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How do you tell what size fuse to put in your amp? I thought I could just add upp the amps the tubes were drawing and that would do it, but that can't be right; the 845's draw over 6 amps just by themselves. Is there a rule of thumb?
Vince
Follow Ups:
Here's the rule from Electra-Print:total VA rating divided by line voltage equals line current drawn, times 133% is the fuse rating ( example, 200va/117v =1.7A times 133% is 2.2A or 2A fuse)
http://electra-print.com/custompower.php
Don't use jarthel's method as it requires you to power on without a proper fuse, or with at best a guess, and that is plenty of time to burn up the power transformer.
measure voltage drop across resistor. use ohm's law to derive current.
Hi.Taking into the consideration of instantaneous spikes & surges of non-sinusoidal natures riding on the static load, better put at least 50% margin of what is measured across the resistor.
A slow-blow fuse should be used.
For example, a single 845 filament consumes 32.5 watts (10V x 3.25A). If the anode has 1,000V and draws 70mA (0.07A), that's 70 watts. After you add everything up like this, divide by 120 (or 240 if that's your line voltage) to get the approximate amperage on the AC side of the amplifier. To accomodate the inefficiencies of power transformers and rectifiers (and to include a window for the fuse to operate in), double your result. That's the size fuse to start with. Note that if you're using a SS rectifier and large value caps in a cap-input HV filter, you might need a higher rating and/or or a Slo-Blo fuse.
nt
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