Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #105249  by claytushaywood
 Tue Nov 15, 2011 2:31 pm
I've been looking into getting a "jerry" speaker for my jerry rig. I was gonna go ahead and get a new speaker knowing how much variation i've heard among vintage speakers of the same kind. But after hearing soooo many people on this board say nothing is like an original JBl, i've decided i would indeed like to get a real JBL. I am worried about buying a speaker off ebay though. i dont really know what to look for at all when looking at a picture of a speaker. does the ohmage matter? i've seen 8ohm speakers with people taking pictures of their measurements and listing measurements ranging from 6.2ohms to 8.3 ohms for an 8ohm rated speaker. does this matter?

can you all guide me in picking a good ebay auction to purchase an old jbl off of? maybe a sort of how to? any tell tale signs that I can see in a picture?
 #105272  by hippieguy1954
 Wed Nov 16, 2011 4:16 am
I had good luck just buying a couple of used E120's off ebay. They were fine with no problems. Just check out the sellers "feedback". Then I wanted brand new, so I did a little research and found the Beyma Liberty 8 which is a E120 clone. The Beyma Liberty 8 is manufactured to the exact spects on the E120. Brand new JBL E120's are not made anymore by JBL. That's why Beyma makes the Liberty 8.
As far as ohms go, 8 ohms usually works well with most amps, but you need to concider the amp you are going to use because when you wire two 8 ohm speakers in parallel (which I perfer over series) it becomes a 4 ohm load. So, your amp must be able to handle a 4 ohm load. Slight variations in the ohms of a speaker will not matter.
:smile: :smile: :smile:
 #105433  by claytushaywood
 Sat Nov 19, 2011 1:04 pm
yeh i know about wiring speakers and feedback. i was asking about variations in impedance among different individual speakers and if there is any way i can look at a picture of a speaker and tell how good of shape its in. as in an 8ohm speaker read at 6.4ohms versus 6.2ohms.

some speakers are lighter or darker in color than others, some have different looking cones, etc.
 #105436  by ugly rumor
 Sat Nov 19, 2011 2:38 pm
The range in ohms is very standard. 8 ohms often translates to anywhere from 5.5 to 8.5 or so, though usually in the 6-7 range. 4 ohms can be 2 or 5; that is why I consider a 4 ohm load to be amp abuse; 4 ohms is seldom 4 ohms. The less of a load you have, the greater the chance of the amp blowing, and to my ears the less warmth in the tone. Heat is the enemy of electrical components.
 #105437  by strumminsix
 Sat Nov 19, 2011 2:45 pm
ugly rumor wrote:The range in ohms is very standard. 8 ohms often translates to anywhere from 5.5 to 8.5 or so, though usually in the 6-7 range. 4 ohms can be 2 or 5; that is why I consider a 4 ohm load to be amp abuse; 4 ohms is seldom 4 ohms. The less of a load you have, the greater the chance of the amp blowing, and to my ears the less warmth in the tone. Heat is the enemy of electrical components.
Always a good idea to test the ohms on a cabinet with a multi-ohm meter!
 #105737  by TI4-1009
 Mon Nov 28, 2011 6:28 pm
I was just watching the Dale Erlewine DVD that came with my Guitar Player Repair Guide book. He shows a neat little tool that he uses to check the pickup resistance, but could be used for a speaker cab too. It's a 1/4" plug wired with two alligator clips off the two wires. If you plug it into the guitar jack and connect your ohm meter you can measure the resistance across the pickup to check if all the pickups are working. Should work if you plug it into a cab jack too, right?
 #107258  by Smolder
 Sun Jan 01, 2012 1:32 pm
Measuring resistance typically gives you a number that is 70-80% of the load. Typically if measured resistance is over 4, it's an 8 ohm. Over 8 and it's likely a 16 ohm. That said, not all transformers are wound for exact 4, 8, or 16 ohms... Some are wound at 6.5-6.9 so that either 4 or 8 ohm speakers can be used.
 #108001  by claytushaywood
 Tue Jan 17, 2012 11:52 am
I've heard that old fender amps can easily handle speakers one "notch" above the amp rating. like my 4ohm pro reverb could handle an 8ohm speaker load. is this true?

I've been looking at an old altec lansing 14b that says 8ohms on the speaker but measures 4.8ohms. seems like it shouldnt be a problem at all for my amp... what do you guys think? what about a typical JBL that measures around 6ohms... would that be too high for my 4ohm transformer?

Id really appreciate some insight on this. I've read a lot of stuff on it. But most of the guys are saying things like yeh its a cool sound to run an 8ohm speaker with a 4ohm fender... makes the tubes work harder so you can get more crunch at lower volumes. is that an accurate description of what will happen?

Thanks a ton in advance! I never really liked forums until I joined this one
 #108086  by Smolder
 Thu Jan 19, 2012 11:09 am
The presumption in bf/sf amps is that you would run a single speaker set at the appropriate load, but there is a second speaker jack. If you run an addition cab, it will be in parallel, effectively halfing the load on the output transformer. You could safely assume that the designers/engineers were aware of this. What you can also assume is that they really didn't think we'd ever be running these amps full out... so proceed with some confidence and some caution.
Last edited by Smolder on Fri Feb 03, 2012 10:05 am, edited 1 time in total.
 #108868  by claytushaywood
 Thu Feb 02, 2012 6:51 pm
Yeh I kinda realized that from the above post. so the amp must be able to handle a 2 ohm load if their gonna make a paralell speaker out.

But my question is... would i be able to run an 8ohm speaker with my 4ohm fender? most of the speakers I'm looking at that are 8ohms are actually 6.2ohms. would this be okay? would i lose a ton of headroom and volume? i was thinking i'd like to just run one jbl e120 out my 40 watt fender, keeping in mind kimock's one speaker 2 tubes theory, and the idea of being able to push an e120 at a decent volume, and be able to carry the amp while i still have it in combo form.
 #120734  by Vin-Tone
 Fri Nov 09, 2012 4:54 am
First, any speaker has a Impedance 4/8/16 ohm as well as a Resistance. The Impedance is a nominal or average number (actual impedance may vary quite considerably from the nominal figure with changes in frequency) Resistance is the static resistance of the voice coil as measured by a ohm meter (typical reading for a 8ohm speaker can very from under 5 ohms to over 7 ohms but should never be above the nominal of 8 ohms). So how does this affect the output of a tube amp? Too low impedance, for instance a 2 ohm speaker cabinet hooked up to an amp expecting 8 ohms, will overload the output tubes and the output transformer. The lower impedance will force lots of amperage through the tubes and out through the transformer. You would get a very loud amp for a short period of time. Something would blow. The tubes probably would go first, but you could also blow the output transformer or the speakers. It is possible that a small power transformer would not take the high amperage and blow up. Bottom Line, I personally would not use a lower impedance speaker because of the greater possibility of damaging the amp. As far as higher impedance speakers the two most common examples are 8 ohm speakers with a 4 ohm amp and 16 ohm speakers with a 8 ohm amp. My personal experience has been as follows. In both of these instances the higher impedance speaker did no damage what so ever to the amp. In fact in most cases they seemed to sound better. The amp was able to reach the Sweet Spot in it's power curve at a more reasonable volume. Now this has mostly been done with Vintage 16 ohm Alnoco Speakers and I am sure the lack of efficiency played a part it the results as well.
 #121637  by ugly rumor
 Fri Nov 30, 2012 10:22 pm
Excellent post, Vin-tone! Very good recap of what and why. Thanks!
 #121826  by Rick Turner
 Thu Dec 06, 2012 8:36 pm
A quick aside re. measuring guitar pickups DCR: You will not get a good reading at the output jack. The volume and tone controls are likely in the circuit and act as loads unless you have "no-load" pots. You'll be reading the pickup resistance with the added value of some number of pots in parallel even if the pots are turned all the way up. See this for a quick way to figure it

For a simple one pickup with volume and tone it might be:

pickup @ 12 K
Voume pot @ 500 K
Tone pot @ 500K

DCR at output jack with both pots full on around 11.45 K, an audible difference when then in parallel with the input impedance of an amp.