Update :73 Silverface Twin

[Tom Phillips]

I agree with Jazz's replies. Following are some additional comments:

...With the volume at 3 I'm getting my 100 watts?...

This comes up quite often. If you drive the amp hard enough with the tone controls dimed, and 50 to 100mV is hard enough, then Fender BF & SF amps will produce full clean power at the volume control setting you described. When you turn the volume control up higher the circuits start to distort more but the amplitude shown on the scope trace doesn’t increase much more.

...I checked the load resistor value after the fuse blew ,and it was reading near 0 ohms ,but as it cooled it returned to 4. Is this normal ?...

That’s not normal at all. I’m assuming that you remembered the earlier discussion and had the resistor disconnected from the amp when you made both of those measurements.

The resistance of all resistors changes as the temperature changes. The temperature coefficient of resistance (TCR) of a resistor like the one you are using is such that the resistance increases as the resistor heats up. The percentage change is modest and the resistance should not decrease as you described. You could do an experiment to test the resistor as a standalone part just to make sure there wasn’t a problem with your setup.
Just connect the resistor to your Ohmmeter and monitor the resistance value while you heat the resistor with a heat gun or hairdryer. You should observe the resistance increase a little as you heat it and then return to the starting value as the resistor cools back to room temperature.

You can also do an experiment to investigate the apparent low amplitude display on your scope. Just connect both the scope and the DVM (set to AC volts) directly to the output of your signal generator and compare the results. Remember that the meter is displaying Vrms and the scope is showing the whole peak-to-peak sine wave (Vpp)
The conversion equation is Vpp = 2.8 x Vrms.
The differences in the readings you described (and showed) seem way off.

One last comment...Under normal conditions your amp should not be blowing the fuse (not even a 2.5A Slo-Blo) when putting out continuous full power.

Cheers,
Tom

[g1]

Hook your scope and meter up to your signal generator, how do the voltage readings compare? Are the cal knobs in the voltage knobs fully clockwise to clicked position? Exercise them a bit in case the switches are dirty. Also try your other probe in scope Ch.1, try Ch.2 also.

[gtrplayr1976]

I agree with Jazz's replies. Following are some additional comments:

This comes up quite often. If you drive the amp hard enough with the tone controls dimed, and 50 to 100mV is hard enough, then Fender BF & SF amps will produce full clean power at the volume control setting you described. When you turn the volume control up higher the circuits start to distort more but the amplitude shown on the scope trace doesn’t increase much more.

That’s not normal at all. I’m assuming that you remembered the earlier discussion and had the resistor disconnected from the amp when you made both of those measurements.

All readings were done with the speaker jack unplugged.

The resistance of all resistors changes as the temperature changes. The temperature coefficient of resistance (TCR) of a resistor like the one you are using is such that the resistance increases as the resistor heats up. The percentage change is modest and the resistance should not decrease as you described. You could do an experiment to test the resistor as a standalone part just to make sure there wasn’t a problem with your setup.
Just connect the resistor to your Ohmmeter and monitor the resistance value while you heat the resistor with a heat gun or hairdryer. You should observe the resistance increase a little as you heat it and then return to the starting value as the resistor cools back to room temperature.

I will try this his evening.

Cheers,
Tom

You can also do an experiment to investigate the apparent low amplitude display on your scope. Just connect both the scope and the DVM (set to AC volts) directly to the output of your signal generator and compare the results. Remember that the meter is displaying Vrms and the scope is showing the whole peak-to-peak sine wave (Vpp)
The conversion equation is Vpp = 2.8 x Vrms.
The differences in the readings you described (and showed) seem way off.

I did this in the beginning ,and it all seemed to be equal. I have the probe set at 10x also.

One last comment...Under normal conditions your amp should not be blowing the fuse (not even a 2.5A Slo-Blo) when putting out continuous full power.

I agree ,and it didn't do this when connected to the speakers.

Thanks Tom

[gtrplayr1976]

Hook your scope and meter up to your signal generator, how do the voltage readings compare? Are the cal knobs in the voltage knobs fully clockwise to clicked position? Exercise them a bit in case the switches are dirty. Also try your other probe in scope Ch.1, try Ch.2 also.

I had done this when I first got the scope just to get familiar. They seemed to be pretty accurate then.

[gtrplayr1976]

The resistance of all resistors changes as the temperature changes. The temperature coefficient of resistance (TCR) of a resistor like the one you are using is such that the resistance increases as the resistor heats up. The percentage change is modest and the resistance should not decrease as you described. You could do an experiment to test the resistor as a standalone part just to make sure there wasn’t a problem with your setup.
Just connect the resistor to your Ohmmeter and monitor the resistance value while you heat the resistor with a heat gun or hairdryer. You should observe the resistance increase a little as you heat it and then return to the starting value as the resistor cools back to room temperature.

I performed this yesterday. Never doubted you ,but I made a short video. Proves exactly what you were saying.

One last comment...Under normal conditions your amp should not be blowing the fuse (not even a 2.5A Slo-Blo) when putting out continuous full power.

What could be blowing this fuse ? Should I hook it back to the speaker ,and check it ?

Took a few more photos. #3 is the test of the probe using the 500mv port. #2 is the probe connected to the signal generator set at 100mv +/- 5mv. .1vpd . #1 is a reading I took the day before of the resistor just after the fuse blew . It was disconnected from the amp.

[Tom Phillips]

I performed this yesterday. Never doubted you ,but I made a short video. Proves exactly what you were saying.

I think it’s interesting and useful to do experiments like this. It really helps a person to understand and remember the principles. This is also why many science classes include laboratory sessions. More related to this thread is that this seems to prove that there was something else wrong with the set up or with measurements that gave you the weird results reported in post #116

What could be blowing this fuse ? Should I hook it back to the speaker ,and check it ?

There was some weirdness going on with your test setup. I would just put in the proper value fuse and proceed for now since the amp has never blown the fuse when played through the speakers.

... #2 is the probe connected to the signal generator set at 100mv +/- 5mv. .1vpd ...

OK. It’s a nice clean sine wave shown but you need to record the scope vertical gain setting and do the math to determine if the scope agrees (or still agrees) with the meter reading.

[gtrplayr1976]

I think it’s interesting and useful to do experiments like this. It really helps a person to understand and remember the principles. This is also why many science classes include laboratory sessions. More related to this thread is that this seems to prove that there was something else wrong with the set up or with measurements that gave you the weird results reported in post #116

I enjoy it ,and learn from it.

OK. It’s a nice clean sine wave shown but you need to record the scope vertical gain setting and do the math to determine if the scope agrees (or still agrees) with the meter reading.

Vertical gain setting ? I don't see that on my scope. Unless I'm overlooking it ,or it's labeled as something else ?

[Tom Phillips]

...Vertical gain setting ? I don't see that on my scope. Unless I'm overlooking it ,or it's labeled as something else ?

Sorry. It's the VOLTS/DIV control.

[gtrplayr1976]

Sorry. It's the VOLTS/DIV control.

It was set at .1 with 10x probe. I was using about 96-98 mv signal. Do you read p-p ,or from the baseline ? Either way looks like its off with the meter.

[Tom Phillips]

... Do you read p-p ,or from the baseline ? ...

Volts peak-to-peak (Vpp) is independent from the baseline so you would read from the lowest point to the highest point no mater where the signal is shown on the scope screen.

[gtrplayr1976]

Volts peak-to-peak (Vpp) is independent from the baseline so you would read from the lowest point to the highest point no mater where the signal is shown on the scope screen.

What I thought. So that would mean the scope is reading 300mv with a 100mv signal ?

[Tom Phillips]

What I thought. So that would mean the scope is reading 300mv with a 100mv signal ?

But those are two different types of voltage reading. Remember from post 221 that Vpp = 2.8 x Vrms.
You are seeing about 300mVpp on the scope and 100mVrms on the meter. Ideally 100mVrms = 2.828Vpp (I see that the equation I posted earlier was rounded off) Your 300mVpp reading is within 6% of the 100mVrms reading. The 6% could easily be a combination of calibration errors and how you eyeballed the scope reading.

Remember that when you are working with two different references you can't just say 100mV and 300mV. You must state the measurement system used. It's somewhat like a person telling you how much an object weighs. If they just say 16 you don't have enough information. However, if they state "16 ounces", then you know exactly. Someone else may say "1 pound." We know that 1 does not equal 16 but 1 pound does equal 16 ounces just like 100mVrms = 282.8Vpp.

HTH,
Tom

[gtrplayr1976]

But those are two different types of voltage reading. Remember from post 221 that Vpp = 2.8 x Vrms.
You are seeing about 300mVpp on the scope and 100mVrms on the meter. Ideally 100mVrms = 2.828Vpp (I see that the equation I posted earlier was rounded off) Your 300mVpp reading is within 6% of the 100mVrms reading. The 6% could easily be a combination of calibration errors and how you eyeballed the scope reading.

Remember that when you are working with two different references you can't just say 100mV and 300mV. You must state the measurement system used. It's somewhat like a person telling you how much an object weighs. If they just say 16 you don't have enough information. However, if they state "16 ounces", then you know exactly. Someone else may say "1 pound." We know that 1 does not equal 16 but 1 pound does equal 16 ounces just like 100mVrms = 282.8Vpp.

HTH,
Tom

Very true. I'll learn. The meter I used for this may not be true RMS. I have two meters both Fluke ,but only the one states it is true RMS.

[g1]

Any chance your probe has a switch that is set to 1x or is not actually a 10x probe?
When you have it plugged into the cal port on the scope, it looks like the V/div is set to 5V for the 10X.

Edit: ignore that, I was thinking backwards. And you probably had the probe set to 1x when doing the probe cal.

[gtrplayr1976]

Any chance your probe has a switch that is set to 1x or is not actually a 10x probe?
When you have it plugged into the cal port on the scope, it looks like the V/div is set to 5V for the 10X.

I had the switch set to 1X with the probe test.