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They don't need to be operated at these limits though, you can easily have them put out less - or a bit more if you're willing to push the ratings a bit hard... or be creative with how you define and measure it .
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
No, most 40-50 watt valve amps will use around 150 - 250 watts or so.
I think most of the power use is by the valves and converted to heat. Transformers are very efficient devices, though there is a small loss there too, circuit components will not use much, speaker a little if your running very loud.
Dissipation is generally heat mostly from the valve dissipation + heaters. There is a tiny amount from the transformer but seeing as they are pretty much the most efficient things humans have made you can safely ignore it.
From memory each output valve of an AC-30 draws about 18W at full chuff, to deliver about 7W audio. The other 11W is heat. Multiply this by 4 and you can see why they get so toasty.
You can break many rules in rock and roll but not the laws of thermodynamics.
Complicated only in the sense that you cannot just look at an amp and say "Ah" two EL34s, that'll be a 50 watter then!"
Also complicated by the fact that SOME amp makers tell porkies or at least make the name suggest a power level the amplifier is unlikely to achieve in practice such as the "BogRock 25" when it contains but two EL84s. Such a pair in the vastly more common cathode biased mode (for 84s) will kick up to about 20W with a high HT and the mains on top but 15-18W is a fairer description.
A good set of examples (if I might be spared accusations of spam?) is the HT range?
The HT-20 uses 34s but with a fairly low HT and part cathode bias. The amp will only deliver around 20 watts before the sine wave gets nasty.
Up the HT a bit and you have the HT-40 which with fixed bias now easily hits 40W and quite a bit beyond.
The HT-60, still only 2 EL34s will easily deliver 80watts but you run into power supply cost issues (I guess!) because the HT-100 only puts out a bit over its honest specc'.
Note too that all these figures are for a PROPER nominal 230V mains input, most folks get around 240 so win-win!
Mind you, these numbers are also for clean power out. Start to push amps into heavy distortion and the actual "rms" power climbs and as the output gets ever "squarer" its destructive power increases. This is why I always want a speaker to be rated at least 50% higher than the amp driving it.
In my last days at B's I was asked to check the PO of some chassis at 10% THD. Is the guitar amp industry starting to get some decent standards?
Dave.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Further to the valve specc'/power output thread (and the HT-40 debacle).
I mentioned testing output power to the 10%THD point?
This is the only fair way IMHO to test valve guitar amps. Some of you will be familiar with "hi fi" specifications will look at 10%THD with horror but in fact it is only just detectable on a sine wave and hardly at all on music and speech . Therefore a guitar amp hitting that figure will be judged to still be "clean" and 10% is handily the level that most valve makers quote for their valve power output point, e.g. the EL84 delivers about 5W at 10%THD.
Power output is often quoted at "clipping"? This can be very difficult to call on a scope and in fact if you are looking at a waveform and reading THD the trace often barely changes from 1 to 10% distortion.
Dave.
As stated above I had a Dynachord amp that used 2 x EL34 at around 800 VDC on the anode and this measured at around 95W at clipping.
"Surely the THD goes up as you approach clipping, i.e. by the time you're clipping THD is high (by definition)? If I understand correctly (which may not be the case), valve power will continue to go up as THD increases while solid state designs typically are operating near their ceiling power before THD starts to rise (and then it rises fast), due to the higher level of negative feedback in most solid state designs. Or do you mean clipping because they've hit protection?"
Yes, sorry! My point should have been that it might be difficult to FIND a 10% THD point for many sstate designs. They tend to produce buggerall distortion then whammo! Off the scale. A valve amp with little or no feedback can be pushed progressively, 1,2,3,....10% and so on.
Err? What did that Service man Terminator SOUND like J?
Dave.