had electronics school about 15 years ago and I used to know a lot more, but most of it is gone, or maybe at best hidden deep in memory.
I don't know what I'm missing here.
I or many here could give you the answer, but try it, then if the light bulb in your head doesn't go off an explanation is in order.
You can really use any light bulb. You will certainly be a better electrician for it.
Here is a link to the data sheet for a 6L6 tube. http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/6L6-GC.PDF If you look at the top of the first page where it says "heater", under that it says "current at 6.3 volts"... 0.9 amp. This is the heater current it will draw. All tube data sheets will give this info, usually at the beginning of the data sheets. Since heaters are generally wired in parallel, at least most modern amps are, the currents add up. So in my example, two 6L6Gc tubes will draw 1.8 amps. The data sheet gives 0.3 amps for a 12AX7 at 6.3 volts, so three will be 0.9 amps for these. Added together, it comes to 2.7 amps.
This will be for a solid state rectified, two 6L6, three 12AX7 tube amp, around 50 watts output. I just used this as an example to illustrate how it works.
This link should get you info on most any tube you will come across.
TDSL Tube search
You need to know what your total current draw for your heaters is to determine if your transformer will be able to handle the load. So get the current draw for all the tubes in your amp and add them up. I suppose you need to add the draw of the pilot light if it is wired to the heaters but shouldn't add much. Maybe the voltage was 5.6 because your tranny is rated 240V/12v, so 120 volts on the primary will get you only 6 v on the secondary, so you are starting .3 volts low. This probably why your control circuit transformers had low voltages. You said it got you to exactly 6.3 volts unloaded but transformers will have lower output when you start to load them so 5.6 v is pretty realistic. The only other wild card is if you have a 6 volt rectifier tube. You also need to add its current in as well.
It seems as though filament transformers are the way to go since they are engineered for the purpose.
This will be for a solid state rectified, two 6L6, three 12AX7 tube amp, around 50 watts output. I just used this as an example to illustrate how it works.
This link should get you info on most any tube you will come across.
TDSL Tube search
You need to know what your total current draw for your heaters is to determine if your transformer will be able to handle the load. So get the current draw for all the tubes in your amp and add them up. I suppose you need to add the draw of the pilot light if it is wired to the heaters but shouldn't add much. Maybe the voltage was 5.6 because your tranny is rated 240V/12v, so 120 volts on the primary will get you only 6 v on the secondary, so you are starting .3 volts low. This probably why your control circuit transformers had low voltages. You said it got you to exactly 6.3 volts unloaded but transformers will have lower output when you start to load them so 5.6 v is pretty realistic. The only other wild card is if you have a 6 volt rectifier tube. You also need to add its current in as well.
It seems as though filament transformers are the way to go since they are engineered for the purpose.
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I have an idea.
The resistance seems way too low. I googled an article that refers to filament resistance at room temperature. Is that what I missed? My 60 watt bulb measured about 20 ohms, which would be 6 amps at 120vac. So the bulb should really be about 240 ohms through the filament when it's turned on?
Yes filaments do not follow ohms law. They heat up and the resistance increases. So a simple ohm meter will mislead you. Now for my next magic trick how much current does a light bulb draw when it is blowing out? (Since you can't measure this easily, the answer is that arc flash can draw ten times or more the operating current!)
Both useful things to know when circuit breakers trip for no apparent reason!
Also useful to know circuit breakers often handle current surges without tripping!
Next topic, how do you tell when the 4160 VAC line is disconnected? (Wrong answers will kill you!)
4160v isn't in my job description but the big transformer in the vault has an air switch. 13,300v? 12,600v? Can't recall right now.
The answer to that one is to use your eyes. Air switch open. Then we use a pole and ground all the phases.
That's the procedure here anyways.
The answer to that one is to use your eyes. Air switch open. Then we use a pole and ground all the phases.
That's the procedure here anyways.
You'll live. Sad tale of the fellow who opened the disconnect switch that was wired in the neutral leg not the hot.
The there was the fellow who wanted to check the minimum spacing of a 22KV line using a steel ruler...
The point is that there are bits of knowledge that actually can be useful!
So now you know Ohms law isn't always.
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