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Post by gotmojo on Mar 27, 2011 18:15:12 GMT -7
I'm no engineer, but I did take physics in college about 40 years ago! And that makes me about totally useless for my question which has to do with Z Brakes, of which I have quite a few.
Why is it that the Z Brake can only be used with 4, 8 and 16 ohm amp loads? What is it that keeps them from being able to be used on a 2 ohm load? Can you put two Brakes in series and use it on a 2 ohm load? I have an old Fender Bassman I'd love to attenuate. I bought one of the THD Hotplates thats a 2 ohm unit, but it is heavy and has lots of knobs. Would love to be able to use the simpler and lighter Z units. Please help!!!! Inquiring minds want to know!
Thanks!
jj
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Post by Scott on Mar 27, 2011 19:06:59 GMT -7
My money is on that it will also work with a 2 ohm load just fine... The only reason a 2 ohm load is not listed, is because the Doc doesn't make an amp, with a 2 ohm out. But wait for someone who knows for sure...
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Post by mickey on Mar 27, 2011 23:45:54 GMT -7
A few years ago I emailed the Doc to ask if it was OK to use an Airbrake with the Fender Vibroking I was using at the time, which with three 8ohm speakers worked out at 2.8 ohms. THD make a Hotplate specifically to work at that impedance, and Doc's advice was to go with that, rather than the Airbrake. I seem to remember he said don't go below 4 ohms, but I'm afraid I don't have a technical explanation.
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Post by Phil (aka Phil) on Mar 28, 2011 7:05:30 GMT -7
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Post by gotmojo on Mar 28, 2011 8:24:13 GMT -7
Um, ..... Phil, thanks. But I agree wif Steve C (Bentop): brain cramping on that graph. Didn't seem to answer my concern except to worry that at one click down I'd be running at like 1 ohm? Still waiting for an "expert" to give an opinion. Although I'd already had an expert opinion from the Z folks, and that was to get the THD unit. =^). So really I'm just being obnoxious with this repeat inquiry. Was just wondering if running two Brakes in series would allow safe attenuation for the Bassman! jj
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Post by benttop (Steve) on Mar 28, 2011 10:35:15 GMT -7
It wouldn't be safe. Don't do it. Nothing lower than 4 ohms for the Airbrake. Of course, it's your output tranny to burn.. BTW, I have three Hot Plates here, and they are pretty good. On Z amps I prefer the Airbrake (or Brake Lite), but the Hot Plate has a bit more attenuation before it gets to the bedroom level, and a cool light that as far as I can tell, does nothing but glow as you play (hoo boy, that's really cool!) Of course the down side of the Hot Plate is that you buy the impedance to match the job at hand, then if you have a different job, you have to buy a different Hot Plate. That's why I have three here - a 4 ohm, an 8 ohm, and a 16 ohm. I coulda bought a Ghia Head for all that!
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Post by Phil (aka Phil) on Mar 29, 2011 6:25:01 GMT -7
I know there's a lot of confusion about the physics of attenuators. The reactions between a tube amp and a loudspeaker can get pretty complicated. I think I'll go with the pragmatic, simplified explanation Tube amp output sections and speakers operate through the output transformer on a principle called "reflected impedance". What this means to us guitar geeks is that the tubes in your amp want to "see" their ideal impedance through the "lens" of the output transformer. That's the output transformer's job: match the output tube's impedance and high voltage to the speaker's relatively low impedance and lower voltage of operation. It's usually "best" to hook up the "correct" impedance speaker to the "correct" transformer tap (i.e. 8 ohm speaker to 8 ohm tap) as we all know. This is so the output tubes are happy; they may be looking for a load of say 10,000 ohms but your speaker is 8 ohms. Through the magic of the output tranny, the tubes "see" 10K ohms. What attenuators do is pure physics. They add resistance in series and/or parallel between your amp and speaker, dissipating some of the energy that would have gone to your speakers as heat. Yes, this means that your output tubes are not seeing the "optimal" load anymore. So at this point a discussion of various amp's construction must come into play. Depending on your amp's output section design and robustness, it may or may not tolerate a large impedance difference well. Many (if not all) tube amps do just fine with 2-1 mismatches on the "high side", i.e. running a 16 ohm speaker from an 8 ohm tap. The power output will be a little less. It's pretty much never a good idea to go "the other way", running a lower impedance load (speaker) than expected, say a 4 ohm speaker in an 8 ohm tap. This presents a lower impedance to the output tubes through the reflected impedance of the output transformer, drawing more current. More current means more heat, and heat in electronics is bad. So, as Randall Aiken's chart shows in the link I posted earlier, the Airbrake's design makes the amp "see" a higher impedance value than the connected speaker at 4 and 8 ohms. For a lot of amps, adding this kind of additional resistance isn't a problem. If you add too much resistance, however, the output section starts to see this as an open circuit (infinite resistance), which is what happens when you run a tube amp with no speaker connected at all. This is bad. That can destroy your output tranny and tubes. Dr. Z amps have very well built overspec'd trannies...as some of you know my 1998 Ghia was turned on once with no load for three days with no ill effects. Do NOT try this at home, kids I'm just sayin. A lot of amps would have been a lot less forgiving. This is why running two Airbrakes in series is a bad idea. When you start adding resistance on top of resistance on top of speaker impedance, you're getting farther and farther away from what the output tubes want to "see" through the tranny's multiplication effect. You've got to add a lot of resistance before the amp sees the load as an open circuit, but it's still not good on the output section. The Airbrake is designed to be in the "safe" area for amps when used with the 4-16 ohm loads that Z's and a lot of other amps (Fender Super Reverbs and the like excepted). The Airbrake can "get" up to 24ohms of resistance...which is why you don't want to use it with a 2 ohm amp. That's a 12X mismatch. 2X or 3X is okay, but I don't think I'd want to go 12X safely. Also, if you notice on Randall Aiken's plot (assuming it's correct), for a 16 ohm load the actual impedance is lower than 16 ohms because of the Aribrake's design. That's why in the Airbrake manual it says to run 16 ohm vintage Marshalls and Voxes in the amp's 8 ohm setting. For 4 and 8 ohm loads the impedance the Airbrake presents to the amp is always greater than the speaker's rated nominal impedance. And I know what you're thinking....hey, I'll run two Airbrakes in series into a 2 ohm speaker load and then plug it into the 16 ohm tap- so that the additional resistance and impedance they have will add up and be safe since the amp's looking for "more" impedance to start with. I've got three words for ya...don't do it. How did that old commercial go? "You can't fool Mother Nature" I'd definitely stick with the 2 ohm Hotplate for Fenders. I hope this is what you were looking for Mojo. Sorry about the earlier cryptic Akien post reference.
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Post by gotmojo on Mar 29, 2011 6:27:57 GMT -7
Thanks Steve. Hope all is well with you, my man. I've fallen off the face of the forum here of late - too much other stuff going on in life. Playing in a band, getting ready to start a new job after 25 years and moving! Sheesh. Thanks for the good advice on the the attenuator question. I actually have the THD 2 ohm'er, just have never gotten around to getting Todd to make me up the cable! Maybe I'll get around to that today, hmmm... Now, can we get the good Doctor to make a 2 ohm Airbrake?
jj
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Post by Phil (aka Phil) on Mar 29, 2011 6:32:59 GMT -7
James, I doubt Z will ever make a 2 ohm Airbrake since all of his amps and cabs are 4-16ohm. You could modify your own though.... ;D
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Post by gotmojo on Mar 29, 2011 6:39:25 GMT -7
Thanks Phil! Nice explanation, dude! And good point on loading up with 2 Airbrakes, although I seem to remember Benttop saying he has done this. And good one on running your Ghia for 3 days! As much as I am finicky about my gear, have left old tweed amps inadvertently on for days at a time several times, luckily with no ill effects! I'm an idiot, I admit it.
jj
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Post by benttop (Steve) on Mar 29, 2011 6:58:04 GMT -7
Yeah, I've done it with my Remedy, but never for very long and I wouldn't call it my normal mode of operation. Just fiddling around here to see what's what...
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Post by Phil (aka Phil) on Mar 29, 2011 7:30:25 GMT -7
It's not that you *can't* hook up two Airbrakes in series...you can. If you use them both at moderate levels of attenuation you're OK. I don't think I'd crank my Mazerati GT into two Airbrakes set on "Bedroom level MAX" for a long time though. Plus, as I said in some other post, lots of attenuation on a tube amp sounds pretty bad to me. As far as my Ghia goes James...I think you missed one important aspect of the story...it was on for three days *with no speaker connected*. I guess a good analogy would be taking a normal-production car engine with no special synthetic oil or other enhancements, putting it in neutral (i.e. no load to slow it down), disconnecting the radiator, then putting a brick on the gas pedal and letting it rev at redline for three days straight, then reconnecting the radiator, changing the oil and driving away with no ill effects for 100,000 more miles. That's my Ghia
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Post by gotmojo on Mar 29, 2011 9:36:29 GMT -7
Yeah, I got that. But you did that on purpose just to see if it'd take it, or was it a lil oversight, oops kinda thang?
jj
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Post by Phil (aka Phil) on Mar 29, 2011 9:56:54 GMT -7
Yeah, I got that. But you did that on purpose just to see if it'd take it, or was it a lil oversight, oops kinda thang? jj My youngest son turned it on without having a speaker connected. He did it on a Thursday and Don and I didn't discover it until Sunday. It was so hot that we couldn't touch the Ghia, the wall behind it, or the Maz 18 NR on top of it. After a lengthy cool down and a new set of EL84's she was good to go. The wall 5" behind it was so hot I was shocked it hadn't started the house on fire.
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Post by gotmojo on Mar 30, 2011 5:33:19 GMT -7
Wow! And bummer. But awesome that it survived. Speaks volumes for the tough-as-nails build quality and components of Z's amps!
jj
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Post by Jan on Mar 30, 2011 16:40:57 GMT -7
Also, if you notice on Randall Aiken's plot (assuming it's correct)Phil may have felt a little wink about to surface when he said that. Maybe not. For anyone who did not make the connection, I would trust Randall Aiken's chart. I highly recommend the Tech Info section on his web site for anyone who owns a tube amp. My ex Dr Z dealer, Midtown Music, was also an Aiken dealer before Dave retired and closed the shop. The Aiken Tomcat was in serious consideration before I finally decided on getting the Maz Jr NR. www.aikenamps.com/PS: Phil is no slouch on the subject of flowing electrons either. ;D
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Post by Phil (aka Phil) on Apr 1, 2011 5:52:01 GMT -7
There was a little winking involved in the Randall Aiken comment I've never met Mr. Aiken but I have a heck of a lot of respect for him. He definitely knows what he's doing. I'd love to sit down with him sometime over some coffee (I don't drink beer ) and talk about electrons. Jan's right; his web site has an *excellent* set of technical documents written on every level from novice to expert. It may be the second best source of good guitar amp technical info next to Myles' site. What I should have said was that I hadn't run the numbers myself to verify them. I'm sure they're accurate. Here's something else to add to the discussion. I found this one the 'Net; I think it's a version of Weber's Z Matcher that's no longer in production...maybe it never did go into production. Ted discusses the exact same scenario we're talking about here. taweber.powweb.com/store/attenmatch.htm
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