Determine output power

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Determine output power

Post by Bieworm »

Is there a correct way to determine the actual output power rating of my 36W amp? I searched online but there seems to be no consensus on the issue.
I wonder because I would like to know if I will blow my favorit speaker when I would run it on the amp. I'm talking about my ol' G12h30
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Re: Determine output power

Post by Bieworm »

Raezzordaze wrote:
Wed 04/28/21 3:04 pm
Uncle Doug has a good video on this. Think DLab mentioned some methods as well iirc.

Here is Doug's video.
I've seen it and tried it too. But I must be doing something wrong. I measure only .5V at the output.. but it's across the dummy load resistor .. dunno if that's correct.
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Re: Determine output power

Post by geoff 1965 »

this is something i want to learn as well, i've watched that video a couple of times and have'nt grasped it yet,when i start learning to use the oscilloscope i'll give it a go.
Mitch Colby in the interview about the Jimmy Page amp say's "it's 8 watts clean then starts to break up" and the only way you can check that is learning to measure the power output with a scope,unless there is another method?
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Re: Determine output power

Post by TriodeLuvr »

Sorry, but that video is garbage. To begin, the maximum output of the amp is not determined by the point at which crossover distortion begins to appear on the scope. Second, although he mentioned clipping at the beginning, the scope sweep was set too slow to be able to see the shape at the +/- maximums of the waveform. It wouldn't have been possible to accurately determine when the waveform began to clip, even if he was looking for it. Then there's the cheapy meter, which isn't capable of wideband RMS measurements (the factory claims its AC response to be 40-400 Hz). Finally, the specification for "peak power" as it has traditionally been applied to audio gear has never been about peak signal levels calculated from a sine wave. It's a term that was used for years by unscrupulous manufacturers to boost the numbers based on brief duration (fraction of a cycle) measurements. In fact, peak power claims became so outrageous in the '70s that the FTC (Federal Trade Commission) had to step into the audio industry and require that output power be measured in RMS over minimum periods of time and with conditioning.

As to why you're only measuring 0.5V, there are a lot of variables. Check the dummy load to be sure something isn't shorted. Be sure your scope is setup correctly - vertical sensitivity is in the CAL position and the probe is what you think it is (10:1 or 1:1).

You should also be aware that speaker power handling claims are really all over the place and difficult or impossible to correlate with output power. Unless the test procedures are well defined, there are too many variables. Is power handling based on maximum possible excursion in free air? If so, at what frequency? (For a given applied voltage, the excursion is much greater at resonance than elsewhere.) And what if power handling is based on power dissipation in the voice coil - does the manufacturer specify how long a given power can be applied without damage, and again, at what frequency?

I really think you have to assume the manufacturer's rating in a case like this is only a general guide, and that there's no way to be 100% certain regarding compatibility at full volume. If the rating is 30W, and your amp is called 36W, that's probably close enough.

Jack
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Re: Determine output power

Post by JMPGuitars »

TriodeLuvr wrote:
Wed 04/28/21 5:39 pm
Sorry, but that video is garbage. To begin, the maximum output of the amp is not determined by the point at which crossover distortion begins to appear on the scope. Second, although he mentioned clipping at the beginning, the scope sweep was set too slow to be able to see the shape at the +/- maximums of the waveform. It wouldn't have been possible to accurately determine when the waveform began to clip, even if he was looking for it. Then there's the cheapy meter, which isn't capable of wideband RMS measurements (the factory claims its AC response to be 40-400 Hz). Finally, the specification for "peak power" as it has traditionally been applied to audio gear has never been about peak signal levels calculated from a sine wave. It's a term that was used for years by unscrupulous manufacturers to boost the numbers based on brief duration (fraction of a cycle) measurements. In fact, peak power claims became so outrageous in the '70s that the FTC (Federal Trade Commission) had to step into the audio industry and require that output power be measured in RMS over minimum periods of time and with conditioning.
applause.gif
Yes! Thank you, Jack. I'm not a fan of Uncle Doug's videos. Some of the stuff he tells people to do is downright dangerous...this one's just ignant. ;)

Thanks,
Josh
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Re: Determine output power

Post by TriodeLuvr »

Bieworm wrote:
Wed 04/28/21 4:01 pm
But I must be doing something wrong. I measure only .5V at the output.. but it's across the dummy load resistor .. dunno if that's correct.
Another thought on this - be sure you haven't accidentally attached the ground clip from the scope probe to the wrong lead on the dummy load. If one side of the amplifier's output is grounded through the mains, and you connect the ground clip of a grounded scope to the wrong lead, it shorts the amplifier's output. 36 watts should produce 48V peak-to-peak on the scope. Also, testing with a signal generator will probably be easier if your amp has an effects loop.

Jack

EDIT: "48V peak-to-peak" into an 8 ohm load...
Last edited by TriodeLuvr on Thu 04/29/21 2:03 pm, edited 1 time in total.
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Re: Determine output power

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TriodeLuvr wrote:
Wed 04/28/21 10:34 pm
Bieworm wrote:
Wed 04/28/21 4:01 pm
But I must be doing something wrong. I measure only .5V at the output.. but it's across the dummy load resistor .. dunno if that's correct.
Another thought on this - be sure you haven't accidentally attached the ground clip from the scope probe to the wrong lead on the dummy load. If one side of the amplifier's output is grounded through the mains, and you connect the ground clip of a grounded scope to the wrong lead, it shorts the amplifier's output. 36 watts should produce 48V peak-to-peak on the scope. Also, testing with a signal generator will probably be easier if your amp has an effects loop.

Jack
I'll have to dig somewhat deeper into this stuff. Here's what I did:

- dummy jack 1/4" at the instrument input, with a probe connected to the lugs of that jack. This probe transfers the signal from my signal generator set at 1kHz
- dummy load resistor 8Ohm 50W soldered to a 1/4" jack between hot and ground.
- scope probe connected to the solder joints of the dummy load resistor. Hot on 1 side, ground on the other side.
- sent the signal in at 120mV and , unlike Uncle Doug, looked for the signal sine's breaking point while magnifying the sine until I have 1 cycle on the screen.
- then measured with the DMM and got something between 0.3V an 0.7V AC. On the scope the Vpp was 620mV. What am I doing wrong?
- all probes were set to 1X, not 10X and the scope was set to 1X too..
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Re: Determine output power

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Raezzordaze wrote:
Wed 04/28/21 11:13 pm
I love people saying how Doug was wrong, yet no one posts anything about the correct method to measure output either. :roll: I mean, ok, maybe he wasn't engineer level correct but, from other videos I've seen, he's pretty damn close. Granted, I'm no engineer myself, so if someone wants to post the actual power output measurement methods feel free. But I feel Doug's done a lot to educate and inform people about tube amps on a basic level so why all the hate? I mean, if he's legitimately telling people to do dangerous things why not highlight that either on his videos as a comment or here in regards to that video?
I find the way Doug explains things pretty amusing, but clear. I also question the nonchalance he has ..especially when pointing things out with his fingers inside a running amp. Think he has regular contact with current doing so ;).. but I can imagine amateurs copying his behavior and the potential severe accidents that might occur. Tecnhically I'm not too familiar with electronics, but I have a healthy sense for technical stuff and hands-on enough to educate myself on certain topics. This is why I can really apreciate Josh, Jack, Geoff, Craig, Phil,... to enlighten me on the technicalities... even if it is the hard way sometimes :), but hey, we're grown ups and I consider you people really as friends, helping out one another.
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Re: Determine output power

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Bieworm wrote:
Thu 04/29/21 12:45 am
I'll have to dig somewhat deeper into this stuff. Here's what I did:

- dummy jack 1/4" at the instrument input, with a probe connected to the lugs of that jack. This probe transfers the signal from my signal generator set at 1kHz
- dummy load resistor 8Ohm 50W soldered to a 1/4" jack between hot and ground.
- scope probe connected to the solder joints of the dummy load resistor. Hot on 1 side, ground on the other side.
- sent the signal in at 120mV...
So maybe the best approach is to determine whether the problem is at the input or the output. One place to start is to disconnect the probe from the open plug at the input and simply touch the tip connection with your finger. The scope should register a huge line-frequency waveform (hum and noise) at the output of the amplifier.

Jack
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Re: Determine output power

Post by TriodeLuvr »

Raezzordaze wrote:
Wed 04/28/21 11:13 pm
I love people saying how Doug was wrong, yet no one posts anything about the correct method to measure output either.
Why should a warning about this video obligate someone to provide a treatise on amplifier theory and the use of test equipment? It's not my fault the video doesn't serve the purpose it claims. There are dozens, maybe hundreds, of videos and white papers online covering this subject. No one needs to reinvent the wheel.
... maybe he wasn't engineer level correct but, from other videos I've seen, he's pretty damn close.
The instructions in this video can't be used to measure amplifier power. How is that "close"? I suggest you Google this topic for more accurate resources and stop blaming the messenger.

Jack
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Re: Determine output power

Post by geoff 1965 »

Get yourself one of these bnc to phono cables Bieworm,then if you get a phono to mono jack adapter and a phono to bnc adapter you can plug into the amp or scope directly without messing around with clips
3B996D87-157E-4209-9C14-4BE0E33589A5.jpeg
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Re: Determine output power

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geoff 1965 wrote:
Thu 04/29/21 4:34 am
Get yourself one of these bnc to phono cables Bieworm,then if you get a phono to mono jack adapter and a phono to bnc adapter you can plug into the amp or scope directly without messing around with clips
3B996D87-157E-4209-9C14-4BE0E33589A5.jpeg
Thanks Geoff! I've come to remember having some old "bajonette" cables lying around somewhere. They fit the scope perfectly. I'm gonna start trying to solder a 1/4" jack to one and check that out...
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Re: Determine output power

Post by JMPGuitars »

geoff 1965 wrote:
Thu 04/29/21 4:34 am
Get yourself one of these bnc to phono cables Bieworm,then if you get a phono to mono jack adapter and a phono to bnc adapter you can plug into the amp or scope directly without messing around with clips
That's too many parts. Each additional piece can degrade your connection, and the phono part of that can slip off. Instead I use a simple / good quality BNC to BNC cable, with one of these: Pomona 1297 Adapter

I have those and other useful tools listed in my tools thread:
viewtopic.php?f=25&t=25173&p=243490#p243490

Thanks,
Josh
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Re: Determine output power

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JMPGuitars wrote:
Thu 04/29/21 7:51 am

That's too many parts. Each additional piece can degrade your connection, and the phono part of that can slip off. Instead I use a simple / good quality BNC to BNC cable, with one of these: Pomona 1297 Adapter
That's a nice adapter. I didn't know Pomona made those.

You know, a more basic issue about all this has to do with the generator itself. I've seen a lot of generators that don't have a suitable attenuator for working at these levels (musical instrument input). Ideally, the output of the signal or function generator would be variable down to 1mV or less. Many are only useful if the work is essentially line level, and the output is only easily adjustable in the range of maybe 100mV to several volts. Also, most generators are rated with a 50 ohm or 600 ohm output impedance. Attaching them into the input of a typical musical instrument amp, which is nearly an open circuit, causes the generator's output level to increase even more. Some generators have an internal load that can be switched in, but some don't. The latter type need to be terminated externally, and that means you can't use a simple adapter. What's needed is a box with an input jack, an output jack and a switch for 600 ohm and 50 ohm terminations. It might also need to have a switched divider network (attenuator) to make the generator's output useable. If output level accuracy isn't important (or if it will always be checked with a scope when testing), a 100K linear pot can replace the divider.

Jack
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Re: Determine output power

Post by Bieworm »

I don't have that lab stuff... I 'll better stick with the things I really know. An amp with a potential 36W clean output..an old 70s speaker rated for 30W... Naaahh not doing that. I'll stick with the 50 to 80W speakers for that one.. so I can sleep at night 😄😄😄

Anyway, if I am to believe the tube bias calculator the amp is near 40W dissipation. Dunno if the 36W OT is allowing that to happen, but I'm not taking that risk.

Side note: the tremolo TMB reverb has no clean output unless you play really soft.. ahaha..that's what I like about you!!!😃
That is why I think a lot of people should play this amp.. the majority of Rock guitarists have that 'always on boost pedal'.. the tremolo TMB makes that pedal obsolete.. I mean it!!!
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Re: Determine output power

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 04/29/21 2:00 pm
You know, a more basic issue about all this has to do with the generator itself. I've seen a lot of generators that don't have a suitable attenuator for working at these levels (musical instrument input). Ideally, the output of the signal or function generator would be variable down to 1mV or less. Many are only useful if the work is essentially line level, and the output is only easily adjustable in the range of maybe 100mV to several volts. Also, most generators are rated with a 50 ohm or 600 ohm output impedance. Attaching them into the input of a typical musical instrument amp, which is nearly an open circuit, causes the generator's output level to increase even more. Some generators have an internal load that can be switched in, but some don't. The latter type need to be terminated externally, and that means you can't use a simple adapter. What's needed is a box with an input jack, an output jack and a switch for 600 ohm and 50 ohm terminations. It might also need to have a switched divider network (attenuator) to make the generator's output useable. If output level accuracy isn't important (or if it will always be checked with a scope when testing), a 100K linear pot can replace the divider.

Jack
I don't think that kind of accuracy is too important in this case. There's such a broad range of input levels from guitars, that going from 100mV to 300mV is perfectly reasonable. It's probably worth testing the input level before it gets amplified to see what the actual levels are, regardless of what the signal generator says, and maybe compare that to a couple guitars on hand. In some cases, testing levels is required anyway, like the Tenma unit on my list (since there's no display to determine any amplitude claims). That said, while the Tenma unit is acceptable, the Siglent units I have allow for much finer tuning. The inbuilt AWG in my scope allows you to go as low as 4mVpp. That's well below any useful voltage for a guitar amp. It even allows setting amplitude to 100ths of a mV, not that I would in this case, but it's cool:
SDS2504X Plus_PNG_137.png
This inbuilt AWG is less robust than their standalone unit I have (SDG2042X)...but it's more than I generally need, so the SDG2042X usually sits in the box. 😂 Ha! That reminds me, I have another one that works well too. A UNI-T UTG932E. The UTG932E goes down to 2mVpp, but can't do any decimal places in the mV range, though it does go in increments of 1mV.

You're absolutely right for more sensitive equipment, but why be concerned with something as variable as guitar stuff? Test some vintage strat pickups and compare them to some modern high-output ceramic humbuckers and see what kind of range you get.

Thanks,
Josh
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Re: Determine output power

Post by TriodeLuvr »

Josh, my point was that many analog bench generators have a minimum output level that's too high for this test. They don't go to zero, and if they do, the pot is so touchy just as it comes off the rest that's it's not usable at these levels. I think the minimum level from my Wavetek function generator is 1/2 volt or so. It can't be used with either a phono preamp or a guitar amp without an external attenuator. This is a common issue.

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Re: Determine output power

Post by TriodeLuvr »

Bieworm wrote:
Thu 04/29/21 3:42 pm
Anyway, if I am to believe the tube bias calculator the amp is near 40W dissipation. Dunno if the 36W OT is allowing that to happen, but I'm not taking that risk.
The output of an amp is often significantly less than the specified output of the tubes themselves. Transformer losses and power supply sag eat up a lot more RMS power than you might think. Too bad you're not in the States, I'd be interested in that 30W for a spare. :)

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Re: Determine output power

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TriodeLuvr wrote:
Fri 04/30/21 12:19 am
Bieworm wrote:
Thu 04/29/21 3:42 pm
Anyway, if I am to believe the tube bias calculator the amp is near 40W dissipation. Dunno if the 36W OT is allowing that to happen, but I'm not taking that risk.
The output of an amp is often significantly less than the specified output of the tubes themselves. Transformer losses and power supply sag eat up a lot more RMS power than you might think. Too bad you're not in the States, I'd be interested in that 30W for a spare. :)

Jack
Haha.. yeah right. Unfortunately I have some 18 watt and 24 watt amps too. So it's hardly a spare 😄😄😄
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Re: Determine output power

Post by geoff 1965 »

39A2584C-810D-4591-80F5-0D33E9C0E0C7.jpeg
So Jack & Josh,have I just wasted £24 on this? I looked for 1Khz sine and low distortion and did’nt consider minimum output.
Type: Function Signal Generator
Frequency Range: Sine wave 1Hz-500kHz, Other waveforms 1Hz-20kHz

Resolution: 1Hz

Output Waveform: Sine, square, triangular, sawtooth, reverse sawtooth

Sine Wave Distortion: <1% below 1kHz, <0.5% above 1kHz

Output Amplitude: Maximum ± 10V(P-P)

Output Impedance: 50Ω

DC Offset: Maximum ± 10V, with shutdown function

There are filters that can be turned on and off, which can well adapt to sine and pulse waveform output

Power: DC3.5-10V

It is recommended to choose DC5V when using adapter, and 3.7V lithium battery when battery powered

Machine Size: Approx. 12.5 x 8 x 3.5cm / 4.9 x 3.1 x 1.4in
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