Determine output power

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Bieworm
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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

Post by Bieworm »

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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Re: Determine output power

Post by JMPGuitars »

TriodeLuvr wrote:
Fri 04/30/21 12:10 am
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.

Jack
I'm not disagreeing with you on that. My point was that there's plenty of modern generators (including some analog devices) that give much better control. I know you're old school, and that's cool, but you should give some new toys a try. ;)

Thanks,
Josh
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Re: Determine output power

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geoff 1965 wrote:
Fri 04/30/21 3:49 am
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.
It's maybe fine. You would need to test it and see how clean the signal is. I know there are complaints regarding the accuracy of it, but that's not really a big deal. What you would need to do is figure out the correct settings since the display on the device will probably be wrong (unless you mysteriously got a less buggy version). For example, if you set it to 1kHz, 200mVpp, you will need to test and see what the actual reported frequency and Vpp are on your scope before applying the signal to your DUT.

To test it directly with your scope, you'll likely need a 50Ω terminator to connect on the scope side.

All that said, I haven't seen any particularly good reviews for the FG-100 DDS. Mostly people saying it's noisy, and inaccurate.

If you can still return it, and you want a fairly inexpensive device that's much better, look for the Uni-T model I posted above. It's excellent, not just for the price.

Thanks,
Josh
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Re: Determine output power

Post by geoff 1965 »

Thank’s Josh,too late to return it but waiting for mouser to get some Arcol non-inductive resistors in for the dummy load then I can start testing.hopefully by then Bieworm will have sussed the output power test!
Here’s a snip of my latest project,check out the genuine mullards and add them to something you take your sandwiches to work in!
8824FD8F-4A6F-4635-8E1E-FA7BA4C3BC05.jpeg
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Re: Determine output power

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geoff 1965 wrote:
Fri 04/30/21 8:49 am
Thank’s Josh,too late to return it but waiting for mouser to get some Arcol non-inductive resistors in for the dummy load then I can start testing.hopefully by then Bieworm will have sussed the output power test!
Here’s a snip of my latest project,check out the genuine mullards and add them to something you take your sandwiches to work in!
8824FD8F-4A6F-4635-8E1E-FA7BA4C3BC05.jpeg
You may be fine with it. Test it out as I mentioned above. You don't need a dummy load yet, you just need the 50 ohm terminator to connect it to your scope before you use it with a DUT. IIRC, the noise was more of an issue at higher frequencies, so you might be okay at a 1k sine wave. Test it and you'll know. Worst case, it wouldn't be a huge loss if it doesn't work out.

I hope you can still afford lunch after getting those Mullards. ;)
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Re: Determine output power

Post by geoff 1965 »

Are you ready for this, the EZ81 is NOS and the ECL’s test very strong on both the triodes and pentodes and closely matched.
£38.70 including delivery,cheaper than new crap JJ’s!
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Re: Determine output power

Post by TriodeLuvr »

JMPGuitars wrote:
Fri 04/30/21 7:41 am

I'm not disagreeing with you on that. My point was that there's plenty of modern generators (including some analog devices) that give much better control. I know you're old school, and that's cool, but you should give some new toys a try. ;)
I'm old school? Aren't you building stuff with vacuum tubes?? Ha, ha, ha. My bench includes a PC with a sound card that can measure THD/IMD into the low triple decimal places and output just about any signal type you could want. The discussion isn't about me, though. It's about using a generator to measure amplifier output power. The issues and procedures I described apply to many generators. If they don't apply to yours (or other readers), please just ignore it. :)

Jack
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Re: Determine output power

Post by TriodeLuvr »

geoff 1965 wrote:
Fri 04/30/21 8:49 am
Thank’s Josh,too late to return it but waiting for mouser to get some Arcol non-inductive resistors in for the dummy load
You absolutely don't need non-inductive resistors for this work. NONE of the high power loads on my bench are non-inductive. This creates no issues whatsoever for testing below 100 kHz.

Check this place out, and remember you can series/parallel multiple resistors to get the exact value you want.

https://www.surplussales.com/homenew.html#Resistors

Jack
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Re: Determine output power

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TriodeLuvr wrote:
Fri 04/30/21 12:25 pm
I'm old school? Aren't you building stuff with vacuum tubes?? Ha, ha, ha.
Hahahahaha touché!
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Re: Determine output power

Post by geoff 1965 »

i don't have the technical experience/background that you obviously have Jack so thank's for confirming the "non-inductive" issue,someone else said that as well but i was unsure which to buy.also those non-inductives are expensive,£20 here in the UK for an arcol 8 ohm 100W!
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