Download Calculating Wattage.pdf

Wednesday, April 10, 2002 10:02:37 AM
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Ivan ([email protected])
4/1/2002 10:08 AM
How many Watts?
Hello,
I just finished my 2*el34 catode biased amp. But not sure how many watts (how much power) it gives?
Problem is can I use single jensen c12Q (35W)?
Imput impedance of output transformer is 3K, plate are 320V, Screen grids 300V. Tubes are biased at
52ma and 54.6ma. screen grid resistors are 1K2 and katode resistors 271Ohms. Pre amp has more than
enough gain. I think power should be somewhere around 35W but I´m not sure.
And there is also triode switch. How much less power this gives?
Thanks for all help!
yours,
Ivan
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kg ([email protected])
4/1/2002 2:49 PM
Re: How many Watts?
ivan,
do you have a dummy load resistor and a digital multimeter, or an oscilloscope?
no better way to determine power output than to measure it!
ken
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Max ([email protected])
4/1/2002 6:23 PM
Re: How many Watts?
Hi everybody!
Ken,how do you measure power with a scope?
Best regards.
Max.
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kg ([email protected])
4/1/2002 7:18 PM
Re: How many Watts?
Wednesday, April 10, 2002 10:02:37 AM
max,
essentially, just use the scope as a voltmeter. after all, that's all a scope is--a fancy voltmeter that displays
in pictures instead of numbers.
attach dummy load.
attach 1 scope channel across dummy load.
power up amp.
inject sine wave test signal.
increase signal gain until waves start to flatten, then back off a bit.
measure (by counting the divisions and multiplying by the units of voltage per division set on the scope)
the output voltage peak to peak.
divide peak to peak voltage by 2 to get peak voltage.
divide peak voltage by 1.41 to get RMS voltage.
square the RMS voltage and divide by load impedance to get power in load.
(if you don't have a sine wave input signal, you can kinda eyeball this process with a guitar used as input.)
a true rms reading voltmeter makes it easier to measure output power when the waveform isn't sinusoidal.
ken
From:
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Ivan ([email protected])
4/1/2002 7:19 PM
Re: How many Watts?
Hello,
I can make dummy load and I got digital multimeter. So how can I measure power?
yours,
Ivan
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kg ([email protected])
4/1/2002 8:33 PM
Re: How many Watts?
ivan,
do you have a signal generator?
-or-
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Wednesday, April 10, 2002 10:02:37 AM
do you know someone with a signal generator?
-ordo you have a CD player, and a test CD?
-ordo you have a CD player, and a CDRW drive w/a blank CD?
-ordo you have a computer with a sound card?
ken
From:
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Ivan ([email protected])
4/2/2002 2:53 PM
Re: How many Watts?
Hey,
I have computer with soundcard (and some program that works as signal generator) and test cd also.
yours,
Ivan
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kg ([email protected])
4/2/2002 7:18 PM
Re: How many Watts?
ivan,
that's all you need! most test waveforms are sines at 1khz, but you have the means to generate different
waveforms/frequencies which can help a LOT. the only downside to the computer source is that you (most
likely) won't get it above 22khz due to the D/A conversion.
get some adapters/patch cords and hook up your sound card's output to the amplifier input. that will
provide you with the means to generate the test tone and pass it through the amp.
ken
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Thanks!
yours,
Ivan
3 of 6
Ivan ([email protected])
4/2/2002 7:57 PM
Re: How many Watts?
Wednesday, April 10, 2002 10:02:37 AM
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Ivan ([email protected])
4/4/2002 7:08 PM
Re: How many Watts?
Hello,
I measured ac voltage across 8 ohm dummy load with guitar input and it was 12Vac. Should it really be
that low, because this amp is loud. It sounds like 35W... If I divide it by 2 and then by 1,41 and then
square and divide by 8 ohms I get something that could not be true.
yours,
Ivan
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kg ([email protected])
4/4/2002 8:08 PM
Re: How many Watts?
ivan,
is your DMM a true rms sensing meter?
ken
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jaybo
4/5/2002 1:04 AM
Re: How many Watts?
If I divide it by 2 and then by 1,41 and then square and divide by 8 ohms I get something that
could not be true.
Didn't you say in an earlier post that you were using a DMM. If so need for all that math (ken described
how to calculate it using a scope which is read in volts peak to peak.).
volts squared divided by resistance is what you want.
12 squared=144
144 divided by 8 = 18
keep in mind that the guitar signal you used for the input is not a true sine wave so you results will be
skewed some (unless as ken stated, you have a true rms meter...usually right expensive)
jaybo
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Dave H. ([email protected])
4/5/2002 9:07 AM
Re: How many Watts?
I measured ac voltage across 8 ohm dummy load with guitar input and it was 12Vac. Should
4 of 6
Wednesday, April 10, 2002 10:02:37 AM
it really be that low
It’s not easy to do with a guitar input. The guitar signal is very peaky and intermittent compared to a
continuous sine wave. Hold down a big barre chord and strum it rapidly and repetitively to give the meter
a chance to get a reading. A sig gen and ‘scope is what you really need.
I have a cathode biased EL34 amp with plate and screen voltages a bit higher than in your amp. It has a
6k p-p OT and the tubes are biased at 50mA. I get about 22W at the onset of clipping. I’ll measure the
power with the OT set to 3k over the weekend and post the results on Monday.
Dave
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Dave H. ([email protected])
4/9/2002 9:25 AM
Re: How many Watts?
I measured 22W with a 6k OT primary. Plate to cathode voltage is 325V, EL34s biased at 50mA each.
When I measured it with the OT set to 3k I didn’t get any more power it was still about 22W but I did get
more crossover distortion. With the 6K primary I can’t see any crossover distortion below clipping but it
comes on well before clipping with a 3k OT. If you want to run it at 3k I’d turn up the bias current or double
your speaker impedance and run it at 6k. Either way I don’t think you’ll get more than 25W.
Dave
From:
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James M
4/5/2002 4:40 AM
how do i hook up...
Ken
I've got this program called wavetools that has an oscilloscope and spectrum analyzer. I'd like to know
how to patch the output from an amp to my soundcard input and vice versa without nuking anything? Also,
how are you supposed to interpret the spectrum analyzer output?
From:
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Ivan ([email protected])
4/5/2002 2:25 PM
Re: how do i hook up...
Thanks very much!
I will try to measure it again with putting sine wave to input. I just have to make suitable cable.
seems that I can use it with my 35W speaker element...
Yours,
Ivan
From:
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gareth ([email protected])
Wednesday, April 10, 2002 10:02:37 AM
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4/9/2002 12:26 PM
Re: how do i hook up...
ivan,
don't forget that if you use the amp flat out with the power amp distorting the actual power to the speaker
will be a lot more than your approx 20W - maybe twice that, which will be close to the power rating of the
speaker, if you don't crank it all the way you shouldn't have a problem, but if you do have it blasting for a
while, you might have problems with a 35W speaker as the voice coil may not be able to take it.
hey, I may be wrong ! others know more than me.
gareth
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Anybody?
6 of 6
James M
4/6/2002 11:46 PM
Re: how do i hook up...