• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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
razorrick1293
 
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?

The Valve Wizard -Push-Pull
 

kevinkr

Administrator
Paid Member
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..
 
Last edited: