Overdriving my SWR Grand Prix -- is this okay?

STOP IT!!!!!!!! You're killing the speakers.

That +20dB light is more often called the "Clip light". A clipped signal sends your speakers some seriously nasty stuff that will zap the speakers voice coil if you do it too often.

Like the manual says, the clip light can cone on once every now and then, but if it's on more than it's off, it's just a metter of time.........

Buy an overdrive effect, pedal, preamp, whatever.........

 

Distortion is distortion. There's very little reason why solidstate preamp distortion would be any more dangerous to your speakers than just using a solidstate distortion pedal.

However, you're not doing the right thing to get tube distortion out of your GP. AFAIK the valve is a fixed gain stage and the gain control is a pad AFTER the valve to ensure that the solidstate sections of the preamp don't overdrive. The level meter is just before the master volume and after all the gain stages and EQ - if you're hitting +20dB you're overloading the solidstate circuitry of the preamp to get your distortion.

If you want to get valve distortion you need to put a signal booster before the GP so you're driving a very hot signal into the valve which overloads that gain stage and then keep the gain control low to pad the output from the valve so there isn't any clipping in the rest of the preamp (i.e. even when you're getting huge distortion from the valve the level meter need only be at 0dB).

Alex

 

I humbly disagree. Distortion is not distortion. The clean amplification by a power amp or later gain stage in a pre-amp of a fuzz pedal or earlier stage in a pre-amp is not the same as a clipping solid state power amp. Example: crank your stereo with some Grand Funk Railroad or Jeff Ament or Mountain or Cream. Sound good huh? Sound like the recording? Now plug your bass into your stereo and crank it. I knew you wouldn't do it because you know it would sound like dog doo and blow your speakers. You see it's about signal level down the chain and what sees what level of voltage. When guitarists get a good sound out of many pedals on a pedal board it is after much tweeking to balance what effect sees how much level from the one before it. Even still follow SWR's advice and get you distortion some other way.

 

Although the character of different gain stages distorting varies, they are all fundamentally similar distortions - there is no more risk of damaging your equipment by using overdriving a preamp than using a distortion pedal. There is a risk of damaging things if you overdrive the power stage but this is not due to the distortion being any different, it is due to the increased average power level of an overdriven power amp.

They key point is that any amplifier system cannot put out any more power than its weakest link - assuming your amp and cab are suitably matched for a bass signal (twice RMS rating of cab - amp power is a good yardstick) then even if you put a aignal into the power amp which is a perfect emulation of power amp distortion there is no increase in risk of blowing the speakers because the power amp's output is limited by virtue of it running within its low distortion limits. If however you overdrive the power amp so that the speaker is seeing exactly the same tone as with the emulated power amp overdrive, you will run a risk of blowing your speakers because AN OVERDRIVEN AMP CAN PUT OUT AN AVERAGE POWER OF APPROXIMATELY TWICE ITS RMS RATING. That is the only reason (ignoring that a distorted signal is compressed and will thus have a higher average signal level for a given degree of loudness)

The reasons behind that are that program sources are fullrange and complex so you'll clearly hear when the amp is being pushed too hard and back off, but more importantly that consumer audio is heavily compressed. A bass signal on the other hand is very dynamic, and a significant amount of its power is below the -3dB point of many home speakers, consequently for a given volume level you'll have to push the amp (and thus the speakers) twice as hard. This problem is then made much more serious because most home speakers are ported and their resonant frequency is above the lowest notes on a bass, consequently you'll be trying to reproduce notes that are so low that the speaker is probably more than -10dB (i.e. demanding 10 times the power for a given volume) and the cones are unloaded and thus liable to over-excursion damdage.

Correct - it's about signal level down the chain, but the limiting factor will always be the weakest link which is almost always the power amp. Assumuing that you do not choose a distortion with an identical sound character to power amp distortion (thus allowing you to overdrive the power amp without noticing the change in tone) you will always be running the power amp clean and thus the maximum average power it can put out will be litle more than its RMS rating.

Bottom-line: power amp distortion is dangerous, not because it is distortion but because it increases the average output power of the amp to potentially dangerous levels. A smaller amp driven into distortion could thus be putting out a higher average power than a larger amp which sounds the same volume because the extra dynamics of the bigger amp will allow it to cut through much better even though the average output power is lower.

Alex

 

Yeah! Like he said. Thanks for the elucidation. (Elucidation Breakdown: a famous hit by Heavy Balloon)
Also you said you where using headphones. That may change everything because it's only possible to model the acoustics of the inside of your head in an obscure lab run by retired and deceased JBL techs in the deserts of Norway during Mardi Gras. So distortion may be in the ears of the Behearer.