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Deminishing Output?

banjo5

banjo5

Dec 8, 2009
145
4
Cincinnati
#1
So my outdoor rig is an SVT5 (which is like 1000 watts into 4) and a Mesa PH810 cab. Seems like I used to be able to knock over small kids, plow snow, move furniture, etc. with it. Here lately, it seems like it just doesn't have what it use to. I checked to make sure all 8 speakers are ok, and they seem to be. My question is, is there a chance my amp could be loosing power? Is there any way I can measure the output to detemine if it's putting out what it should be? I understand that volume or "loud" is subjective, but It seems like there should be some way to measure output power.
 
#2
FWIW, I've gigged an M-pulse 600 into a PH810 ALOT, and have never been able to crank it to even half power - and that was in a LOUD 2 guitar modern rock/metal band, with a Triple Rec halfstack on either side of the stage... I typically played with my Gain between 12 and 1 o'clock, and my Master volume around 9-10 o'clock... Once, I tried to approach 12 noon with my Master volume, and freaked out the venue owner and staff in a 400 seater - I was knocking stuff off the walls, and making bottles of booze fall - they weren't happy AT ALL...

I'm guessing the cab isn't the issue...


- georgestrings
 
#4
OK, I just looked into the SVT 5 Pro's specs, and it isn't anywhere near what the OP claimed(1000 watts into 4) - it's actually:

"500W x 2 @ 2 ohms, 340 x 2 @ 4 ohms, 255 x 2 @ 8 ohms"

It doesn't say anything about bridged capability, so I'm assuming that he's only putting 340 watts into that PH810 - and IME, Ampeg's power claims of their hybrid and SS amps are often optomistic at best... My M-pulse 600 puts out 625 watts @ 4 ohms, so I'm sure it would be significantly louder - although it is still possible that the OP has an issue with his amp - and very well may be a preamp tube on it's way out, as CL400Peavey suggested to look into...


- georgestrings
 
#5
Pre amp tube is easy enough to swap to see if that helps.

I typically set my gain until it just just hits the preamp limiter, then set the master to the volume I need. Here lately, it seems like I'm well over 10:00 with the master, then I'm getting the output limiter light occaionally. It should be pealing paint at that level, I would think.
 
#6
Here's what the manual says as far as specs go. I run it in the mono bridged mode.

OUTPUT POWER RATING 1350 Watts Mono-Bridged @ 4 Ohms (1000 Watts Continuous)
840 Watts Mono-Bridged @ 8 Ohms (680 Watts Continuous)
2 x 675 Watts @ 2 Ohms (500 Watts Continuous)
2 x 420 Watts @ 4 Ohms (340 Watts Continuous)
2 x 255 Watts @ 8 Ohms (205 Watts Continuous)

I can't say whether their specs are overstated or not. The other heads I've had in this power range, (including a Mesa M9) I didn't run with this cab.

My question is really about what I perceive as a deminishing output, over the couple years I've owned this head/cab combo.

I'll try and replace the tubes, which seems easy enough to start the troubleshooting process.
 
#7
banjo5 said:
Here's what the manual says as far as specs go. I run it in the mono bridged mode.

OUTPUT POWER RATING 1350 Watts Mono-Bridged @ 4 Ohms (1000 Watts Continuous)
840 Watts Mono-Bridged @ 8 Ohms (680 Watts Continuous)
2 x 675 Watts @ 2 Ohms (500 Watts Continuous)
2 x 420 Watts @ 4 Ohms (340 Watts Continuous)
2 x 255 Watts @ 8 Ohms (205 Watts Continuous)

I can't say whether their specs are overstated or not. The other heads I've had in this power range, (including a Mesa M9) I didn't run with this cab.

My question is really about what I perceive as a deminishing output, over the couple years I've owned this head/cab combo.

I'll try and replace the tubes, which seems easy enough to start the troubleshooting process.
Click to expand...

With modern tubes an annual is a good idea.
 
#8
BawanaRik said:
With modern tubes an annual is a good idea.
Click to expand...
Eh...

:D

Pre tubes don't last forever, but throwing out perfectly working tubes just because a year's passed is a waste of money.

Another thing you can check is corrosion in the effects loop (spraying Deoxit in them fixes that right up), but I'd suspect a tube most of all in your case.
 
#10
banjo5 said:
Here's what the manual says as far as specs go. I run it in the mono bridged mode.

OUTPUT POWER RATING 1350 Watts Mono-Bridged @ 4 Ohms (1000 Watts Continuous)
840 Watts Mono-Bridged @ 8 Ohms (680 Watts Continuous)
2 x 675 Watts @ 2 Ohms (500 Watts Continuous)
2 x 420 Watts @ 4 Ohms (340 Watts Continuous)
2 x 255 Watts @ 8 Ohms (205 Watts Continuous)

I can't say whether their specs are overstated or not. The other heads I've had in this power range, (including a Mesa M9) I didn't run with this cab.

My question is really about what I perceive as a deminishing output, over the couple years I've owned this head/cab combo.

I'll try and replace the tubes, which seems easy enough to start the troubleshooting process.
Click to expand...

Sorry - I just did a quick search on it, and MF's site came up - I grabbed that directly off of it... Whatever's going on, I hope you get it sorted out - it would've been interesting if you had compared it to the M9...


- georgestrings
 
#11
banjo5 said:
Is there any way I can measure the output to detemine if it's putting out what it should be?
Click to expand...

You'll need an oscilloscope, a tone generator and a four ohm resistor with a massive and efficient heat sink. HAM guys put their test loads for kw+ transmitters into what looks like a house paint can, but it's filled with mineral oil to conduct heat off the resistor to keep it from burning up. Using a 40Hz sine wave test tone into 4 ohms, bridged, your amp should be able to put out 63.25 volts without clipping. You can also check each side separately. They should be 41 volts at 4 ohms. It's not easy, but you'll have the answer definitively.
 
#15
JimmyM said:
Eh...

:D

Pre tubes don't last forever, but throwing out perfectly working tubes just because a year's passed is a waste of money.

Another thing you can check is corrosion in the effects loop (spraying Deoxit in them fixes that right up), but I'd suspect a tube most of all in your case.
Click to expand...

I've found that modern power tubes do have the long term ability the older tubes had. And while I have preamp tubes from the fifties still working it seems that the ones that die are newer.

I would definitely check the bias after a year. If any of the tubes has drifted differently from the other it's time for a new set.
 
#16
Psychobassguy said:
You'll need an oscilloscope, a tone generator and a four ohm resistor with a massive and efficient heat sink. HAM guys put their test loads for kw+ transmitters into what looks like a house paint can, but it's filled with mineral oil to conduct heat off the resistor to keep it from burning up. Using a 40Hz sine wave test tone into 4 ohms, bridged, your amp should be able to put out 63.25 volts without clipping. You can also check each side separately. They should be 41 volts at 4 ohms. It's not easy, but you'll have the answer definitively.
Click to expand...

That's exactly the answer I was hoping to get, although I don't think I'm equipt to perform the test as you described. I do have one more question though, wouldn't the output voltage be effected by the output of the preamp section? You said to pump in a 40 hz sine wave, but at what amplitude, and where should the gain control be set? Or, is that a dumb question?
 
#17
The amount of input signal voltage that's required to produce the amp's rated output, as well as the control settings that are used for testing the amp's rated output, are unique to each model of amplifier. There is no universal standard. To get the service docs for your particular amp you should contact Ampeg tech support.
 
#18
banjo5 said:
That's exactly the answer I was hoping to get, although I don't think I'm equipt to perform the test as you described. I do have one more question though, wouldn't the output voltage be effected by the output of the preamp section? You said to pump in a 40 hz sine wave, but at what amplitude, and where should the gain control be set? Or, is that a dumb question?
Click to expand...

That is where the oscilloscope comes in. Flat topping (clipping) at low output levels indicates excessive gain in or from the preamp.
 

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