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Old 4th November 2011, 09:29 PM   #1
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Default Power tube maximum current draw from a power supply

Hey there,

I was hoping someone could clear something up for me, what are the maximum current draws for tubes and what should the power transformer be rated for.

for example in a EL84 SE class A amp how much current should i have available in the transformer? i read somewhere that it uses most current at idle, yet that doesnt sound right at all. and if i was to add another EL84/6V6 what current will i need?

in a PP class AB amp or SE class A, what would be the maximum current required for
EL84/6V6/EL34/6V6? (not bias current)

This seems like a simple question, yet i havnt found anything out about it, information on it seems to be as scarce as designing a power stage from scratch.

Ricky
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Old 4th November 2011, 09:42 PM   #2
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Your power transformer should be rated to deliver more then the maximum current demand of your amplifier.
Before anyone can answer your question, you will need to give us more information.
What is the plate voltage? What is the screen voltage?
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Old 4th November 2011, 10:05 PM   #3
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How big is a ball of string and would you use it indoors or outdoors?

Seriously, though, have a look at VVT Transformers Ltd. For instance,
www.vvttransformers.co.uk/vth12922-1240_datasheet.htm

Take these outputs.
410-0-410 VRMS 200mA 6.3ct 3A 6.3ct 2A 5v 2A

This gives a theoretical 82 watts HT.
18.9 watts on the 3amp heater winding.
12.6 watts on the 2amp heater winding.
10 watts on the 5 volt rectifier heater winding.
The above is for a 20/25 watt amplifier.

Compare this with Valve Data Sheets for typical anode & screen currents and Heater current. In an Amplifier, forget anode & screen voltages. Its the HT supply that's important as the current going through the anodes & screens goes through resistors which also dissipate power. So, taking the highest HT voltage (output of HT rectifier / HT capacitor), add up all the current flows, multiply Volts x mA = power.
Do the same sort of thing for the heater power. These are what your power transformer needs to output without overload at the mains input voltage where you are (230/240v).

Ok?

Last edited by payitforwardeddie; 4th November 2011 at 10:08 PM.
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Old 4th November 2011, 10:24 PM   #4
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Ok, 300V HT, i think ive just about grasped PP designs, and i know higher voltages mean bias currents can be lower, but then would maximum current be higher than what is available?
And i just want to check on the class A bias thing, is it true, that the current wont go higher than idle? (or at least wont go much higher?) checked some datasheets and it seems some are much more useful than others...
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Old 4th November 2011, 10:48 PM   #5
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Class A - that's simple. Constant power. If it varies, that's distortion.

Look up some amplifier designs, there's loads here and elsewhere try to see where the current flows are. Then try to do the sums.

PP - these are usually Class AB, ie at low power they are essentially Class A and the power used is constant. One valve passes more current as the other passes less. Up to a point the two current flows will balance. (Imagine a child's seesaw) At some however one valve turn tend to turn off (as it approaches zero anode current) but the other can keep increasing. This is Class B.

Now, the higher voltages bit.
Take 300v. if the current flow is 200mA then the power used is 60 watts. If you use 600v and redesign the amp to take 100mA then the power used is still 60 watts. This is why Data Sheets can be confusing. It all depends where the designer puts his Load lines.
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