aphawki Posted February 1, 2009 Posted February 1, 2009 Ok here is one of the first of many questions to ensure I don't break anything. I did a search and couldn't find the right answer. I have the 112 cab...what is the difference in the ohm outputs on the back of the head unit? From my understanding the higher ohms are for larger cabs? And also the most that I want to use is the 4 ohm plug with my 112 right?
cGil Posted February 1, 2009 Posted February 1, 2009 A difference of only 4 ohms between the jacks may not sound like much, but plugging into the wrong one can be bad for the tube in the long run! In a worst case scenario, it can lead to early amp death in the short run. Every speaker cab, whether it's a single speaker (large or small) or a whole bunch of 'em wired together in a cab, will have a specific impedance. Impedance means resistance in an AC circuit, since the "resistance" can vary with the frequency of the AC signal. The OT itself converts a high impedance, high voltage, low current signal from the tube into a low impedance, low voltage, high current output signal for the speaker load. Furthermore, every tube type will need to see a certain impedance on the primary winding side of the OT. The secondary winding will need a load impedance that matches the impedance rating for the tap, to result in the spec'd impedance on the primary winding of the OT. Therefore, you need to match your speaker load to the same impedance tap on the amp. If your speaker load is 8 ohms, you need to use the 8 ohm jack on the amp. In spite of the mere 4 ohm difference at the jack, plugging into the wrong jack will result in a 2500 ohm difference in the reflected impedance that the power tube sees on its side of the OT. Not so small a difference after all, huh? Unless you're technically savvy and know what you're doing inside a tube amp, and you understand the why's and when's of certain instances when not following the standard rules won't destroy the amp, then yes, it IS a big deal! So do your amp, your tubes, and your wallet a favor, and use the speaker jack that matches your speaker load. Refer to your speaker or speaker cab for information on the impedance load it will present. If you don't have that, you can measure it with an ohmmeter. Always expect to see a slightly lower measured DC resistance when checking speakers, so a speaker rated for 8ohms impedance, for example, should actually measure up to around 6 or 7 ohms of DC resistance on the ohmmeter. Here's a few important "DO NOT" do's you may not be aware of that apply to tube amps. Primary Rule # 1 DO NOT turn on the amp without an appropriate speaker load attached. The OT may survive this momentarily, but only if there's no signal present on the input. (be advised, even a hum is enough of a "signal" to kill an OT) A signal present without a speaker load attached will cause the OT to overheat, which will melt the insulation between the windings. When that happens, the OT is now suitable for duty as a doorstop, paperweight, or boat anchor. It sure won't work in the amp any more. Primary Rule #1.1 DO NOT plug in a speaker while the amp is "ON". NEVER NEVER NEVER! Turn the amp off first! If you turn the amp on without a speaker, and then plug in a speaker, the flyback voltage will melt the OT's windings and completely destroy it in far less time than it takes to say "Oh crap!". Gil...
aphawki Posted February 1, 2009 Author Posted February 1, 2009 Hey thank you very much for the heads up information on the amp! I need all of the information that I can get. But I still don't think I understood...do I need to use the 4 ohm jack for my 112 extension cabinet? It's the Epiphone one that comes with the stack. There really isn't any info in the owners manual about it either.
cGil Posted February 1, 2009 Posted February 1, 2009 The cab impedance should be marked on the jack plate on the back of the cab. I believe it's a 16 ohm cab. If so, plug it into the 16 ohm jack on the amp. If it's an 8 ohm load, use the 8 ohm jack. It won't be a 4 ohm load, so either way, it would not be a good idea to use the amp's 4 ohm jack with this speaker. Here's the big scoop from Celestion. http://professional.celestion.com/guitar/features/loadingyourspeakers/index.asp Gil...
aphawki Posted February 1, 2009 Author Posted February 1, 2009 Alright...thanks Gil. I was using the 4 ohm jack...good thing I asked!!! Switched it over to the 16. Thanks for all the info and the link!!
Steven Lister Posted February 1, 2009 Posted February 1, 2009 There really isn't any info in the owners manual about it either. That's curious. Has Epi changed their manuals and/or 112 cab backplates? I have two V3 heads (2007 & 2008) and both came with a 4 page B/W glossy manual for head and 112 cab (photo on front). Page 2 shows specifications of both and toward the bottom are the 112 specs. listing 16 ohm' date=' Eminence lady Luck speaker. The back page has a photo of the backs and while small you can read 16 ohms on the backplate of the 112 cab. A tip also worth following is be sure to use good speaker cable not guitar cable to connect the head/cab. Using guitar cable can result in "the appearance of a connection" when in fact there isn't one and whooops there goes another OT plant. Hit every BLUE NOTE baaaby..., I'm going to play on:-"
aphawki Posted February 1, 2009 Author Posted February 1, 2009 Well my lack of the correct packaging doesn't really surprise me on this one...I just had the warranty paperwork and that was it. Nothing but nice! I am just glad I didn't break anything! It's all sounding good and really only played it hooked up incorrectly for about 5 minutes or so...hoping that I didn't break anything.... However....I will admit that I am now feeling like a bit of a dumba*#.....I was so excited once I finally got my speaker wire (thank you for the heads up by the way...fortunately got that one right!) and missing power cable... I just plugged into the 4 ohm thinking lower=safe...oops...but when I looked at the backside of the cab there it was...a nice little sticker on the plate saying 16ohm. Well that was my one dumb move. Again hoping that I didn't break anything in there....but it all sounds really good and not breaking up any earlier than what I have read they are supposed to.
cGil Posted February 2, 2009 Posted February 2, 2009 Don't worry about it. The OT probably only got warm and the tube still works, so it survived and now you know what to do. All's well. But be sure to stock up on el84's. They burn out just like lightbulbs, except they'll go right when you want to play. You keep spare light bulbs around, right? You'll need spare tubes, too. It's the price of real tone; but then, real tone is easily worth the trouble of swapping tubes occasionally, which is really no trouble at all. Gil...
aphawki Posted February 2, 2009 Author Posted February 2, 2009 Will do! Thanks for all the help Gil...appreciate it!
Robo Posted February 2, 2009 Posted February 2, 2009 I never got a manual for my Special or VJr combo, just warranty papers. I wonder what the deal is? Manuals only come with the stack maybe?
aphawki Posted February 2, 2009 Author Posted February 2, 2009 I bought the stack and only got the warranty information. Kinda funny. I was talking with a guy from Guitar Center when I went back in to get my missing power cord...and he said they had just got a drum kit in that they were going to display and it had a bag of fries from McDonald's in it. So if someone could talk to Epiphone and let them know that if they are going to leave the power cord out the least they could do is leave a Big Mac in there for me.
Recommended Posts
Archived
This topic is now archived and is closed to further replies.