Selecting correct wattage speakers!

[Seegs]

When hooking up two speakers in parallel how much power is delivered to each speaker? Series?

Is it equally delivered and how much wiggle room should one allow when selecting speakers for an amps rated power output?

ie. if an amp is rated at 50watts would two 20watt speakers blow or what is the safe bet between performance and longevity?

I want to take a BF/SF Fender head and build it into a 2X10" combo and want to get some 10" alnicos with later breakup...

Goal is clean headroom at 4-5 vol. (40-50watt amp and 2X10" speakers) and then nice smooth breakup as you turn the amp louder...at which point turning the guitars volumn down will result in nice chimey cleans with a bit a hair on em...

yea I'm making my own Vibrolux but don't want a Vibrolux:p

what speakers would you recommend for either a SF Bassman 50 head or a BF/SF Bandmaster?

Chow,
Seegs

[JimH]

correct - an 8 ohm parrallel cab with 2x 75W speakers is a 150W cab. also each speaker is 16ohms idividually. example - Marshall 1936. As for wiggle room.. hmmmm, depends on head room required - the standard answer is somewhere above the amp rating - though if you're running the amp hard you need even more. This also depends how the amp's been rated, pre or post distortion. It's all pretty complicated. I'd go for minimum 2 x 35W. Jensen P10Q. But I wouldn't be entirely comfortable with cranking.

[Seegs]

Thanx Jim...thats what I don't know...if the amp ie. is a SF Fender 50watt Bassman is that pre or post distortion...and if it's pre how much power is running it post gonna deliver?

I think the spkr. outs on the Fenders I'm looking at is 4 ohms so I would go with 2X 8 ohms in parallel...

Chow,
Seegs

[JimH]

Hopefully someone can help you who has more familiarity with the amp. I know that matamps, for instance, rate thier amps very low - I'd guess Fender have had various schemes over the years. I'd not be happy using a 50W plexi with much under 100W cab.

[Southbay Ampworks]

The accepted method I always heard about was running the amp at 8 ohms with a clean signal, and measuring wattage with an oscilloscope before any clipping occurs. My amp tech says that's how Marshall, Fender, etc. used to do it, so that's what he does.

Most amps he's tested are "clean" around 2-4 on the volume knob for their rated output.

50 watt Marshall plexis, are 55-60 clean (4) and around 90 when dimed. 100 watt Marshalls are 100 watts around 4, max. They will put out considerably more at 8, like 170-190. One old 100 watt Plexi my amp tech tested was 140 clean, and 235 dimed.

So you have to get speakers to handle what volume level you're using the amp at, then add on 50% to cover your butt.

Examples:
18 watt amp, 25-30 watt speaker, minimum.
30 watt amp, 50-60 watt speaker, or two 25-30 watt speakers.
50 watt amp, 100 watts of speaker power handling, like a 4x12 w/four 25-30 watt speakers.
100 watt amp, two 4x12's w/25-30 watt speakers.

Re: Seegs amp & speakers. I've heard of good results with my 10" speakers with a Bassman from previous clients. PM Hey! You! on this forum, he's got a pair he uses with his old 64 Bassman head, he'll be able to answer questions about it better than I could.

[JimH]

So you're saying basically x2 when dimed? I'm not trying to be clever but that's not +50%, it's +100%.

[Seegs]

Thanx a bunch Jim...I love your 12s in Chris's cab....he'll be here Friday and were gonna do the 55/75 X pattern in his 4X12" Germino cab. as well as checkin out that GVCG you so kindly sent over;)

right now I am just conceptualizing but I was definitly considering a couple of SB 10's as one of the front runners...but unfortunatly I waited too long to take advantage of your holiday special:p

I don't have the head yet but I am watching a SF 50 watt head w/MV and a BF Bandmaster...

which speaker would you recommend for either of those heads...the 30 or 75 watt model?

fwiw on their own in a 2X12" cab. I really love your 75H hz cones and the 55s sound great when mixed...

Chow,
Seegs

[Southbay Ampworks]

So you're saying basically x2 when dimed? I'm not trying to be clever but that's not +50%, it's +100%.
Well, here's how it can work. If you're playing your 50 watt amp at 2-4, you're not typically running more than 40-50 watts., since it isn't going into clipping, or above the "clean" rating that established the RMS wattage. So having double the power handling (100 watts) is overkill. If you had 60-75 watts power handling you should be fine.

But if you ran your 50 watt plexi (clone, or similarly wattage structured amp) at 8, you're probably well past 50 watts, closer to 70-80. At that point, 2 30 watt speakers are getting exercised fully, and just like when you run an engine at 8000 rpm for a length of time, failures occur. Same with running your speakers full out, or "on the ragged edge". Lots of players do it this way, and they blow speakers.

If you want to go that way, then you'll be buying or reconing speakers to handle it.

Personally I use the 175-200% speaker power handling to wattage rule. So my 30 watt amp gets two 30 watt speakers to run through, or one 65 watt speaker. My experience is that you can get a speaker working in it's dynamic range using 1/2 the power of the speaker rating. This means you have a safe zone, without sacrificing tone, and without sacrificing speakers!

Now, you can run your amp any way you want with your speakers. It's just a question of how often you want to replace/recone them.

To be on the safe side, and you set your amp all over the dial, I'd go with 200% of the amps rated clean output in RMS watts.

[Boogs]

I think it's worth noting that it depends not only on how the amp is rated, but how the speakers are rated.

Weber, for instance, rates his speakers conservatively. This is by user accounts, and is what Ted told me himself. Therefore, I feel better about running closer to the line with one of his speakers than with, say, an Eminence. Nothing against Emi, who make great speakers, but everyone rates things differently and I assume 50 watts mean 50 watts (i.e. a dimed 25-30 watt amp) unless I have reason to think otherwise.