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Estimating power output

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I think I may have asked this before, but I don't think I ever got a real answer and it remains one of the eternal mysteries to me. So, how does one estimate the output power of a tube amp.

I know this requires context, and to make it the simplest possible example, assume a tube with a certain Gm=x and mu=y, a plate voltage=Vp, biased to voltage=-Vb via a resistor and cap, or with an LED, from the cathode to ground and transformer coupled to a speaker. Also, assume an input voltage of Vi.

An externally hosted image should be here but it was not working when we last tested it.


Any quick and dirty way to figure this out. I am not looking for any precision here, just a rough guess to know if a tube is appropriate for a particular project.

Thanks
 
Here is something that will form an "upper bound" on a triode circuit.

A class A1 circuit is has a maximum theoretical efficiency of about 25%
Real world, somewhat less.

Class A2 Circuit is has a maximum theoretical efficiency of about 50%
Real world, somewhat less.

Example, An EL84 plate dissipation is 12 watts.
Absolute max for an EL84 SE class A1 would be about 3 watts. Add a transformer or follower and re-bias for A2, and the max is about 6 watts.

Myself, I just Fire up TCJ tools.

Hope this helps;

Doug
 
Hi Doug,

Here is something that will form an "upper bound" on a triode circuit.
A class A1 circuit is has a maximum theoretical efficiency of about 25%
Real world, somewhat less.

This assumption is not valid given sufficient voltage. An A1 300B @ 450V anode, 80ma with -97V fixed bias and a 2,000 ohm OPT can make 17.8 watts according to the Western Electric Datasheet. This gives a plate efficiency of 49%. Distortion is high and the tube is run hard, but these are not the issues at hand.
 
Class A does indeed have a maximum theoretical efficiency of 50%, by definition. Here we mean plate/anode efficiency rather than overall, which tends to be rather lower. Usually triodes arent run SE with low loads and high volts, too much distortion (especially if your a feedbackaphobe.) When you do this in PP you tend to end up in AB1 or AB2.
 
One more question -- power dissipated by the tube is current drawn * voltage dropped across the tube, I assume. But, some of this is audio power, and thus not dissipated as heat, right?

So, if a tube's rated plate dissipation is 3W and I have B+ of 330V, a current draw of 10mA, and it is biased to -30V (assuming the schematic above), can I assume at least some of the power is going to the output and thus I am not right at the 3W level, or am I missing some fundamental concept here?
 
dsavitsk said:
One more question -- power dissipated by the tube is current drawn * voltage dropped across the tube, I assume. But, some of this is audio power, and thus not dissipated as heat, right?

Right.


So, if a tube's rated plate dissipation is 3W and I have B+ of 330V, a current draw of 10mA, and it is biased to -30V (assuming the schematic above), can I assume at least some of the power is going to the output and thus I am not right at the 3W level, or am I missing some fundamental concept here?

In a case like this, you have: P= (330)(10E-3)= 3.3W of DC going in. If you're 25% efficient, then you get 825mW of audio out. That'll leave 3.3 - 0.825= 2.475W to be dissipated as heat. Still, it is not good (although it's done a lot with RF amps) to rely on output power to get you below the nominal Pd rating of the VT in question. Running very hot like that will reduce the service life of the VTs.

The usual rule for Class A1 and AB1 operation is to set the Q-Point current so that the (Vpk)(Ipq) is about 70% of the rated dissipation.
 
hey there - some links for calculation of SE output power.

http://www.valveheart.com/theory/triodeSE.html
http://members.aol.com/sbench101/

I guess you'll have seen the last one already.

One other thing about where the audio power, fits in to things. Surely, there is only audio power when there's input signal? If you're over the limit at quiescent then, as I understand it, you're in a bad place.

Andy
 
Actually, the given schematic shows cathode bias (although I have never seen a 30 volt LED, SY has proven that this can be done). Therefore you do have 300 volts across the tube with 10 mA the tube dissipates exactly 3 watts at idle. The dissipation in the tube will be reduced by the power delivered to the load. So if you could squeeze 1 watt out of this amp the tube would dissipate 2 watts. This assumes that the bias current doesn't change when signal is applied. In theory it shouldn't, but in practice, it does. The dissipation in a class A amp is always worst case at idle with no signal applied.

I have experimented along these lines with some of my glowing Chinese 6L6GC experiments. I can set the bias (in excess of the max spec) on the 6L6 to the point of a dull red glow in the plates. When the amp is then cranked to the max, the glow will disappear. However when operated at a normal listening level the tube is still glowing. Most music has a 10 to 20 db peak to average ratio. Therefore if you crank the amp up so that it is just clipping on music peaks, the average power level is 10 to 20 db lower (1/10 to 1/100 lower power). If the amp makes 10 watts the average listening level is in the sub watt level, so not much power is being delivered to the load (averaged over time).

As stated before most tubes should be operated at 70 to 80% of there max spec at idle for reasonable tube life. Now that I have said that, I will tell you that I have operated some tubes at 110% of max and higher. Some tubes seem to handle this without issue, while others will die in short order. The previously mentioned Shuguang "Coke bottle" 6L6GC served a life sentence in a Fender Bandmaster before I got them. I have been abusing these poor tubes for about 2 years, often with glowing red plates, and they still work good. Some of the EH guitar amp tubes can be cranked up without issue.

The Tubecad SE amp cad software provides a decent simulation of a SE amplifier and the power output and efficiency estimates are pretty accurate. As far as efficiency goes, I have built SE amps from 5% to 45% efficiency. Often tweaking an amp for best distortion performance lowers the efficiency considerably.
 
The total dissipation in any amplifier, tube or solid state, is the DC power being supplied to the amplifier minus the power delivered to the load. In a pure class A amplifier the power being supplied from the power supply to the amplifier is fairly constant regardless of the signal level. This is true up until the clipping point where the amplifier is no longer operating in class A. This means that the dissipation is worst case when the amplifier is undriven since it is not delivering any power to the load. All of the energy from the power supply is burned up as heat.

The situation is somewhat different in a class AB or class B amplifier. The same rules still apply. Dissipation is still the power drawn from the power supply minus the power delivered to the load. However the power delivered from the power supply to the amplifier is not constant. The current drawn from the supply increases an the amplifier is driven harder. The dissipation can be computed if you can measure the voltage across the output tubes, the current through the output tubes, and the amplifiers output power. The dissipation in each output tube is the voltage across the tube, times the current through the tube, minus 1/2 of the output power (push - pull). The dissipation changes as the power level changes. The worst case condition is dependent on the bias level and the load impedance. It is usually somewhere between idle and max power, but can occur at max power in a heavilly loaded amp (low output impedance). The dissipation in a class B amp is usually worst case at maximum power output.

The efficiency of an amplifier is the ratio of power drawn from the power supply and the power delivered to the load. This also changes with power output and is obviously zero at idle.

Example (all numbers are made up for ease of calculations):

A push pull amplifier operates from 430 volts of B+. It is cathode biased with 30 volts on the cathodes. The idle current is 100 mA total, and the current drawn at full power is 250 mA. Power is measured at 50 watts RMS with some reasonable distortion figure (5% or so). A high distortion makes power measurements difficult without a true RMS meter.

At idle the amplifier dissipates (430 - 30) volts * 100 mA - 0 watts of output power or 40 watts. This is split between the two output tubes giving 20 watts per tube. At full power the amplifier dissipates (430 - 30) volts * 250 mA - 50 watts of output power which is 50 watts or 25 watts per tube. The efficiency is 50 watts / 100 watts or 50%.
 
tubelab.com said:
Actually, the given schematic shows cathode bias (although I have never seen a 30 volt LED, SY has proven that this can be done). Therefore you do have 300 volts across the tube with 10 mA the tube dissipates exactly 3 watts at idle.

This isn't a real tube, it was just a constructed example. I am actually trying to use a 6V6 as an output tube and am running up against its dissipation limits, so I was just looking to know how hard to push it.
 
The spec for a 6V6 is 12 watts for the plate and 2 watts for the screen grid this implies that you could do 13 or 14 watts in triode mode. I have some nice looking 6V6G tubes that I wouldn't even push that hard. I am running them at about 300 volts and 30 mA they sound really nice, and make almost 2 watts. Next time I get the amp out, I will measure the exact settings.

I also have a bunch of some late vintage wafer based junk (Sylvania I think) that doesn't glow until over 20 watts. The problem is they sound like crap at any bias level. The Sovtek tubes can probably take a lot of abuse, but I haven't tried them.
 
I mistyped -- I mean 6W6 which is 10W + 1.25W. I have a box full of them that I was planning to use with a CCS loaded 6BQ7A, which I also have a box full of, as a driver. The 6BQ7A looks to be almost the same as a 6N1p.

The idea was to design for this tube (6W6), but to try to do so such that 6V6's, or EL34's or any number of other tubes could be used. A watt or two is all I need for the project, so I think power requirements are met. My sense was to use the rectifier to adjust for various tube types -- something that drops lots of volts for the 6W6, and maybe a 5ar4 for el34's.

Schematic is almost the same as the SimpleSE except the CCS is of the 2 transistors and 2 led variety (digikey is out of the 10m45 for a while) and the driver will be LED biased.
 
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