Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boogieWoogie on February 09, 2020, 10:10:12 am
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Hi folks,
I'm working on a Vox AC30C2. Currently there are no real problems I'm aware of (although my bandmate believes it's quieter than a 30W amp should be). The thing that concerns me is the current draw & power consumption from the perspective of the power outlet. From the outlet, how many watts should a 30W amp consume? I'd assume more than 30W, due to efficiency loss / heat / etc, but 75W doesn't feel right to me. Thoughts?
Conditions: ~4 mins of warm-up time, no inputs, volume & master-volume are on 10.
Thanks,
BW
Current / Power consumption from wall-outlet
(https://i.imgur.com/hkE0O81.jpg)
Vox AC30C2
(https://i.imgur.com/g4kX4HR.jpg)
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Sounds very realistic to me. Heck, those EL84s are wasting over 19 watts just to burn the filament.
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Ok then, I'll package this thing back up & enjoy it! Thanks for the reply sluckey.
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Where did you get the idea to mount the meter on your variac?
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Not unusual for the power supply to consume 3x or 4x the amount of rated output power of a tube amp. The current draw at the mains is the sum of the V.A. on all the PT secondary windings, which includes the heater winding (and in tube-rectified amps, also the rectifier winding).
Whereas, the power output at the speaker jack is a different kettle of fish - determined by the voltage and current from the output tube plates and the VAC/impedance ratio of the OT.
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Gotcha tubeswell. I'd never thought to look before, but when I think about the heat coming off tubes, it should have been clear that these are not efficient devices.
sluckey, there's a youtube channel where a fella (calls himself the "guitologist") repairs amps, and on one of his episodes, he showed how he patched that $15 digital multimeter into his variac. Here's the episode: https://youtu.be/2mrlha-uzro (https://youtu.be/2mrlha-uzro). It's been handy to have a visual indicator as I power up an amp on the variac.. if I see the current taking off, I can shut it all down before I burn through a transformer or tube.
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Filament light bulbs are maybe 4% efficient, Class A amps are especially inefficient... I'm impressed that your tubes are doing so well!
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The plate circuit of a tube amp (smaller than a car) is 45%-56% efficient at-best.
So *when making full "30 Watts"* the plate circuit sucks 53-67 Watts of DC.
There is some loss converting your AC to DC. Say 59W-76 Watts.
Four EL83 heaters are 19 Watts extra. 78-95 Watts total.
Your "75.5W" meter number is at the low end of the range because (I assume) you are reading it silently, not full-blast. Even a class A amp (ideally constant input power) typically sucks a bit less at idle than at full roar.
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Filament light bulbs are maybe 4% efficient
maybe for light, but use north folk are happy with the heat waste :icon_biggrin: