wall outlet watts vs. amplified watts (Vox AC30C2 seems too hungry)

[boogieWoogie]

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


Vox AC30C2

[sluckey]

Sounds very realistic to me. Heck, those EL84s are wasting over 19 watts just to burn the filament.

[boogieWoogie]

Ok then, I'll package this thing back up & enjoy it!  Thanks for the reply sluckey.

[sluckey]

Where did you get the idea to mount the meter on your variac?

[tubeswell]

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.

[boogieWoogie]

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. 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.

[Greenwichpaul]

Filament light bulbs are maybe 4% efficient, Class A amps are especially inefficient... I'm impressed that your tubes are doing so well!

[PRR]

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.

[shooter]

Filament light bulbs are maybe 4% efficient

maybe for light, but use north folk are happy with the heat waste