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Post subject: power ratings
Posted: Thu Jul 08, 2010 10:26 am
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So when they say that, for example, a JTM45/100 head is 100 Watts, I thought that meant absolute peak.

However I talked to Jim from Southbay Ampworks, the creator of the scumbacks, who I would really trust on this issue, and he said that if you have a 100 watt amp, you should as a general rule double the wattage and run through that. So for a JTM45/100 head, your speaker configuration should be rated for 200 watts. I guess following the same logic you need a 36 W speaker for an 18watt head.

Here's a PDF where he discusses this:
http://www.southbayampworks.com/sd/speakermounting.pdf

Is this everyone else's understanding of it too?

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Posted: Thu Jul 08, 2010 4:39 pm
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My rule-of-thumb for a speaker load's power handling capability is, 50% more rated capacity than the amp can possibly produce.

I haven't blown a speaker in thirty-five years.

HTH

Arjay

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Posted: Thu Jul 08, 2010 5:24 pm
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Yeah, guess I run mine at 100%, ie. two 50watt speakers in parallel for a 50watt amp. My reasoning being, that if one speaker should fail ,the amp would still have a good load on it until I can shut it down. Art

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Posted: Fri Jul 09, 2010 10:56 am
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Retroverbial wrote:
My rule-of-thumb for a speaker load's power handling capability is, 50% more rated capacity than the amp can possibly produce.

I haven't blown a speaker in thirty-five years.

HTH

Arjay



+1 But it's forty years for me. :wink:

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Post subject: Re: power ratings
Posted: Sat Jul 10, 2010 8:44 pm
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supah wrote:
So when they say that, for example, a JTM45/100 head is 100 Watts, I thought that meant absolute peak.


Amp output wattage specified by manufacturers is RMS clean output before the tubes start to saturate (distort the signal). Depending on the model of amp with tube output stages, the point where "clean" starts to distort can be 7 or 8 on the volume knob. At 10 the amp can be pushing 120 to 140 watts to the speakers. Mike's rule of thumb is good advice, speakers (cabs) should be rated at least 50% more than the amp's rated RMS output.

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Posted: Sun Jul 11, 2010 10:13 pm
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Supah, I think there is some wiggle room here with the speaker(and for that matter power tube) ratings. Some are rated very conservatively, ie-blue alnico Celestion Bulldog 15watt. The Blues hold up quite well, with two running out of a Vox AC30, Matchless DC30, and the Badcat 30watt amps. I'd have to say that my approach is probably overkill, and very possibly not letting the amp breath correctly. I do know that some NOS power tube spec sheet's data are"centered", and state a"max" for some parameters, that are just low ballpark figures, for designers. Art

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Posted: Sun Jul 11, 2010 10:34 pm
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aclempoppi wrote:
Supah, I think there is some wiggle room here with the speaker(and for that matter power tube) ratings. Art


Right you are, Art.

Leo's amps were all conservatively designed and built. I've personally witnessed a '67 Super Reverb pushed to 52 watts and a '66 Showman driven to 105 watts on a bench-test rig, using an Eico signal generator, Heath power meter, and a fistful of wire-wound resistors to load the output tranny. Those figures are easily 20% in excess of the respective design specs. Conversely, we've all seen speakers with a claimed power handling capability of 75 watts program material fail when used in pairs with a 50-watt amp.

It really all boils down to the reputation of the company building the component......and periodically we learn -- the hard way -- that their PR hype doesn't always equate to street cred.

Arjay

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