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Old 28th November 2010, 10:41 PM   #1
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Default max current draw according to loadlines and reality

Hello all,
ok this may seem quite a stupid question but if i was building a power amp with 4 output tubes, do i need a transformer that can supply every milliamp of current shown by loadlines? the datasheet for an el84 using 8ka-a output needs max 150mA (at 300v/Za-a/4), and then halving the output transformer and adding another tube means i would need about 300mA. am i thinking along the right lines? (no pun intended)

thank you in advance
Kind regards
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Old 28th November 2010, 11:04 PM   #2
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I won't say this is a stupid question, but I don't understand it. You need to be more specific and maybe post a schematic.

Perhaps, there are folks who know what you are talking about and they can help you. I can't.

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Old 28th November 2010, 11:09 PM   #3
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sorry, i dont have much of a schematic, im trying to design based on the instructions on this website. the loadlines drawn on the datasheet graphs go up to 150mA. then for a second tube in parallel with the first one i would assume you need twice as much current. but then do you need a transformer that can supply exactly that amount of current? more? or could i get away with supplying less current?

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Old 28th November 2010, 11:59 PM   #4
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Do you mean to parallel output toobs for twice of power?

Sure, you will have twice of current: both idle and peak, so transformers should have twice lower primary impedance and twice higher current capabilities, for twice higher power.
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Old 29th November 2010, 04:26 AM   #5
kevinkr is offline kevinkr  United States
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I assume you are actually asking about the current rating of the power supply and transformer? Many hifi manufacturers scrimped on the power transformer current ratings and used transformers rated for continuous operation at about 70% of maximum power consumed, and this may be reasonable in your application depending on what it is being used for.

In your case a 250mA (DC) CCS rated transformer ought to do the job fine if average power consumption is no more than that. Short term peaks can be accommodated provided the winding resistances are not high enough to adversely impact short term voltage regulation - in guitar amp applications sag is considered desirable. (I'd experiment with a 200mA transformer for guitar use..)

Higher current ratings are very reasonable in that they result in cooler transformer operation and lower noise. If ventilation is marginal an upsized PT is recommended and this would also be the case if line voltages tend to be high.. (I tend to go this way..)

Note that there is common precedent, well actually worse than this - just look at any Dynaco Stereo 70 amp, each channel idles at 100mA, maximum current consumption per channel IIRC is >200mA and the rectifier is rated at 250mA, and I doubt the original power transformer is rated any higher than this.. And they don't break all that often..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

Last edited by kevinkr; 29th November 2010 at 04:29 AM.
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Old 29th November 2010, 06:30 AM   #6
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ok thanks for the help, i just dont want my transformer to turn into a few useless coils
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