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26th July 2010, 04:42 PM  #1 
diyAudio Member
Join Date: Jul 2010

How do I calculate output power?
Hi all,
I'm looking for a formula for calculating the output power for SE amps. I understand the OT reflected load impedance and the valve's gm have to do with it but cant find a formula. .Can anybody help? Thanx ! Mario. 
26th July 2010, 06:21 PM  #2 
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Join Date: Nov 2004
Location: Vancouver

Hello Mario,
Most of the tube's datasheet would tell you the output power of the tube in SE mode with different B+ voltage. What kind of power tube you intend to work on? Johnny 
26th July 2010, 06:35 PM  #3 
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Join Date: Jul 2010

Hi Johnny,
Thanx for your reply. I do use data sheets as a guide but I wanted a formula of sorts so I can know what to expect with new builds and to study and learn what variables affect the output power and why. I basically have a new amp I built and although I can calculate the disibation of each tube by measuring the Plate voltage and cathode current I dont know what power output to expect. 
26th July 2010, 06:46 PM  #4 
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Join Date: Jul 2010

A quick edit......My new amp uses one half of a ecc83 as a line stage amp and a kt88 on each side SE UL. The screens are connected to the OT from tabs close to 4550% shared winding with the anodes.(the output tranformer is not designed for distributed loading I improvised). Screen grid resistors are 1k. B+ is 395v Plate Voltage is 375v and the primary load resistance is 8k(high I know for kt88's). Bias is fixed and the idle plate current is 40ma. I would like to know how much output power to expect.
Thanx again! Mario. 
26th July 2010, 07:16 PM  #5 
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Location: Johnson City, TN

What is the impedance of your output transformer, Plate and Speaker load?

26th July 2010, 07:30 PM  #6 
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Join Date: May 2007

The maximum possible power output from singleended Class A is half the power used during nosignal conditions. In reality you never get this much, so for a good design assume somewhere around 3040% of DC power at most.

26th July 2010, 07:47 PM  #7  
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Join Date: Feb 2009

Quote:
Pout(RMS) = Vout(RMS) * Iout(RMS) Just multiply the two bits you read off the diagram and that's it !
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26th July 2010, 09:55 PM  #8 
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For a single ended triode amp of conventional design, a rough estimate of power output is one quarter of the idle dissipation. For pentode/tetrodes it is one half the idle dissipation.
Then knock a bit off, 'cuz we're engineers...
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26th July 2010, 10:50 PM  #9 
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Join Date: May 2010
Location: Ball Ground, GA

It would take a set of curves with a load line portraying the particular loading you are using to generate an accurate estimate of the design. The next best thing would be to perform a power output test and measure it. Lacking that capability, here is another approach:
1. Your plate B+ is 375 vdc. 2. Assume that the tube can draw that down to 60 v at full power output. 3. That represents a peak voltage of 315 volts developed across the OPT pri. 4. Divide that by 1.414 to obtain an rms value, and then multiply the result by 90% to account for OPT losses. 5. That means that around 200.5 vac will developed across the OPT winding. 6. Applying the standard power formula of E x E / R to your design yields: 200.5 x 200.5 / 8000 or 5.0 watts RMS into an external load. You can also estimate power output from the dc power consumed by the stage as well, but for lack of any better information, 5 watts will be a pretty accurate estimate of what you can expect your design to produce. Dave 
26th July 2010, 11:22 PM  #10 
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Join Date: Apr 2004
Location: 3rd rock from the sun

A question for dcgillespie (Dave). The 315 V you calculated  is that the peaktopeak voltage or the peak voltage?
I'm used to solidstate, OTL (output transformerless) designs, and those the voltage you calculated would be the peaktopeak output voltage (not the peak voltage). I'm guessing, however, that using a transformer allows the voltage at the plate to swing both above and below the B+ voltage, doubling the output voltage swing for a given supply voltage? That would make your calculation correct, and quadruple the output power compared to a transformerless design. In the fastmoving world of electronics, it's hard to believe the best technology we have for amplifying an electric guitar is now over a century old. Lee De Forest received his patent for what we now call the triode in 1908! Flieslikeabeagle 
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