Test Bench Procedures
Essential Math Formulae For the Audio Circuit
Some Test Bench work on a Tube Audio Amp. For those of you not afraid of a little Algebra, this listing of Formulae in order to analyze what your voltage is - either the
"RMS" voltage ( root mean square ), Peak to Peak, etc. It is adviseable to commit these to memory, or copy it off, in order to try to get an idea how to read the meter, scope, what wattage is developed, and so on.
AC voltmeters will give you the RMS of a given AC voltage. It helps to understand this "positve negative" swing as passing through theoretical "Zero", and thus there is a certain "average" electromotive force at work. A sine wave can also be imagined as swinging around and round in a Circle. It happens that a 1/4 way round is the mathematical factor "point 707" or .707 . This is the RMS that your voltmeter is reading. These formulae pictured here can help you reason out what you're seeing on the scope - which is showing you the actual peak to peak trace. Examples below. Click on the Formula sheet here to enlarge in a new window.
Now take a look at the Scope -with both the music signal at approx 100 mv; and the pure wave audio signal . The "Range" on the O'scope is set to .1, or 100 mv...each horizontal division line on the scope represents 100mv...so counting the divisions and, the "ticks" bewteen the lines, we have what looks like 280 mv for the pure , steady audio signal. Multiply by the factor above, .3535 X 285 mv = 100.75 millivolts
On another page, I plan to attempt to explain what is actually taking place inside the vacuum tube, and provide a diagram of my own invention which helps to visualize what is happening with more emphasis on images, and less on math.
For now, starting with this first Trace on the Oscilloscope, I am applying 300 MILLIVOLTS , three tenths of one volt RMS by audio signal generator, to the input of the Blueberry Amplifier. The scope image below is reading the trace of that 300 Mv signal on the anode of the first stage of amplification.
With the Oscilloscope range set to a half volt per division, the 300 Mv applied to the BB Amp is indicating 2.6 VAC Peak to Peak on the scope. To determine the RMS VAC, again, we multiply .3535 X 2.6 VAC= .9 VAC.
Now to determine what amplification took place at the first stage, divide .9 VAC RMS by .3 = 3. The First Stage amplified the incoming audio signal by three times.
My "Set Up" in the pair of images below here...with Bench Test Equipment showing 300 millivolts from the Audio Signal Generator...and .95 VAC RMS on the Meter, with the Range on the HP AC Meter set to the "1 Volt Scale"...the upper scale
Now...Below here, we will read the RMS on STAGE TWO, and also display the same Signal simultaneously on the Scope. The Scope Range is set to 2 Volts Per Division. Now we have changed the setting on the scope probe from X-1, or "Times One"...to X-10...or "Times Ten". This means the 2 Volts Per Division on the scope will actually now be 20Volts Per Division. The Scope displays a 76 Volt Peak to Peak Amplitude, at "20 volts Per Division".
See the RMS Meter reading below that which indicates 26.8 VAC. This, of course, is the 76 Volts P~P on the scope,...multiply the factor .3535 X 76 = 26.8 VAC RMS
Here below now are the images which display the 76 VAC P~ P on the Scope; and the 26.8 Volts on the HP AC Voltmeter with Range on 30 Volts...the lower scale this time. I should explain there are two pointers, as you can see. the one on the "30" range is the pointer we are concerned with. The other pointer is a calibration adjustment, and is ignored.
Note in the image above, that the "Range" on the Scope is set to "1". The scope probe is on X-10...meaning there are TEN Volts Per Division. Counting the divisions from P~P, it indicates approx 76 VAC P~P. Now look above at the AC Meter, with the Range set on "30 Volts'...so the RMS would appear to be 76 VAC X .3535 = 26.8 VAC
Again, let's see how much amplification of the original 300 Millivolts RMS is now amplified at 26.8 VAC RMS....26.8 divided by .300 = 89.3 times larger !
Note: ignore the knob with an arrow pointer in the center of the HP AC voltmeter range knob...that stays put throughout all these measurements. It is a calibration knob
Now let us take the same readings at the last stage of amplification right at the anode of the Output Tube, which does not feed to a load resistor, but directly to the output transformer - which in turn is what pushes the speaker .
Now observe that the scope range is on "5"...and the probe at "X-10"...thus there are 50 Volts Per Division . The scope indicates a P~P of approx 230 VAC.
In the next image take a look a the HP AC Voltmeter, which indicates what looks like 81 VAC RMS. This, of course, is, again, 230 VAC P~P X .3535 = 81.3 VAC RMS
Again, let us divide the original 300 Millivolt RMS Signal into the 81.3 Volts RMS =
an amplification of 271 times in only three stages. This is , in part, what makes the tube amp so special. It can take an average signal from an audio source, and amplfiy it to a thunderous Sound Pressure Level in the Shortest Signal Path known to man. THIS , together with care in selection of , and arrangement of components, to reduce anything being added to or subtracted from the original studio recording...is what provides us with bona fide , true "Hi Fidelity"...or "faithful to the recording ". This is what "Brings the Musicians NEAR to you" !
Here at last is the HP AC Voltmeter Reading of 81.3 VAC RMS. Isn't it nice to have some good bench test equipment which provides precision measurements? Now if you have made it this far, I would like to start on my next page, and explore what is going on inside the vacuum tube. Assembly is one thing, taking measurments is another...but understandign how the tubes manage to do the phenomenal things it does is even more intriguing.
Range is set to 100 Volts...thus the top scale indicates 81.3
For info or comments, or inquiry about puchasing a Blueberry Amp, contact frank-elliott@hotmail.com