Update :73 Silverface Twin

[gtrplayr1976]

Yes. However the DVM will not cause a problem even if the leads are reversed because the inputs are floating. The hookup of the DVM is important when you are measuring DC voltage and you want the polarity sign on the display to be correct. Also when you are testing diodes.

If you can hear the OT singing that's a pretty good indication that the amp is developing significant power. It's apparently not getting through to your test load resistor.

Always good to check everything.

Measure the resistance of your test load with it disconnected from the amp and verify it is correct.

It's 4 ohms disconnected. Read 0 when connected ,and reading across the jack. Also seems to read 0 to ground ?

Verify that you are connecting to the main speaker jack, not the external speaker jack.

Yes sir. Main jack

Verify that the amp is still loud when you play into the real speaker.

One of the issues was that it didn't seem to be as loud as it should be. That's why I wanted to test the output. What kind of voltage would I expect at the output in a 100 watt SF Twin ?

[Jazz P Bass]

One of the issues was that it didn't seem to be as loud as it should be. That's why I wanted to test the output. What kind of voltage would I expect at the output in a 100 watt SF Twin ?

V x V / R= W
20Vac output (RMS, but let's not get picky) into a 4 ohm load = 100 Watts output.

[gtrplayr1976]

V x V / R= W
20Vac output (RMS, but let's not get picky) into a 4 ohm load = 100 Watts output.

So theoretically I should see 20 volts ?

[g1]

19 to 20V RMS would be about right. That would be about 53 to 56V p-p on the scope.
Did your speaker wire turn out to be connected correctly?
When you measure the load resistance with it connected to the amp, you will measure roughly 0 ohms due to the winding of the OT secondary being in parallel, so that is normal.

[JoeM]

So theoretically I should see 20 volts ?

Yes, that's RMS, so if your viewing the scope (sine wave before clipping) about 28v

[gtrplayr1976]

19 to 20V RMS would be about right. That would be about 53 to 56V p-p on the scope.
Did your speaker wire turn out to be connected correctly?
When you measure the load resistance with it connected to the amp, you will measure roughly 0 ohms due to the winding of the OT secondary being in parallel, so that is normal.

I'm not sure if I had it connected wrong or not. I had disconnected it after the fuse blew ,but it very well could have been. I know I had the ground from the scope wrong.

So looking at the photos I posted, the last few p-p looked like about 3 divisions at 2 volts per X 10 ? This would be correct ? Although I had the scope ground wrong. I am going to do this again anyway to see if I get a different reading/waveform.

[g1]

So theoretically I should see 20 volts ?

Yes, that's RMS, so if your viewing the scope (sine wave before clipping) about 28v

Sorry to nitpick Joe, but that would be 28V peak, 56V peak to peak on the scope.

So looking at the photos I posted, the last few p-p looked like about 3 divisions at 2 volts per X 10 ? This would be correct ?

No, I don't think it was correct. It looked like your scope was showing 2V/div WITH 10X probe, so that was around 6Vp-p total output.
You will probably see it properly when you redo it.

[gtrplayr1976]

Sorry to nitpick Joe, but that would be 28V peak, 56V peak to peak on the scope.

No, I don't think it was correct. It looked like your scope was showing 2V/div WITH 10X probe, so that was around 6Vp-p total output.
You will probably see it properly when you redo it.

Yeah ,I guess I was thinking multiply the 2 x 10. I hope it shows correctly this time around.

[JoeM]

Sorry to nitpick Joe, but that would be 28V peak, 56V peak to peak on the scope....

Not at all, thanks for making that clear. I wasn't sure if I stated peak to peak would make it more confusing or not, but that is more technically correct.

[gtrplayr1976]

Not at all, thanks for making that clear. I wasn't sure if I stated peak to peak would make it more confusing or not, but that is more technically correct.

I kind of figured since g-one stated 56 p-p that the 28 was from the baseline.

[gtrplayr1976]

OK. Here's what I found last night.
Replaced the fuse (could only find a 2.25 amp ) , powered up .
Looking at the pictures 28 ,and 29 are at a volume setting of 2 ,
30, 31,32 are a volume of around 2.75 ,
33 ,and 35 are just above 3 ,and the voltage didn't change after that.
At the last voltage 24.6 ,the fuse blew again. This happened twice. 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 ? Why is this fuse blowing ? It never blew when connected to the speaker ,and the volume maxed. The scope didn't seem to be reading correctly either.

If you put the pointer on a photo the last two numbers are the photo numbers.

[Jazz P Bass]

I do not see anything wrong with the signals.
They appear normal.
If you are getting a clean sine wave at 20 volt output, that is great.
The slammed to the wall, distorted 24 volt output may be pulling more than 2.25 amps from the mains.
A 3 amp should suffice.

[gtrplayr1976]

I do not see anything wrong with the signals.
They appear normal.

Shouldn't they match the voltage ? I don't see 20 volts on the scope unless I'm reading it wrong. It was set at .5 vpd.

If you are getting a clean sine wave at 20 volt output, that is great.

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

The slammed to the wall, distorted 24 volt output may be pulling more than 2.25 amps from the mains.
A 3 amp should suffice.

I had a 3 amp in when it blew the other day ,but I think the scope was connected wrong. Did that blow the fuse ?

Thanks Jazz !

[Jazz P Bass]

I would believe the Fluke.
Learn how to set the scope by using the Fluke.
What is the problem with the amp?
Maybe quit testing it & play it, no?

[gtrplayr1976]

I would believe the Fluke.

Learn how to set the scope by using the Fluke.

I turned every knob ,and switch to see if it would read the 20 volts ,but couldn't get it there

What is the problem with the amp?

I wanted to check the output. Didn't seem like it was 100 watts also the vibrato channel sounded very thin compared to normal channel ,but a big part of that was that I missed connecting the .047 cap in the normal channel

Maybe quit testing it & play it, no?

Yes sir !