Fender M80 Rated Output Test? Feel Like Amp Is Anemic?

[JSpina911]

Nevermind and thank you.

[The Dude]

Firstly, welcome to the place!

Yes.
1) Measure the peak to peak output just below clipping.
2) Multiply by .3535 for RMS.
3) Square the result.
4) Divide by the load impedance.

I'd recommend using a dummy load. It's going to be quite loud!

Edit: The first step should actually be: Insert a test tone.

[JSpina911]

Hello Dude....How does one measure peak to peak? Is that with an oscilloscope which I don't have? Thx..

[The Dude]

If you don't have a scope, you can use AC volts on a DVM. Most DVM's already give you RMS voltage, so ignore the x .707. That assumes your meter reads RMS. You might want to read the manual for your meter to be sure. Not as accurate as a scope, but it will certainly get you in the ballpark.

Edited: I misread your first post and didn't catch the "w/o". Apologies.

[JSpina911]

No problem.Thx again for the replies.Not sure if my meter reads RMS? Guess I won't be able to read the rated output..LOL..Supposed to be 72 watts@ 8 ohms.

This M80 amp seems like about the same overall volume as my little 30 watt Vox AD30VT..

[Enzo]

Is your test signal a sine wave, or reasonably close to one? If so, the AC scale on your meter will measure it accurately enough, RMS rating or not.

Most meters are OK up to at least 400Hz, so run a test signal into the power amp and see.

[The Dude]

Yes, by all means, give it a whirl. If you don't have a tone generator, you can use your computer, phone, MP3 player, whatever, as a source. There are test tones all over the web and on YouTube that you can find with a simple Google search.

[JSpina911]

Hi Enzo..Actually my meter is a true RMS meter...I'd have to use a program via my phone to generate the test tone and a dummy load at 8 ohms.

The amp needs a load at full volume to get the readings right?

[JSpina911]

The amp may be fine but I just think it's not quite got the output? Or the stock blue label 12" speaker just sucks?

[JSpina911]

Enzo...So do you think that the low readings I'm getting at the 4 .47 5 watt resistors can't contribute to low output? I look at it like a tube amp that has cold output tube settings..

Doesn't give the amp more wattage and output with the cold settings,but the amp is beefier with more current running through the tubes.

Just wondering why the voltage readings are so low when most of the complaints with alot of the SS Fender amps were too high a mv reading,hot trannies,and blown fuses...I'm just the opposite!

[Enzo]

I'd say MOST never even check that. Cold biased SS amps just sit there working, so why would someone write in? Overheating amps get attention, and here they are. This isn't a tube amp, so drawing parallels gets you in trouble fast. A cold amp can have excess crossover distortion. You warm up the setting until the crossover notch disappears. Any further warming just turns into heat in the amp. The amp will put out its full power whether the crossover distortion is there or not.

What you read about on the internet is NOT a cross section of the amps. It is a cross section of amps WITH PROBLEMS.

I don't usually associate anemic sound with the power amp so much as I do the preamp. If a 50 watt amp only is producing 40 watts, you;d barely be able to detect the difference. When someone says his amp is sounding weak, my first reaction is to check for dirty FX loop jacks. Seriously. One that is hard to detect without a scope is an oscillating amp.. It is screaming at RF, so you cannot hear it as sound, but the poor amp is cranking for all it is worth, an the audio that does come out usually sounds washed out and weak, maybe a little more hummy than usual.

Actually you can detect that without a scope, google signal tracer, and many of the articles that result will include a simple RF detector. It amounts to a diode in the probe lead. You use it with your meter.

Do you have a synth or an electronic organ? Find a clean sound on that and use it for a test tone generator. A match book poked between two keys will hold one down for a continuous sound. Or set a roll of solder on top of a black key.

[JSpina911]

Thanks for the reply Enzo....I Own several guitar amps and I can say that this amp isn't putting out 72 watts into 8 ohms at even full volume.

Hence my questioning about the output.

My question again though...Why the low readings to all of the TIPs? I'll continue working on it...Thx

Possibly that opamp(The discontinued one needs replace)..The MC1436 PI.

I'll take measurements off the transformer and in and around the circuit and see if the voltages are inline with the schematic.

I payed $10 for the amp and I have about $12 invested in some new components...Been a fun project so far because the amp didn't work at all when I got it.

Thx all..

[JSpina911]

I'll also try running the preamp of one of my vox SS amps into this M80 and also vice versa as far as preamp/output stages between these 2 amps and see what happens?

[Dave H]

When I measure the dc voltages at the .47/5 watt resistors,I'm seeing less than 20 m.v.to the TIP 142/147 output trannies..

Specs I'm seeing state the bias idle current should be more like 40 to 80 mv and I'm wondering if this is the issue(If there is one?)

I've switched out the R74,76 resistors that were 1K to 2.2K,but went back to the stock 1Ks when I saw these low readings.

...My question again though...Why the low readings to all of the TIPs?

OK, are you sure that 40 to 80mV spec for idle bias current isn't 40 to 80mA? 80mV would be 160mA across 0.47R which is 30W at idle in the transistors. I think that's far too high.

Changing R74,76 to 2.2k will reduce the current through the bias diodes which could reduce the output bias current.

I think you need to measure the power output to see how wimpy it really is.

[52 Bill]

Specs I'm seeing state the bias idle current should be more like 40 to 80 mv and I'm wondering if this is the issue(If there is one?)

How are you measuring these voltages? Don't confuse current draw with voltage measurements.

Edit: Dave beat me to it, we need more info.