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

[J M Fahey]

Firstly, welcome to the place!

Yes.
1) Measure the peak to peak output just below clipping.
2) Multiply by .707 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.

Sorry, itīs multiply Peak to Peak by .3535 for RMS

AS of detecting distortion without an oscilloscope. inject 400/440Hz sinewave, at low levels it will sound like a flute, above certain level it will become harsher, at extreme distortion reedy ... with some practice, rising/lowering volume a little you will detect start of clipping with some precision.

Tube amps are harder, because distortion is smoother and more gradual, but on SS ones itīs quite noticeable.

I can say that this amp isn't putting out 72 watts into 8 ohms at even full volume.

No you canīt and youīll lose if you bet on that .
You are *listening* to it which is not the same.

It has the cheapest speaker Fender could get and its efficiency is around 94/95dB, to boot itīs a single one in a small open back cabinet ... are you comparing it to multiple 99/101dB speakers in larger cabinets?

That said, itīs not bad, and in fact quite usable in a Club situation.

By the way, Eminence often takes its most popular OEM speakers and sells them to the general public under the Legend label, this one is the Legend Blue Tick Hound.

[JSpina911]

Hi Dave....These readings and info I got from what I believe was a Fender repair tech that has worked on the series..He mentioned that the TIP 142/147s should be in the range of 40 to 80 mv..

This was listed under the same area that I'm guessing was tech bulletin to change R74,and 76 from 1k to the 2.2Ks because the voltages with 1k resistors were creating voltages of 80 to 120 mv and people were complaing of blown fuses and too hot TIPs in their amps..

When I saw the voltages being in the 20 mv ranges at those .47 resistors,I just felt the voltages were too low and I was questioning that?

I don't have an accurate way to measure the wattage output of this amp because I lack a scope...Thx for posting.

[JSpina911]

I'm measuring in dc millivolts across all of the 4 .47/5 watt resistors...They all measure pretty evenly but I questioned the lower voltage readings..

[JSpina911]

I'll plug this amp into my Vox V30 2x12 and see if it has some balls,plus I'd be at 4 ohms now and not 8 so it will produce more wattage.

I know the stock speaker sucks,but I'm not gonna dump a bunch of money into another speaker..LOL...

I have an Eminence Rajun Cajun in one of my Vox 30 watt amps and it's great...I know it's SPL is 100 or higher and efficient..

I have taken that into consideration..

I'm just learning on this amp guys and appreciate all the replies.

[JSpina911]

I ran the M80's send into my Blue Vox VTH 120's head and into the FX Return and it sounded great with plenty of volume..This put the M80 into my Marshall 4x12 with V30s...LOL

Then I did the opposite and ran the M80's return using the preamp from the Vox and same scenario..Sounded great..I mean the stock Fender speaker isn't that great but the Vox sounded really good through the M80s power amp.

So my thoughts are that the M80 I did a nice job fixing and the stock speaker sucks! LOL..

I'm gonna add another speaker out jack that would allow me to disconnect the stock 8 ohm speaker and then I can use that jack to run into a seperate cab at 4 ohms..

I'll bet this clean channel on the M80 would sound great through the Weber 12s that are in my Fender BF Pro Reverb amp.

The seperate cab is also closed back and gives me more balls and projection,though I lose the clean openess of the open back M80 cab..The other cab is the blue vox series match to the VTH 120 and it's 2x12 with V30s also.

I may even put a back panel on the M80 and have a half or 3/4 open back cab like Mesa does..Still the stock Fender speaker is lacking and flat sounding to my ears.

This is just a practice amp and it'll reside at our drummer's house..

Thx again for all of the replies!

[JSpina911]

Dave...The schematic for the M80 shows an AC MV reading of 4.90 at Tap 22...I'll check this next time I go in to the amp again...

I'm guessing that the readings I mentioned of 40 to 80 millivolts at the .47 resistors wouldn't be an idle bias reading perhaps?

If you have a schematic of the M80,check the voltage readings...

[JSpina911]

...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.

Dave...The schematic for the M80 shows an AC MV reading of 4.90 at Tap 22...I'll check this next time I go in to the amp again...

I'm guessing that the readings I mentioned of 40 to 80 millivolts at the .47 resistors wouldn't be an idle bias reading perhaps?

If you have a schematic of the M80,check the voltage readings...

[The Dude]

......Sorry, itīs multiply Peak to Peak by .3535 for RMS......

Good catch Juan and thank you! I was mistakenly thinking Peak instead of Peak to Peak. I will try to edit the original post so that misinformation isn't handed out.

[Enzo]

Yeah, I have to think that 80mv was supposed to be 80ma, which would be about 18mv across the resistor.


If you apply a sine wave or pretty clean steady signal to the power amp in (FX return), you can measure the AC on the output to the speaker with your volt meter. or a dummy load. And then it is just a matter of voltage squared divided by impedance gives you watts. it may not be a laboratory measurement, but it will be close enough to tell you what the amp is doing.

[JSpina911]

Nevermind and thank you.

Amp sounds great through better speakers...I'm going to re-tolex the cab in white and put a 3/4" plywood back onto the cab..If I actually keep the amp,I'll replace the speaker with some like the Eminence Red,White,and Blue etc..

So far I have $10 that I payed for the amp and about $15 in components..Thx to all for posting and the info..Learning all the time!

[JSpina911]

Yeah, I have to think that 80mv was supposed to be 80ma, which would be about 18mv across the resistor.


If you apply a sine wave or pretty clean steady signal to the power amp in (FX return), you can measure the AC on the output to the speaker with your volt meter. or a dummy load. And then it is just a matter of voltage squared divided by impedance gives you watts. it may not be a laboratory measurement, but it will be close enough to tell you what the amp is doing.

Ok this explains alot and makes sense Enzo...

[JSpina911]

Ok all....Can someone look at the schematic for the standard M80 amp and tell me how I can tame the highs,increase the lows,and lessen the gain on the overdrive channel..

This channel is just unusable...I see myself only playing through the clean channel,but would be good if the other channel could be tweaked a bit..I'm just not sure where to make the tweaks.

If it were a tube amp I'd know where to tweak...

I have the schematic if need be...Thx again to all that have posted to me!