Ken
Well-Known Member
I still disagree. How can you say a higher resistance level is a safer mismatch when an open load, meaing infinite resistance, will blow the OT in short order?
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OHMS mismatch mishap...
- Thread starter mountain2012
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I still disagree. How can you say a higher resistance level is a safer mismatch when an open load, meaing infinite resistance, will blow the OT in short order?
nagha
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Better to resurrect than ask again, right?
So, for the past year, about once a week, JMD-100 has been plugged into a Marshall 412A 8 ohm cab via the 16 ohm output from the amp. Each session has been 2-3 hours. Other than my amp never quite sounding right (always sounded better on my 1960A with proper 16->16 ohm connection), I've not noticed any problems.
It only occurred to me last night to check to see what resistance these 412A Marshall cabs I've been plugging in for the past year.
Is my amp doomed? My understanding of reading this thread is that if it hasn't died, it should be OK. Correct?
Thanks and pardon my stupidity.
So, for the past year, about once a week, JMD-100 has been plugged into a Marshall 412A 8 ohm cab via the 16 ohm output from the amp. Each session has been 2-3 hours. Other than my amp never quite sounding right (always sounded better on my 1960A with proper 16->16 ohm connection), I've not noticed any problems.
It only occurred to me last night to check to see what resistance these 412A Marshall cabs I've been plugging in for the past year.
Is my amp doomed? My understanding of reading this thread is that if it hasn't died, it should be OK. Correct?
Thanks and pardon my stupidity.
Holy necropost Batman!
What you have done is doom us all.
Your negligence has initiated a singularity.
As I said in the DSL40c thread, we only have days to live...
That is, until you rectify the situation and return the amp to the proper setting.
Once that is complete, humanity may survive...
What you have done is doom us all.
Your negligence has initiated a singularity.
As I said in the DSL40c thread, we only have days to live...
That is, until you rectify the situation and return the amp to the proper setting.
Once that is complete, humanity may survive...
ampmadscientist
Well-Known Member
8 ohm amp into 16 ohm load = survivable
16 ohm amp into 8 ohm load = not good, not good at all
because it makes the output draw twice as much current.
When you play the amp wide open, it will start blow'in.
No harm done this time, because you didn't crank it up loud.
Next time be sure to set it correctly.
And remember that mismatches will cause damage.
chuckharmonjr
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If it still works and sounds fine in the 60A at 16 ohms, I would say you got very lucky indeed my friend. That would be an awfully expensive repair had it gone nuclear on you. ALWAYS carry a cheap ohmmeter with you and check the cab before you plug into it.
ampmadscientist
Well-Known Member
It does tend to leave a pretty big crater.
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Jezclayton
Member
OK so my JMP 50 Combi has original 16 ohm speakers wired in parallel. These speakers are now nearly 40 years old. Wired in parallel I would expect the impedance to read 8 ohms but is actually 6.5 ohms. Do I set the amp output at 4 ohms or 8 ohms?
Like Roadburn and Ken my understanding was that you are better to have a slightly lower load and I would therefore set the amp at 8 ohms.
Like Roadburn and Ken my understanding was that you are better to have a slightly lower load and I would therefore set the amp at 8 ohms.
Ken
Well-Known Member
8 ohms because you're measuring DC resistance, not Ohms. An 8 Ohm load will read around 6.5 on your meter.
Ken
Jezclayton
Member
Perfect reply and thank you. There I was expecting someone to tell me my speakers were about to crap out as if I haven't had enough go wrong recently.
More worrying is that I believe you over my (ex) amp tech who insisted on returning the amp (JMP 50 Combi) from its last service set to 4 ohms even though it had been on 8 ohms for the previous 35 years with no ill effect.
I now understand the rough and ready way to measure the nominal impedance of a speaker is to measure the DC resistance and multiply it by 1.3.
FWIW, I ran a PA20, with 16ohm taps, into a 8ohm load for a fair while - not necessarily cranked, but loud. No problems.
100W JMP MV2203 hard wired to 8ohm, 2 tubes pulled, into 16 ohm cab. Cranked often. No problems.
YMMV, but if the Ceriatone is still working and sounds ok with correct impedance now selected, I'd stop worrying about it. Which is easy for me to say, since I didn't plug my new amp into the wrong load (!). But seriously, its probably fine.
As others have said, triple check your connections before flipping the switch, and label stuff (cabs) if necessary.
100W JMP MV2203 hard wired to 8ohm, 2 tubes pulled, into 16 ohm cab. Cranked often. No problems.
YMMV, but if the Ceriatone is still working and sounds ok with correct impedance now selected, I'd stop worrying about it. Which is easy for me to say, since I didn't plug my new amp into the wrong load (!). But seriously, its probably fine.
As others have said, triple check your connections before flipping the switch, and label stuff (cabs) if necessary.
ampmadscientist
Well-Known Member
"100W JMP MV2203 hard wired to 8ohm, 2 tubes pulled, into 16 ohm cab. Cranked often. No problems."
No problems because you did it correctly!
There is no mismatch.
Set amp on 8 ohms, two tubes pulled. Then connect to 4 ohm load.
Then it "will" blow chunks.
That is the incorrect method.
No problems because you did it correctly!
There is no mismatch.
Set amp on 8 ohms, two tubes pulled. Then connect to 4 ohm load.
Then it "will" blow chunks.
That is the incorrect method.
yup - Tube amp book vol III to thank for that tip when I got a Marmac 4x12 to replace use of the 8ohm speaker in a Celestion sidewinder combo!
PA20 was the naughty one....although now with correctly wired 8 & 16 ohm taps.
PA20 was the naughty one....although now with correctly wired 8 & 16 ohm taps.
When I had the builders in last summer and we were very pushed for space I took my DSL100 to rehearsal to stretch it's legs. I decided to check the bias as I hadn't used it for a couple of months. Plugged the lead from a cab into the amp and proceeded as normal. The bias would not settle so checked the lead and it wasn't plugged into the cab as I normally leave it. Amp still works fine....so far 
Ken
Well-Known Member
DC resistance and AC impedance are both measured in ohms.
Good point! Oh well...I get only half a point then dammit!
Ken
