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Post subject: Advice for Ext. Cab ///// NEW Question on this/ Pg. 2
Posted: Sat Sep 04, 2010 8:07 am
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I just got a Blues Jr NOS and it has a jack for the Jenson speaker(1/4 inch)
which can also be used for a ext. cab. I know I have to stay with a 8 ohm
config. So my ? is 2x12 or 4x12 cab, anyone else done this? or can I go with a 16 ohm 4x12 which you can run as a 8 ohm stereo? Not alot of experience with all this. Thanks all

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Last edited by newwt on Wed Sep 08, 2010 4:26 pm, edited 1 time in total.

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Post subject: Ext cab
Posted: Sat Sep 04, 2010 10:37 am
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Hi newwt,

As long as it is 8 ohms, you can use whatever cabinet that you want. Different brand speaker cabinets can have different ohms. When you say a typical 2x12, if it's 2 speakers at 8 ohms each, that's 4 ohms for the cabinet, if it is 2 speakers at 16 ohms each, that would be 8 ohms for the cabinet. It's the same with a 4x12 cabinets, 4 speakers at 8 ohms each would be 8 ohms for the cabinet, while 4 speakers at 16 ohms each would be 16 ohms for the cabinet. And the 4x12 can even be wired different internally to give different ohms. So, any 8 ohm speaker cabinet, or half a cabinet will work. I'd like to hear the BJr plugged into a 4x12, just to see what it would sound like. :)

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Posted: Sun Sep 05, 2010 8:48 am
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shimmilou,
Thanks for the reply, at least now I have enough info to be dangerious.

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Post subject: Ext Cab
Posted: Sun Sep 05, 2010 9:45 am
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Thanks newwt. The ohms value of the cabinet is a "load" on the amp. The smaller the ohms of the speaker, the greater the load on the amp. The ohms value rating is really called an impedance as the output of the amp to the speaker is is AC. The DC resistance of the speaker can be measured with an ohm meter and will be close to the rated impedance in ohms. Really, you can use 16 ohm cabinets as well with the BJr. It will sound different, maybe better than an 8 ohm, but the amp won't be as efficient, and volume levels will be different. You can even run a 4 ohm cabinet, but that's not recommended as your amp will reach full output at a much lower volume than it will with an 8 ohm, and the amp may overheat if cranked up too much. It's just best practice to match the ohms of the speaker and amp for best efficiency. I have a Champion 600 that is a 4 ohm amp and I have used it with an 8 ohm cabinet and a 16 ohm cabinet. It sounds great (better) with the 8 ohm cabinet, and not bad with the 16 ohm cabinet. I wouldn't want to try the C600 with a 2 ohm cabinet though, too much load. :)

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Posted: Sun Sep 05, 2010 12:00 pm
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Thanks again that gives me a better idea of what you ment.
So basically for optimal performance sticking with a cab rated at 8 ohm
would be my best bet overall then.

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Post subject: Re: Ext Cab
Posted: Sun Sep 05, 2010 1:25 pm
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shimmilou wrote:
The ohms value rating is really called an impedance as the output of the amp to the speaker is is AC.


WTF?!?!?

Sorry shimmilou but it's called "impedance" because strictly speaking, such a measurement is the aggregate resistive value of an inductive device (ie: the static impedance of a coil). And the DCR value of a speaker is generally an estimation of its dynamic resistance at a given frequency, usually -- although not always -- at around 400 Hz.

Arjay

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Post subject: Speaker impedance
Posted: Sun Sep 05, 2010 2:53 pm
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Hi Retroverbial,

The impedance of a speaker or other coil is determined by 1 divided by [2(pi)FL] in an AC circuit, where F is frequency and L is the inductance of the coil in the speaker. Yes, the output of an amp is AC, and the impedance is what is estimated, because the F changes many times in an AC signal from the amp, therefore the impedance of the speaker changes with the frequency through it (imedance usually figured using 400Hz or 1000Hz). The DC resistance is what is actually measured using an ohm meter. If you put an ohm meter on an 8 ohm speaker coil, and measure it's DC resistance, you may find it at 6.9 ohms or around 8 ohms depending on the construction of the coil. Hey, I can't sing, I ain't pretty and my legs are thin, but sometimes electronics is all I have. :D

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Posted: Sun Sep 05, 2010 3:44 pm
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An hour and a half to look all that up?

I score this response as "epic fail".

:mrgreen:

Arjay

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Post subject: Ext cab
Posted: Sun Sep 05, 2010 3:56 pm
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:lol: Not sure what you mean Retroverbial, but it doesn't sound nice. :? Yeah, I didn't have to look up anything, I got A's in class while I got my electronics degree, and so far, my memory hasn't completely failed. :D Everything that I said is just electronic fact, no opinion there. The hour and a half was used to run up to GC and buy another Champion 600, as I went a little nuts and modified my first one and they are on sale again this weekend. Just got through playing the new one (unmodified), and it does sound good out of the box. :D

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Post subject:
Posted: Sun Sep 05, 2010 4:00 pm
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You gonna mod the new one as well?

Arjay

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Post subject: C600
Posted: Sun Sep 05, 2010 4:47 pm
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The only thing that I want to do to the new one is to try a different grill cloth. When I had the other one apart, I had to remove the grill cloth to trim the baffle board edge by about 3/4 inch to install the Weber speaker. I found that I could barely blow a little air through the stock cloth, it's restricting the sound of the amp, it's like polyester on the back. The grill cloth reminds me of the material on a toy drum that I had when I was kid. The grill cloth may also be part of the reason that it sounds good, by good I mean in it's own warm muddy sort of sound. I used some Burlap for the new cloth, I think that it looks pretty good, and definitely lets the speaker sound through very well. I had to move the Weber speaker over to be able to get bigger output tubes in, if I want to change later. The stock Chinese tube and the Electro Harmonix tubes both fit with the new speaker in the same place as the old one, but it was a close fit and the JJ tube or a 6L6 wouldn't fit without moving the speaker over. I used the Signature 6 Alnico with the fully ribbed cone for late breakup. Man, the amp is really a different animal now! I love the new, louder, tighter, great sound of the modified amp, but I lost the muddy warm sound that the amp had to begin with, soooooo.....I got another. :D

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Here is a comparison pic.

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Posted: Sun Sep 05, 2010 5:15 pm
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Shimmy, JOC have you compared the RI Champion 600 to an original? I have a 1949 Champion 600 (as well as an 800). Maybe it's the 6SJ7 pentode driver instead of a 12AX7 or the Triad iron, but the 1949 sounds quite a bit different than the 3-4 RI's that I've tried. Even, ones with Mercury iron, changed-out coupling caps, & NOS tubes.

The old Fender Champions are a trip. There's no circuit board. All the caps, resistors, etc are soldered right to the chassis. Despite this, the hum isn't too bad. The tone is killer brown. Iwas lucky enough to scoop up some Weber 6-inch big ceramics before he stopped advertising them. It just fits into the cabinet.

Anyhow, if you get a chance...


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Post subject: C600 comparison
Posted: Sun Sep 05, 2010 5:34 pm
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I have only looked at pictures of the late-40s and early-50s Champion 600s, but haven't had the thrill of playing an original. The new ones look very much like the old ones. The new ones are circuit board though, and probably cheaper components than the originals had. :( What is the grill cloth like on your original? The cloth surely is different, but I bet that the sound difference is partly what you said, different tubes, and partly that the new ones have a solid-state rectifier instead of the old tube rectifier. The tube rectifier in the originals affected the sound somewhat, gave it that "sag". And, really, the new grill cloth is weird, sound restricting/muffling.

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Post subject:
Posted: Sun Sep 05, 2010 6:02 pm
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Shimmy, grille is solid brown cloth. And the tolex covering seems thinner than the RI. Running a metal RCA 6SJ7, gray glass RCA 6V6GT, and Sylvania JAN-CHS VT-197A/5Y3GT. I agree that the rectifier does make a big difference. I kinda like the tone of the octal 6SJ7 over the 12AX7 used in later model Champions. And the RI. At least in this amp.


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Post subject:
Posted: Sun Sep 05, 2010 9:15 pm
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Leaving one of them in stock trim (at least for the moment) will give you a "bench-mark" amp which you can use to effectively compare your mods and upgrades on the hot-rodded unit to. Smart idea.

Arjay

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