"DOes anyone know if its possible to measure ohms at the amp end - if I attach a meter to the + & - of the speaker itself will that give me the reading..."
Yes, that will be close enough IF you know the make and model of the drive unit.
A hi fi bass/mid driver will have a dc resistance of about 75% of the nominal impedance and that is virtually regardless of size, power rating or make.
Guitar speakers generally have a dc R slightly closer to nominal Z, 15's coming out at about 12Ohms but there are anomalies like the G12 EVH which is 15Ohms dc, unless that's a missprint!
But, as has been said, this is only a guide, great precision is neither needed nor possible. A good amplifier will tolerate some degree of error. ( I know we are stuck with it but I dislike the term "matching". It has a precise meaning in the RF and video/data world but nothing, not one thing, is "matched" in the audio world!)
RTFM, look at the amp plate, look at the speaker plate and do as you are told!
Dave.
however the safest way of running a valve amp is with a matched load, and this also produces maximum power.
THIS - is useful and, I think pretty much answers the question I have had for a while. All the technical speak was turning me cartwheels - I am, after all, a complete plank.
I have been GASing to buy a 12" loaded extension cabinet to connect to the 4ohm speaker out jack of my little Champ 600 (which would cut the internal 6" wasp fart speaker). My query was, and sort of still is, do I have to buy a 12" speaker rated at 4ohms (say a trayden true brit, which is available in 4R) or can I get an 8ohm model. I'd prefer the latter option as I may well end up buying a Vox Night Train or similar, which has 8R and 16R out jacks.
Cheers for all the column inches, though. Keeping out of trouble - ie off ebay
McToot the Bat
The easiest way to remember matching ohms is with people and cars:Originally Posted by mctoot
4 people can get in a 4 person car, an 8 seater mpv and a 16 seater minibus, however 16 people cant get in a 4 person car without causing damage....
Don't let somebody elses world make yours unhappy...
"The easiest way to remember matching ohms is with people and cars:
4 people can get in a 4 person car, an 8 seater mpv and a 16 seater minibus, however 16 people cant get in a 4 person car without causing damage...."
Now you're talking my language Dawg!! Fanks
To run the Champ 600 optimally you would need a 4 ohm load, although obviously, as you correctly point out, having a 4 ohm speaker would less use with other amps.
You may get away with running the Champ 600 into an 8 ohm load, however you may also compromise the output transformer. I suspect that the OT in the Champ isn't the most robust.....
Another option may be to change (or get someone else) to change the OT in the Champ so that you can match to say 8 or 16 ohms (or both!). There are plenty of people offering transformer upgrades that could do this for you, so have a look on the internet re pricing.
A final (cheaper) option is to buy an 8 ohm speaker and wire a high power (25W would be fine) 8 ohms or there abouts resistor in parallel with it. You would lose some power (which may be good) but will keep the recommended impedance.
To be honest, whilst I know that vintage Champs used 4 ohm speakers, it would have made much more sense designing the Champ 600 for an 8 or 16 ohm speaker.
Last edited by jpfamps; 4th February 2010 at 01:36 PM.
Thanks JPF - that's really useful and is much appreciated. You're going to hate me though - I've posted another question about the Champ... Sorry.
"To be honest, whilst I know that vintage Champs used 4 ohm speakers"
I suspect J that since domestic radios always had 3-4Ohm speakers someone came upon a shedload of transformers and possibly some (not so!) suitable speakers!
The shunt R idea is a good one. Four watts into a decent guitar speaker is still a fair old racket if the distaff side is about!
Dave.
I expect that's right.
The origins of the Champ predate go back to K&F which predates Fender, and I expect they just kept using 4 ohm speakers in the Champ model (into the 80s!!), although obviously by the mid 50's Fender weren't using of-the-shelf transformers and speakers, so could easily have made an 8 ohm Champ at no extra cost.
Would this also put the output valve under pressure? I sold a SF Champ last year and the buyer said it wasn't sounding good. He'd tried it with the stock speaker and all was good. After this, he'd hooked it up to a 4x12" cabinet running at 16 ohms. At first I thought he might have damaged the OT but it turned out to be the output valve that was giving the poor sound. Could a 4 ohm output going into 16 ohms worth of speakers put the valves under more strain?
"Could a 4 ohm output going into 16 ohms worth of speakers put the valves under more strain?"
It does neither valve or transformer any good Heartfelt. The actual failure scenario can play out several ways.
The basic problem is overvoltage at the valve(s) anode. Now how much over standard rating a particular valve can stand is in the lap of the gods. Valves with a really "hard" (i.e. good) vacuum will stand more than softer ones, note we are talking kilovolts here in many cases! Then, if a valve arcs over momentarily, no great harm might be done or, that arc might realease gas into the valve, that will ionize and like a neon street sign, the valves' resistance becomes bugger all. If you are lucky the HT fuse will blow before the op transformer goes open circuit.
If however you have nice hard valves they will put up with the volts but the tranny might not! That will arc internally, burn the interwinding insulation (often paper) and the result is a nice(!) conductive bit of carbon.
You must always remember that a guitar amplifier is often used well beyond any drive level that would be considered safe (for the kit) and reasonable for domestic or even professional amplification. You don't drive your Ka at and beyond the red line all the time but peeps expect gitamps to perform under similar abuse!
At LEAST give them a fighting chance and get the load about right!
Dave.
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