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Old 8th April 2011, 02:44 AM   #1
santa is offline santa  United States
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Default filament transformer current rating

Hi,
I'm going to power a tube with a heater current rating of 0.8 A, 6.3V.
The transformer I am considering has a secondary current rating of 2A no CT.
I plan to use a bridge rectifier and RC filter to produce DC at 6.3V.

Will the 2A 6.3 VDC transformer be sufficent to power the filament?
I have seen somewhere that the transformer current rating should be 1.8 times the load current when using a bridge.

I normally use a full wave for filaments but no CT in this case.
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Old 8th April 2011, 03:42 AM   #2
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IIRC the factor is 1.4. But you have current excess by over two, so you're good. Are you going to regulate the DC to the filament? It is hard to get regulated 6.3vdc from a 6.3vac winding. You will have to use Schottky Diodes and lots of capacitance. The DC to the filament will have to be well filtered (or regulated) or it will cause hum also. Sometimes worse than ac on the filaments.
Good luck...Daniel
PS. I just thought of something else, which I have done with good results, but your transformer may not have enough current output. What I am talking about is configure the transformer as a voltage doubler. Use 2 Diodes and 2 capacitors over 10,000uf each. This will give you around 12-14 volts DC which can be regulated easily as there is enough voltage.

Last edited by danFrank; 8th April 2011 at 03:45 AM.
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Old 8th April 2011, 03:52 AM   #3
singa is offline singa  Singapore
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Hi, 1.8 x 0.8A =1.44 times.

2A divide by 0.8A =2.5 times. Theory =1.8? More than adequate even if
power loss is incuded. That's for one tube,if it's for two tubes it will get hot.
So double up if that's the case to run cool. (2 tubes,1.6A X1.8 =2.88) singa
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Old 8th April 2011, 02:24 PM   #4
santa is offline santa  United States
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Thanks,
I plan to run one tube on this supply.
I may try a regulator when the transformers arrive but I'm not sure if I can get enough voltage as I only have 6.3 to start with.

I had not considered the doubler but it is a possibility as well.
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Old 8th April 2011, 05:55 PM   #5
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Hi again,
Look at the LT1084, LT1085, LT1086 regulators from Linear Technology. They have low dropout voltage and if you use the above diodes I mentioned with LOTS of capacitance you should be able to pull off regulation.
Daniel
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Old 9th April 2011, 06:57 PM   #6
santa is offline santa  United States
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Having so little headroom has bothered me to the point where I changed the transformer design.
I'll go with 8V@ 4 Amps so I will be certain to have enough power to overcome the losses incurred in rectification and regulation and I can run a second tube if needed.
I was going to go with a 6BM8 mu follower to drive my SET output but now I may go with 6SL7 - 6SN7 in the Aikido configuration.

My mistake was in trying to economize on cost, size and weight of the final product.
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Old 9th April 2011, 08:57 PM   #7
ryuji is offline ryuji  United States
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I personally bought a switch mode power supply with adjustable output and called it a day(cost me $20),otherwise I was going to build one custom. Smps are more stable and 80% or better efficient
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Old 9th April 2011, 10:55 PM   #8
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If you are working on the cheap and only need power for one or two heaters, you can usually find a suitable wall wart at the goodwill for a dollar. Take a DMM with you because you will likely find that half of them are shot. You may want to use a 6 or 12 volt regulator with it.
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Old 9th April 2011, 11:44 PM   #9
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Hello,
In my playing with stuff I have found that 6.3 Volt transformers with full 4 diode bridge rectifiers do not put out 6.3 volts DC let alone have any voltage to spare for a regulator. 12.6 volt AC transformers with a CT can use a 2 diode full wave rectifiers and work ok. The voltage drop across two diodes in series in the bridge rectifier consumes too much voltage.
The transformers that I like with the full bridge rectifier are 8 volt AC. The 8 volt AC through the diodes and capacitors puts out ~ 9 volts. 9 volts raw DC works well with the 1085 low drop out regulators.
Rule of thumb, I select a transformer with a VA rating at least twice the watts used by the connected heaters. This way the transformer does not get real hot and the voltage does not dip too low.
DT
All just for fun!
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Old 9th April 2011, 11:50 PM   #10
ryuji is offline ryuji  United States
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Theres also the fact that heaters consume a lot on power on. My amplifiers 12.6v supply needed to handle 8 to 10 amps for first two seconds and then 4.5 amps or so thereafter.
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