https://theultimatetone.com/ Mon, 11 May 2026 12:47:38 +0000 MyBB https://theultimatetone.com/Thread-AWG-vs-sine-generator Tue, 19 Sep 2023 15:33:52 +0000 K O'Connor]]> https://theultimatetone.com/Thread-AWG-vs-sine-generator
In the past, analog signal generators fell into two categories: low-distortion oscillators (LDO), and function generators.

LDOs were sinewave generators intended for audio testing and more specifically , to be used along with a distortion analyser to test THD (total harmonic distortion). If you were lucky, the LDO distortion was around 0.002% (20ppm) and these would be fairly expensive and require some time to settle. More affordable sinewave generators exhibit 0.3% THD or so, maybe down to 0.1%.

Function generators produce sine, square and triangle wave outputs. THD for the sine is as described for affordable sinewave-only units.

In our digital age, test signals can be generated using computers using data tables and DACs to generate various wave shapes. The resolution of a sinewave depends upon the number of data points per cycle, similar to the sampling rate of an analog-to-digital conversion, just in the opposite direction. The DAC output is typically a current fed into an analog integrator stage (a virtual-earth stage with the virtual-earth node fed directly from the DAC). THD can potentially be quite low.

The digital version of a function generator is called an "arbitrary waveform generator" , or AWG. The AWG is a cheap and dirty signal generator that can produce, sine, square, triangle, pulse and other wave shapes. Those other shapes can be practically any shape you wish to program, limited only by the interface provided by the manufacturer.

For audio testing, the AWG should have a THD spec listed for its sinewave output. Generally, this will be in the 0.3% to 0.1% range. More expensive AWGs will offer sine THD down to 0.03% or 0.02% - the lowest I've seen for affordable AWGs. Cheapo AWGs are often around $60cdn where the units with the lowest-THD sine output are $5-600cdn.

DSOs (digital storage oscilloscope) can have AWGs built in, with sine THD listed (or not) down to 0.05% to 0.03%. It is certainly convenient to have both devices in one box as it makes frequency response tests easier to do.]]>
In the past, analog signal generators fell into two categories: low-distortion oscillators (LDO), and function generators.

LDOs were sinewave generators intended for audio testing and more specifically , to be used along with a distortion analyser to test THD (total harmonic distortion). If you were lucky, the LDO distortion was around 0.002% (20ppm) and these would be fairly expensive and require some time to settle. More affordable sinewave generators exhibit 0.3% THD or so, maybe down to 0.1%.

Function generators produce sine, square and triangle wave outputs. THD for the sine is as described for affordable sinewave-only units.

In our digital age, test signals can be generated using computers using data tables and DACs to generate various wave shapes. The resolution of a sinewave depends upon the number of data points per cycle, similar to the sampling rate of an analog-to-digital conversion, just in the opposite direction. The DAC output is typically a current fed into an analog integrator stage (a virtual-earth stage with the virtual-earth node fed directly from the DAC). THD can potentially be quite low.

The digital version of a function generator is called an "arbitrary waveform generator" , or AWG. The AWG is a cheap and dirty signal generator that can produce, sine, square, triangle, pulse and other wave shapes. Those other shapes can be practically any shape you wish to program, limited only by the interface provided by the manufacturer.

For audio testing, the AWG should have a THD spec listed for its sinewave output. Generally, this will be in the 0.3% to 0.1% range. More expensive AWGs will offer sine THD down to 0.03% or 0.02% - the lowest I've seen for affordable AWGs. Cheapo AWGs are often around $60cdn where the units with the lowest-THD sine output are $5-600cdn.

DSOs (digital storage oscilloscope) can have AWGs built in, with sine THD listed (or not) down to 0.05% to 0.03%. It is certainly convenient to have both devices in one box as it makes frequency response tests easier to do.]]> https://theultimatetone.com/Thread-V-Plate-Voltage-measurement Mon, 07 Aug 2023 19:33:12 +0000 Champ81]]> https://theultimatetone.com/Thread-V-Plate-Voltage-measurement  I hope I have the terminology correct first of all (V+ is B+ is plate voltage).
I hear on forums and websites to measure the plate voltage to calculate the desirable current draw (around 70%).

Referring to TUT 2 p3-27 dissapation bias set procedure it says to measure the V+  with amp under test. The previous step says to have the bias pots to maximum negative voltage to check presence of voltage on pins 5 of the power tubes.  Amp is unplugged. Tubes are out back in and then amp is powered back on to measure the plate voltage.  

My question is. Plate Voltage varies as you bias the tubes.  So then what is the plate voltage since it is not constant with pot adjustment. 
In TUT 2 it doesn't say to turn the bias pots to reduce current draw before plugging the power tubes back  So in that state the tubes would be conducting maximum limit based on R4 (currently at 6.K81) of the RBX.  Mine is drawing past 190mA in that state. Is this where you measure the V+?


Edit:
I realized that the voltage does vary but when you plug the numbers in to get current the voltage differences are in fact negligible.]]>  I hope I have the terminology correct first of all (V+ is B+ is plate voltage).
I hear on forums and websites to measure the plate voltage to calculate the desirable current draw (around 70%).

Referring to TUT 2 p3-27 dissapation bias set procedure it says to measure the V+  with amp under test. The previous step says to have the bias pots to maximum negative voltage to check presence of voltage on pins 5 of the power tubes.  Amp is unplugged. Tubes are out back in and then amp is powered back on to measure the plate voltage.  

My question is. Plate Voltage varies as you bias the tubes.  So then what is the plate voltage since it is not constant with pot adjustment. 
In TUT 2 it doesn't say to turn the bias pots to reduce current draw before plugging the power tubes back  So in that state the tubes would be conducting maximum limit based on R4 (currently at 6.K81) of the RBX.  Mine is drawing past 190mA in that state. Is this where you measure the V+?


Edit:
I realized that the voltage does vary but when you plug the numbers in to get current the voltage differences are in fact negligible.]]> https://theultimatetone.com/Thread-ESR-Meter Fri, 30 Sep 2022 09:21:38 +0000 Strelok]]> https://theultimatetone.com/Thread-ESR-Meter


The wheather here now is much better to create things!


What is your experience with ESR meters?
Are they any good?
They sound to good to be true


So, you can only use them on discharged caps.
When does the voltage rating come into play, is that just a thing to get the right result?


I saw the famous Blue ESR Meter by Anatek, it is stated to measure up to 450 Volts.
Then there is the MESR-100, I don't know what the max rating is of that one.
Is it a problem if it "stops at 250 V"?


The "Blue" seems to be not so good available anymore.
I don't like Amazon but paying $50 shipping when ordering direct is a bit too steep for me.

Neither have reference charts that go above 250 Volts.
So someone suggested to measure a good cap and keep that figure for reference.

Another someone created a frint end to use with a normal multimeter.
The big advantage is the protection circuit if you measure a cap that is still charged by mistake.

What do you use?




Strelok]]>


The wheather here now is much better to create things!


What is your experience with ESR meters?
Are they any good?
They sound to good to be true


So, you can only use them on discharged caps.
When does the voltage rating come into play, is that just a thing to get the right result?


I saw the famous Blue ESR Meter by Anatek, it is stated to measure up to 450 Volts.
Then there is the MESR-100, I don't know what the max rating is of that one.
Is it a problem if it "stops at 250 V"?


The "Blue" seems to be not so good available anymore.
I don't like Amazon but paying $50 shipping when ordering direct is a bit too steep for me.

Neither have reference charts that go above 250 Volts.
So someone suggested to measure a good cap and keep that figure for reference.

Another someone created a frint end to use with a normal multimeter.
The big advantage is the protection circuit if you measure a cap that is still charged by mistake.

What do you use?




Strelok]]> https://theultimatetone.com/Thread-A-Safer-Test-Probe Thu, 27 Jan 2022 15:50:13 +0000 ZeusMC]]> https://theultimatetone.com/Thread-A-Safer-Test-Probe
https://audioxpress.com/article/a-beginner-6bq5-se-amp

Testing your Amp

" In Photo 5 you will see a standard test lead probe that has been made much safer. I slipped some pieces of heat-shrink tubing over the shaft and shrank them with a heat source, leaving only a small portion of the tip bare at the end. This tubing is rated for 600V and will keep you from shorting the probe to ground or other connections while touching the test points on the circuit. This will keep you and your amp much happier."

Stay safe out there ]]>
https://audioxpress.com/article/a-beginner-6bq5-se-amp

Testing your Amp

" In Photo 5 you will see a standard test lead probe that has been made much safer. I slipped some pieces of heat-shrink tubing over the shaft and shrank them with a heat source, leaving only a small portion of the tip bare at the end. This tubing is rated for 600V and will keep you from shorting the probe to ground or other connections while touching the test points on the circuit. This will keep you and your amp much happier."

Stay safe out there ]]> https://theultimatetone.com/Thread-FFT-Software-Spectrum-Analyzers Thu, 05 Aug 2021 18:01:12 +0000 ZeusMC]]> https://theultimatetone.com/Thread-FFT-Software-Spectrum-Analyzers
Thanks.]]>
Thanks.]]> https://theultimatetone.com/Thread-Power-Supply-For-Tube-Circuit-Development Tue, 03 Aug 2021 08:43:03 +0000 ZeusMC]]> https://theultimatetone.com/Thread-Power-Supply-For-Tube-Circuit-Development Are there any alternatives to the Heathkit Ip-17 regulated high voltage dc power supply?
Thanks.
Peter.]]> Are there any alternatives to the Heathkit Ip-17 regulated high voltage dc power supply?
Thanks.
Peter.]]> https://theultimatetone.com/Thread-Spectrum-Analyzers Tue, 09 Mar 2021 03:34:50 +0000 NCtubes]]> https://theultimatetone.com/Thread-Spectrum-Analyzers Thanks,
Steve]]> Thanks,
Steve]]> https://theultimatetone.com/Thread-Multimeter-specs Thu, 14 May 2020 18:16:27 +0000 Strelok]]> https://theultimatetone.com/Thread-Multimeter-specs
I am in the market for a new multimeter, or how you call it nowadays.

Is True RMS useful for AC or is it just more expensive?
I'm thinking about a Voltcraft meter.


Kind regards,

Strelok]]>
I am in the market for a new multimeter, or how you call it nowadays.

Is True RMS useful for AC or is it just more expensive?
I'm thinking about a Voltcraft meter.


Kind regards,

Strelok]]> https://theultimatetone.com/Thread-Frequency-response-using-white-noise-and-FFT Tue, 09 Oct 2018 17:44:57 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Frequency-response-using-white-noise-and-FFT
Spectrum analysers are a specialised oscilloscope with multiple frequency filters at the input. Typically, a single frequency is fed into an amplifier, say, but the amplifier adds distortion harmonics that were not present in the original signal. The analyser shows amplitude on the vertical axis (Y) and frequency on the horizontal axis (X). t each frequency, a vertical line is displayed whose amplitude corresponds to the amplitude of that frequency. In a typical display, the original signal is the "fundamental", or "first harmonic", and is used for the reference level; ideally it has the highest amplitude. The harmonics have much lower amplitudes, so their lines are not as tall as for the fundamental. 

The vertical scale is often in deciBels, but can be in volts. The dB scale is more convenient if the amplifier is very good, exhibiting very low distortion, as the "linear appearance" of the logarithmic dB scale allows the low levels of the harmonics to be visible.Were the scale truly linear, the harmonics might be buried in the noise of the zero line across the bottom of the display. The displayed output is also called a "fast Fourier transform" (FFT), which is a mathematical derivation that would result in the same graphical output.

So, the spectrum analyser is great at showing harmonics of a single frequency input. if we feed white noise into the amplifier we see a displayed line that represents the frequency response of the amplifier. This is also called a "Bode" plot. Spectrum analysers are a bit expensive for most labs to have and unless they use it constantly it is difficult to justify the expense. Fortunately, modern DSOs (digital storage oscilloscopes) have a computer core and computers are good at doing any math the designer wishes to have it do. As a result, most DSOs will do FFTs - maybe not as well as a dedicated spectrum analyser, but well enough for most techs and hobbyists.

The only DSOs I found that specifically lists being able to display Bode plots are from Keysight (formerly Hewlett-Packard, HP - not the brown sauce from England... mmmm), EDU1002G - 50MHz, ~$900cdn and DSO1102G - 70/100MHz; ~$1200cdn. Both have abuilt-in 20MHz arbitrary wave generator (AWG) allowing the scope and generator to be perfectly synchronised to do a frequency sweep. Keysight isavailble through Mouser; HP sauce is availble at your local grocery store.]]>
Spectrum analysers are a specialised oscilloscope with multiple frequency filters at the input. Typically, a single frequency is fed into an amplifier, say, but the amplifier adds distortion harmonics that were not present in the original signal. The analyser shows amplitude on the vertical axis (Y) and frequency on the horizontal axis (X). t each frequency, a vertical line is displayed whose amplitude corresponds to the amplitude of that frequency. In a typical display, the original signal is the "fundamental", or "first harmonic", and is used for the reference level; ideally it has the highest amplitude. The harmonics have much lower amplitudes, so their lines are not as tall as for the fundamental. 

The vertical scale is often in deciBels, but can be in volts. The dB scale is more convenient if the amplifier is very good, exhibiting very low distortion, as the "linear appearance" of the logarithmic dB scale allows the low levels of the harmonics to be visible.Were the scale truly linear, the harmonics might be buried in the noise of the zero line across the bottom of the display. The displayed output is also called a "fast Fourier transform" (FFT), which is a mathematical derivation that would result in the same graphical output.

So, the spectrum analyser is great at showing harmonics of a single frequency input. if we feed white noise into the amplifier we see a displayed line that represents the frequency response of the amplifier. This is also called a "Bode" plot. Spectrum analysers are a bit expensive for most labs to have and unless they use it constantly it is difficult to justify the expense. Fortunately, modern DSOs (digital storage oscilloscopes) have a computer core and computers are good at doing any math the designer wishes to have it do. As a result, most DSOs will do FFTs - maybe not as well as a dedicated spectrum analyser, but well enough for most techs and hobbyists.

The only DSOs I found that specifically lists being able to display Bode plots are from Keysight (formerly Hewlett-Packard, HP - not the brown sauce from England... mmmm), EDU1002G - 50MHz, ~$900cdn and DSO1102G - 70/100MHz; ~$1200cdn. Both have abuilt-in 20MHz arbitrary wave generator (AWG) allowing the scope and generator to be perfectly synchronised to do a frequency sweep. Keysight isavailble through Mouser; HP sauce is availble at your local grocery store.]]> https://theultimatetone.com/Thread-Frequency-response-using-2-ch-Scope Tue, 09 Oct 2018 17:11:45 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Frequency-response-using-2-ch-Scope
Connect the sinewave generator to the amplifier input and connect one probe from the scope to the amplifier input, as well. Connect the bench load to the amplifier output along with the second scope probe. It does not matter what the gain of the amplifier is, just set the channel sensitivity for the scope to be appropriate for the end of the amplifier that channel is monitoring. ideally, both waves should be as close to full-scale on the display as possible.

Turn on the amp and see the waves be essentially overlapped at low-frequencies. As you sweep the frequency of the generator higher, the waves will eventually begin to spread apart.

If the scope has an X-Y function, then you can generate a Lissajous figure, commonly exploited for RF work. The problem with this technique is that the shape of the resulting ellipse depends on BOTH the amplitude of the two waves and their phase difference, so it works best if the generator output is highly stable, which we assumed was questionable at the beginning oft his discussion. if it is stable, checking output amplitude of the amplifier is much simpler, as described in another post here. The other problem with the Lissajous method is that when 45-degrees phase difference is achieved, the ellipse is a very specific ratio of diameters rather than being, say a straight line, as it is for 0-degrees or 180, or as a perfect circle at 90-degrees and 270.]]>
Connect the sinewave generator to the amplifier input and connect one probe from the scope to the amplifier input, as well. Connect the bench load to the amplifier output along with the second scope probe. It does not matter what the gain of the amplifier is, just set the channel sensitivity for the scope to be appropriate for the end of the amplifier that channel is monitoring. ideally, both waves should be as close to full-scale on the display as possible.

Turn on the amp and see the waves be essentially overlapped at low-frequencies. As you sweep the frequency of the generator higher, the waves will eventually begin to spread apart.

If the scope has an X-Y function, then you can generate a Lissajous figure, commonly exploited for RF work. The problem with this technique is that the shape of the resulting ellipse depends on BOTH the amplitude of the two waves and their phase difference, so it works best if the generator output is highly stable, which we assumed was questionable at the beginning oft his discussion. if it is stable, checking output amplitude of the amplifier is much simpler, as described in another post here. The other problem with the Lissajous method is that when 45-degrees phase difference is achieved, the ellipse is a very specific ratio of diameters rather than being, say a straight line, as it is for 0-degrees or 180, or as a perfect circle at 90-degrees and 270.]]> https://theultimatetone.com/Thread-Oscilloscopes-for-Audio Fri, 05 Oct 2018 01:11:48 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Oscilloscopes-for-Audio
Being an analogue guy I've always had analogue oscilloscopes. My first one was a Philips 50MHz 2-ch that I bought new right from their warehouse for about $1500cdn. That was 1984 and it worked until about 2012 or so. I used a borrowed scope for a while, but it was old and temperamental, then bought a new GW Instek GOS-620 which is 20MHz 2-ch with a simple front panel. It worked for 18-months and then the vertical positioning went awry. A sine wave in the bottom 2cm of the display would look okay, but if you moved it upwards using the vertical position control, the top of the sine would distort, then flatten to zero a centimetre before the top of the screen. There was no way to accurately look at large waves or complex waves.

When I bought the Instek, it was about $560cdn and i looked at DSOs, as well. A DSO is a digital storage oscilloscope. They were initially designed and optimised for looking at digital systems where wave forms were mostly square waves and strings of pulses. None of that requires much vertical resolution, which was fortunate because at the time DSO were being born, analog-to-digital-converters (ADCs) were expensive to make. ADCs got better with time, faster, stronger - not bionic, though! - and are at the heart of 90% of all DSO still made. But now, you can get bandwidth out to 100s of GHz and memory depth unheard of a decade or so ago. 

Initially, DSOs had a latency issue, where it would take a short amount of time to build up the waveform before the scope would display it. Modern computer technology is blindingly fast and the latency is pretty much gone The only time you would have an issue is if you set the time base to be many 10s or 100s of seconds - something you could not do with an analogue scope - so, nothing to worry about for what we look at in guitar and hifi amps. I was concerned about that latency a year or so ago when I decided to go with the Instek CRT scope. I did not know what many of the DSO specs meant until the Instek partially died and I had to re-investigate scopes and figured I'd get a DSO this time.]]>
Being an analogue guy I've always had analogue oscilloscopes. My first one was a Philips 50MHz 2-ch that I bought new right from their warehouse for about $1500cdn. That was 1984 and it worked until about 2012 or so. I used a borrowed scope for a while, but it was old and temperamental, then bought a new GW Instek GOS-620 which is 20MHz 2-ch with a simple front panel. It worked for 18-months and then the vertical positioning went awry. A sine wave in the bottom 2cm of the display would look okay, but if you moved it upwards using the vertical position control, the top of the sine would distort, then flatten to zero a centimetre before the top of the screen. There was no way to accurately look at large waves or complex waves.

When I bought the Instek, it was about $560cdn and i looked at DSOs, as well. A DSO is a digital storage oscilloscope. They were initially designed and optimised for looking at digital systems where wave forms were mostly square waves and strings of pulses. None of that requires much vertical resolution, which was fortunate because at the time DSO were being born, analog-to-digital-converters (ADCs) were expensive to make. ADCs got better with time, faster, stronger - not bionic, though! - and are at the heart of 90% of all DSO still made. But now, you can get bandwidth out to 100s of GHz and memory depth unheard of a decade or so ago. 

Initially, DSOs had a latency issue, where it would take a short amount of time to build up the waveform before the scope would display it. Modern computer technology is blindingly fast and the latency is pretty much gone The only time you would have an issue is if you set the time base to be many 10s or 100s of seconds - something you could not do with an analogue scope - so, nothing to worry about for what we look at in guitar and hifi amps. I was concerned about that latency a year or so ago when I decided to go with the Instek CRT scope. I did not know what many of the DSO specs meant until the Instek partially died and I had to re-investigate scopes and figured I'd get a DSO this time.]]> https://theultimatetone.com/Thread-Frequency-response-with-generator-and-meter Wed, 19 Sep 2018 18:58:24 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Frequency-response-with-generator-and-meter
Measuring the frequency response of your amp is pretty straight forward, but can be tedious or simple depending on the test equipment you have.

Most hobbyists have a multi-meter and if they also have a signal generator then it is possible to do a frequency response test. The first thing to know is that when the signal level drops by 30%, you've reached the 3dB-down point, or the accepted bandwidth limit of the unit. 

Set the amplifier to whatever is its "flat" response. If you have a scope you can see this easily, but for now we are hoping there is something close to it that you can set by ear or by knowing the equipment. Connect the generator to the input and the meter to the output. Set the generator to a mid frequency, 400Hz or 1kHz, then adjust the volume for an easy to read value like 1V out.

Now change the generator frequency and watch the meter reading. With an analogue meter-movement type AVO or similar, there is no problem with this sweep but there might be with a DVM (digital volt meter). You might have to sweep the frequency and stop to let the meter get a new reading, then sweep up more and stop, get the new reading, and so on. You will get to a frequency where the meter reading is low and continues to drop as frequency is raised. With the 1V reference, the 3dB point is where the meter shows 700mV. You can do the same towards the low-frequency end to see where the bass roll-off is.

We are assuming that the generator output is constant over the frequency range and most are quite stable in that regard. If you have some doubt, you'll have to check the input signal level to the amp (generator output) as you change the frequency. Certainly at the point where the 3dB limit appears to be, it would be a good idea to verify that the amp is causing the drop rather than the generator.

You might encounter points where the meter reads high or low but you are not at the expected limit of response of the circuit. These indicate peaks or dips in the response that are the result of EQ controls not being at a flat setting, or of frequency shaping built into the circuit, or of points where the circuit might be prone to oscillation (if a peak). The "unstable" peaks tend to be near the 3dB limit or even beyond it. A wideband amplifier that is not compensated correctly could show a rise in response well above the designed roll-off - this is a quirk of some active EQ circuits used for cross-overs and reflects the limitations of the opamps used in the circuit. You are unlikely to encounter this in a tube circuit or a guitar amp.

Of course, now you can do similar tests with a given control set to zero or set to maximum and see its influence.]]>
Measuring the frequency response of your amp is pretty straight forward, but can be tedious or simple depending on the test equipment you have.

Most hobbyists have a multi-meter and if they also have a signal generator then it is possible to do a frequency response test. The first thing to know is that when the signal level drops by 30%, you've reached the 3dB-down point, or the accepted bandwidth limit of the unit. 

Set the amplifier to whatever is its "flat" response. If you have a scope you can see this easily, but for now we are hoping there is something close to it that you can set by ear or by knowing the equipment. Connect the generator to the input and the meter to the output. Set the generator to a mid frequency, 400Hz or 1kHz, then adjust the volume for an easy to read value like 1V out.

Now change the generator frequency and watch the meter reading. With an analogue meter-movement type AVO or similar, there is no problem with this sweep but there might be with a DVM (digital volt meter). You might have to sweep the frequency and stop to let the meter get a new reading, then sweep up more and stop, get the new reading, and so on. You will get to a frequency where the meter reading is low and continues to drop as frequency is raised. With the 1V reference, the 3dB point is where the meter shows 700mV. You can do the same towards the low-frequency end to see where the bass roll-off is.

We are assuming that the generator output is constant over the frequency range and most are quite stable in that regard. If you have some doubt, you'll have to check the input signal level to the amp (generator output) as you change the frequency. Certainly at the point where the 3dB limit appears to be, it would be a good idea to verify that the amp is causing the drop rather than the generator.

You might encounter points where the meter reads high or low but you are not at the expected limit of response of the circuit. These indicate peaks or dips in the response that are the result of EQ controls not being at a flat setting, or of frequency shaping built into the circuit, or of points where the circuit might be prone to oscillation (if a peak). The "unstable" peaks tend to be near the 3dB limit or even beyond it. A wideband amplifier that is not compensated correctly could show a rise in response well above the designed roll-off - this is a quirk of some active EQ circuits used for cross-overs and reflects the limitations of the opamps used in the circuit. You are unlikely to encounter this in a tube circuit or a guitar amp.

Of course, now you can do similar tests with a given control set to zero or set to maximum and see its influence.]]> https://theultimatetone.com/Thread-TEST-WITHOUT-TUBES Wed, 19 Sep 2018 17:06:36 +0000 K O'Connor]]> https://theultimatetone.com/Thread-TEST-WITHOUT-TUBES
When we have a tube amp for repair or we have a new build and want to test it, the first thing we should do is REMOVE THE TUBES.

Of course, we never power anything newly built or of unknown status without connecting it through our Power Limiting Safety Socket (PLSS) - this is the mandatory project in TOT (Tonnes of Tone). This test fixture will save you countless $$ on fuses and will keep things from going up in smoke.

At first we limit power as much as possible since there are no tube loads, just caps charging. The surge will cause the lamp to brighten then dim if everything is okay.

We use our DC voltmeter to check voltages within the circuit, to check their polarity and relative sizes. If this has a power section with fixed-bias, check that the bias voltage is present right on the tube socket pins. Sweep the bias pots to assure their function and leave them set to the maximum negative voltage (bias pot '0' end; zero, cold; low bias; CCW)

If the lamp brightens and stays bright, there is a problem in the circuit.

If the voltages seem okay, you can add preamp tubes. This will require changing to a higher wattage lamp in the PLSS. Powering up should be the same as before but the bulb might glow a little bit. You can measure voltages around the tubes and even send a signal through and scope it up to the empty power tube sockets. Check the bias voltage again with the reduced power limiting. Its proportion to the screen or plate voltages should look more typical now.

Add the power tubes and reduce the limiting further using a larger lamp or parallel lamps.

TEST WITHOUT TUBES any time you modify the bias circuitry AND for any new build AND when you add Power Scaling.]]>
When we have a tube amp for repair or we have a new build and want to test it, the first thing we should do is REMOVE THE TUBES.

Of course, we never power anything newly built or of unknown status without connecting it through our Power Limiting Safety Socket (PLSS) - this is the mandatory project in TOT (Tonnes of Tone). This test fixture will save you countless $$ on fuses and will keep things from going up in smoke.

At first we limit power as much as possible since there are no tube loads, just caps charging. The surge will cause the lamp to brighten then dim if everything is okay.

We use our DC voltmeter to check voltages within the circuit, to check their polarity and relative sizes. If this has a power section with fixed-bias, check that the bias voltage is present right on the tube socket pins. Sweep the bias pots to assure their function and leave them set to the maximum negative voltage (bias pot '0' end; zero, cold; low bias; CCW)

If the lamp brightens and stays bright, there is a problem in the circuit.

If the voltages seem okay, you can add preamp tubes. This will require changing to a higher wattage lamp in the PLSS. Powering up should be the same as before but the bulb might glow a little bit. You can measure voltages around the tubes and even send a signal through and scope it up to the empty power tube sockets. Check the bias voltage again with the reduced power limiting. Its proportion to the screen or plate voltages should look more typical now.

Add the power tubes and reduce the limiting further using a larger lamp or parallel lamps.

TEST WITHOUT TUBES any time you modify the bias circuitry AND for any new build AND when you add Power Scaling.]]> https://theultimatetone.com/Thread-Why-use-sine-waves-for-testing Wed, 19 Sep 2018 16:48:50 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Why-use-sine-waves-for-testing
The sine wave is extremely common in the natural world and is a very useful waveform to use for testing audio equipment. The sine shape is a continuously changing, smooth, curved wave and it is easy to see if it is distorted or relatively pure. It is also symmetrical, so we can see if our circuit is introducing distortion or error if the wave loses its symmetry.

If we feed a signal into a preamplifier, we can use an oscilloscope to probe various circuit points to check the gain of the circuit and see the headroom for each gin stage. As TUT5 (The Ultimate Tone vol.5) demonstrates, we can see if the tube grids are being clipped and can fix those issues. If the preamp has EQ controls, we set these for the most correct sine wave initially, just to see characteristics of the circuit even though this may not be an EQ setting we would use for guitar. In hifi, it would be a setting most used for music play back.

Similarly, when testing a power amp we want to see that the output is symmetrical at the rated power. The amp might clip first on one half of the wave or the other depending on the design, although in general clipping should be symmetric. if clipping is far from symmetrical there might be a problem in the amplifier, either by design or by component defect, or by a mistake in wiring of a new build. 

For hifi equipment a 1kHz sine wave is used, where guitar amps are tested with 400Hz. The difference stems from the fact that a guitar has a lower range of frequencies in its output than does full-bandwidth audio. As TUTs state, the highest note on a guitar is around 1,500Hz, with harmonics that are higher.

The sine wave amplitude when connected to a guitar input is 70mV RMS, which is 100mV peak, or 200mV peak-to-peak. Some guitars produce more voltage than this but it is usually the case the the preamp or integrate guitar amp will have "excess" gain to assure that even the lowest output pickups will drive the amp to full output, if not to clipping.]]>
The sine wave is extremely common in the natural world and is a very useful waveform to use for testing audio equipment. The sine shape is a continuously changing, smooth, curved wave and it is easy to see if it is distorted or relatively pure. It is also symmetrical, so we can see if our circuit is introducing distortion or error if the wave loses its symmetry.

If we feed a signal into a preamplifier, we can use an oscilloscope to probe various circuit points to check the gain of the circuit and see the headroom for each gin stage. As TUT5 (The Ultimate Tone vol.5) demonstrates, we can see if the tube grids are being clipped and can fix those issues. If the preamp has EQ controls, we set these for the most correct sine wave initially, just to see characteristics of the circuit even though this may not be an EQ setting we would use for guitar. In hifi, it would be a setting most used for music play back.

Similarly, when testing a power amp we want to see that the output is symmetrical at the rated power. The amp might clip first on one half of the wave or the other depending on the design, although in general clipping should be symmetric. if clipping is far from symmetrical there might be a problem in the amplifier, either by design or by component defect, or by a mistake in wiring of a new build. 

For hifi equipment a 1kHz sine wave is used, where guitar amps are tested with 400Hz. The difference stems from the fact that a guitar has a lower range of frequencies in its output than does full-bandwidth audio. As TUTs state, the highest note on a guitar is around 1,500Hz, with harmonics that are higher.

The sine wave amplitude when connected to a guitar input is 70mV RMS, which is 100mV peak, or 200mV peak-to-peak. Some guitars produce more voltage than this but it is usually the case the the preamp or integrate guitar amp will have "excess" gain to assure that even the lowest output pickups will drive the amp to full output, if not to clipping.]]> https://theultimatetone.com/Thread-Power-Output-Measurement-with-a-generator-but-no-scope Wed, 19 Sep 2018 16:32:00 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Power-Output-Measurement-with-a-generator-but-no-scope
As you acquire test equipment, you might buy things in the order you can afford them and so might have a sine wave generator before you have an oscilloscope. this would be an odd order for most people intent on having a well-equipped bench. Or maybe your scope has died and you still have to do some testing?

To test output power of an amp with a generator but no scope requires at least an AC voltmeter and a bench load. The procedure is the same as for no-scope and no-generator EXCEPT you have the generator - hehe.

Connect the generator to the amp input. Connect the bench load to the amp output. Connect the AC voltmeter across the load. Set gain or volume to zero.

Turn on the generator and the amp.

Dial volume up until the meter gives a steady reading. Note the value and dial volume back to zero.

The voltages for clipped power, so we can calculate power then divide by two for sine wave power.]]>
As you acquire test equipment, you might buy things in the order you can afford them and so might have a sine wave generator before you have an oscilloscope. this would be an odd order for most people intent on having a well-equipped bench. Or maybe your scope has died and you still have to do some testing?

To test output power of an amp with a generator but no scope requires at least an AC voltmeter and a bench load. The procedure is the same as for no-scope and no-generator EXCEPT you have the generator - hehe.

Connect the generator to the amp input. Connect the bench load to the amp output. Connect the AC voltmeter across the load. Set gain or volume to zero.

Turn on the generator and the amp.

Dial volume up until the meter gives a steady reading. Note the value and dial volume back to zero.

The voltages for clipped power, so we can calculate power then divide by two for sine wave power.]]>