JD01 wrote: ↑Sun Apr 24, 2022 12:20 pm
You finishing him off today?
Almost. Let's nerd out for one more thing....
I'm pretty confident at this point that this amp is rock solid. My voltages are looking beautiful and it's working great so far so I wanna see what the actual output wattage is. Stock 6G6B Bassmans are about 40-ish watts. Let's get a measurement of what mine is doing.
So we need to inject a sine wave, measure the output in volts AC, convert that to vRMS, and then then do a little formula. My scope has a math function that will do the RMS conversion, but we can also do it with a calculator.
So first I'm gonna inject a 500hz/400mv sine wave into the Bass Channel input. I'm using the bass channel because it has one more gain stage than the normal channel. So here's my little signal generator set to a 500hz sine wave and 400mv output. 1Khz is the "industry standard" for testing equipment, but this is a guitar amp and 500hz usually works better. The 400mv amplitude is sort of an average output between what a single coil and humbucker would put out so we're hitting the tubes with a good approximation of a guitar signal.
So here's the setup...signal generator going into input 1 of the bass channel. The blue trace on the scope is the input signal at the first grid of the first tube. That's exactly what the signal generator is putting out. The input jack is a straight shot to that grid. The yellow trace is around back on the dummy load measuring the total output. The PPIMV is cranked full up. So what we do is turn up the channel volume and we see the yellow trace grow gigantic. We want to measure the maximum voltage we can get with the output (yellow) sine wave staying smooth and clean. That's max clean output. Once it starts to distort we've gone too far. So you turn it up until the sine wave starts to clip, then you back off until it just cleans up again. As you can see that's about 11:00 on the vol knob for this particular test.
So here's the screen. Blue trace is our input and it's measuring 400mv peak to peak just exactly as it should. The yellow trace is that same signal after passing through the amp. It's measuring right up to the 40v mark. We've gone from a puny 400mv to a massive full 40 volts AC. That's amplification. 100 times amplification. Actually it's just a hair shy of 40vac. We'll call it around 38 vac. Still, that's big amplification.
So now with this data we can do some calculations to get a general estimate of what the output wattage is.
Wattage = RMS voltage squared divided by the load resistance. We need to find the RMS value of our output voltage.
RMS voltage is peak voltage x .7071.
Our peak voltage is 38VAC, and we'll multiply that times .7071....and that is 26.869....we'll call that 27 VRMS.
So now with the wattage formula we put in our RMS voltage, which will be 27 x 27 / our dummy load, which is 16 ohms.
27x27 / 16 = 45.5 Watts.
The Tender Assman is a 45.5 Watt amp. And that's a long way to verify what we already pretty much knew.
