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Fender Stereo 412 cab/Peavey C30 Combo ohm question here.

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I think I have this figured out but I just want some confirmation before I go and damage my amp.

 

I want to run this cabinet from the speaker extension jack on my Peavey C30 combo. Since it is 16ohm and this cab is 8ohm mono, that would be a no no for all 4 speakers. However since this is a stereo cab that says right on there stereo 16ohms each, I should be good to go as long as I'm only using two speakers in the stereo mode, correct?

 

Question is if I have the C30 plugged into the right stereo jack, do I have to have anything plugged into the left mono jack to still be at 16ohms going to the peavey? Will a "dummy" plug in the right mono jack work just the same as if I have a second amp connected? And if I do have another amp plugged in, which I usually will (Bugera V5 @ 4ohms), does the Bugera need to be turned on to take the stereo jack to 16ohms?

 

Please forgive my ignorance but I just got this fender cab for free, and after reading all the contradicting confusion online for a few hours I've given myself a hell of a headache. I did try the set up briefly with the two amps and it seems to be working, Peavey on the right jack, Bugera on the left. But do I need to turn on the Bugera, or have it plugged in at all, to avoid overloading the C30? If this works out I may look into getting some different speakers down the road.

 

Any help is welcome!

 

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It looks like you can use the right jack and get 16 ohms from that jack without using anything in the left jack. Its likely the right jack is the one that is switched and splits the cab. If you had an volt/ohm meter you could simply test the speakers and read the ohms. Ohm meters are super cheap these days. You can find them for like $10. Its something every musician should own, even if its for only simple tests like these.

 

Maybe you have a friend who owns one?

 

The other simple check is this. Connect your cord to the right jack. Take a 9v battery and touch it its terminals against the end of the jack. You'll see the two of the speakers move and two stand still. This will confirm that only two speakers are connected to that jack and you'll have your 16 ohms.

 

You'll run safe with the internal speaker and a 16 ohm extension for a total of 8 ohms which is the minimum for that amp head but keep this in mind. You'll be running 1 speaker vs 2 in the extension cab. The wattage splits equally for the two 16 ohm loads. If the amp is 30W that means your internal will now see half power of 15W and the extension cab will see 15 watts. The extension cab has two speakers so its going to split its wattage to 7.5W per speaker.

 

Depending on the speakers SPL ratings the extension cab with two speakers may be less efficient then having a single 16 ohm extension speaker. It may be less efficient then just running the internal speaker solo too. You wont know till you give it a try so give it a shot. The bass response will of course be increased because of the increased cabinet sizes. The footprint will be bigger too. Stock Fender speakers on those cabs are those bargain basement eminence type so they might not produce allot of sound.

 

 

Personally what I'd do is add a switch to the amp to disconnect the internal speaker, then just run the 4X12 cab at 8 ohms. Its going to sound allot better with all 4 speakers moving. You can do the test easy enough. Just pull one lead off the internal speaker and put a piece of tape over it so it isn't touching anything then connect the external cab using the 8 ohm jack.

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Thanks for the info!

I did plug in both amps with a Strat through each and everything seems to be working as it should. Like you said though, the combo speaker is much louder than the cab with just the peavey through two speakers. Could hardly tell its on unless I get within a few feet of it, and I have them on opposite sides of the room. I might try it without the combo speaker connected to see how it sounds, but I'm not expecting much with those speakers. I can always rock the little Bugera with the full cab which sounds a hell of a lot better than with the little 8" speaker thats in there.

 

I think what I'm gonna do is treat myself to one of those new Peavey 6505 mini heads and run that into it. From what I've read these cabs were designed for high gain hence the HM for Heavy Metal in the name. I've been neglecting my metal side for many years now, I'll finally get the sound I wanted so bad when I was 15.

 

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I suspected that would happen. Fender doesn't use the best speakers and the SPL level is probably low so the actual loudness isn't there.

 

A rule of thumb I use for speakers is use a head that matches the speaker wattage with solid state heads. If the SS head is 100W you want the cab to be at least 100W or maybe a little more. With tube heads you want the cab to be at least double what the head is. If the head is 50W - the cab should be 100W. This is because tube heads are rated for clean watts and most will reach that at half volume. You still have additional clipped power once the head starts clipping so you need a cab with at least an additional 30~50% wattage above the heads RMS rating.

 

From the looks of the pic you posted the cab is carpet covered. Those were made for their Red Knob series amps in the late 80's early 90's.

I have one of the 100W heads from that era I mounted in a 4X10" combo cab I built. Not a great head but very durable. I think that can you have was paired with an M-80 head which was stereo. The cabs were built well but the speakers were pretty mediocre. They are likely rated for 75W each for a total of 300W which is pretty common for 4X12 cabs.

 

I think your idea of getting a separate head for it is a good idea. That combo isn't going to get those cones moving. I think you'll be able to get a meaty sound out of the cab without peeling the paint off the walls. Too bad we cant trade cabs. I have a Peavey 4X12 that would be a match for that 6505 and you have the match for my Fender red knob.

 

 

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Once the waveform clips, there is no additional "clipped power." That's a myth. It doesn't matter if it's a tube or solid state amp - once you reach clipping, there's nowhere else to go.

 

However, trying to reproduce a square wave (which is what you get when the signal is highly clipped) is extremely hard for any speaker, and attempting to do so at, or close to the speaker's recommended wattage rating is a good way to blow the driver.

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Once the waveform clips, there is no additional "clipped power." That's a myth. It doesn't matter if it's a tube or solid state amp - once you reach clipping, there's nowhere else to go.

 

Phil, I have to disagree. I think you're either mixing up how a peak limiter works with how tube amps. I assure you they are not the same.

 

RMS Wattage is only 70.7% of the peak. What happens when you push a tube amp past its clean waveform into clipping is you flatten the peak first as the RMS value increases.

 

You have to push the RMS value up at least +30% before the RMS levels start attenuating, while all the time its changing its percentage. Eventually the wattage stops increasing when you have a pure square wave which can be up to double the Clean RMS value.

 

(Not that a tube amp would ever sound good being pushed that hard. Few rarely go beyond a 30~50% increase from my experience)

 

Here's why in a nut shell.

 

"For musical instrument application, where distorted (overdriven) output may be a musical requirement, the system should be powered with an amplifier capable of delivering only one-half of the IEC rating for the system." This necessary because, for example, an amplifier normally outputting "300 watts of undistorted sinewave" can reach closer to 600 watts of power when clipping (i.e. when its output is closer to a square wave). If such a scenario is plausible, then for safe operation of the loudspeaker, the amplifier's (RMS) rating must no more than half the IEC power of the loudspeaker.

https://en.wikipedia.org/wiki/Audio_power

 

The wattage rating of amps is based on the maximum clean tone. RMS is calculated using a 1kHz sine wave with max. 1% THD. What you actually feed an amp can have multiple frequencies (Musical power) and cause distortion at lower wattages, especially with bass tones. .

 

Tube amps can get considerably louder (not just perceived because if the distortion) with power tube saturation. Of course Tube amps can be all over the place on how hard the preamp pushes the power amp.

 

For example an Ultra linear amp may produce very little distortion full up so you can get closer to match between the RMS value and the speaker. It doesn't clip so you don't need the additional RMS headroom. On the other hand A tweed may begin to saturate at half volume and increase in clipped wattage as its turned up. The preamp may be saturating too but the power tubes have plenty of open headroom.

 

Connect an ammeter some time in series with the speaker and measure the current and you'll confirm it increases even after the signal begins clipping and when you aren't hearing much change in volume. It will eventually top out from getting audibly louder but that doesn't mean there isn't more current. (Which is what damages voice coils)

 

Again, Tube Amp rated with a 1% acceptable distortion level. Turning the amp up into clipping simply raises the distortion percentage.

 

Typically, an amplifier's power specifications are calculated by measuring its RMS output voltage, with a continuous sine wave signal, at the onset of clipping—defined arbitrarily as a stated percentage of total harmonic distortion (THD), usually 1%, into specified load resistances. Typical loads used are 8 and 4 ohms per channel; many amplifiers used in professional audio are also specified at 2 ohms. Considerably more power can be delivered if distortion is allowed to increase; some manufacturers quote maximum power at a higher distortion, like 10%, making their equipment appear more powerful than if measured at an acceptable distortion level.

 

One spec manufacturers of guitar amps don't publish is maximum Continuous Wattage. They do with many pieces of PA and Hi Fi gear but not Guitar amps. (Mainly because they inflate wattage to make the specs look better) Tubes are old school and had a more realistic rating system. They do have much higher voltages available too so obtaining higher clipped wattages is surely possible.

 

Continuous power measurements do not actually describe the highly varied signals found in audio equipment (which could vary from high crest factor instrument recordings down to 0 dB crest factor square waves) but are widely regarded as a reasonable way of describing an amplifier's maximum output capability. For audio equipment, this is nearly always the nominal frequency range of human hearing, 20 Hz to 20 kHz.

 

This is an important factor to consider when buying speakers. With tube amps It's best to get speakers that can handle more than the amp's RMS rating. (30% minimum is my recommendation) Clipping is a sustained wattage and the chances of a speaker coil overheating is higher then clean intermittent peaks.

 

SS amps, you can usually match the RMS value because the amps are designed to run maximum RMS at full volume.

Clipping power transistors not only sounds nasty, but heat is their enemy so most power amps circuits are designed not to clip. (with a few creative exceptions, but even those exceptions are small percentages compared to how tubes can crush a signal with no ill effects)

 

Edited by WRGKMC

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