I have a 1974 Marshall 50 watt amp. Just the head...So I have been running it through my Fender Blackface Cabinet W/ 2x12 Celestion 8 ohm speakers...Anyway...I've been selecting 4 ohms on the back of the Marshall Amp...Is that cool or am I damaging the transformer...
Ohm's Question
- Thread starter Fattriga
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Ken
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OMG...match the amp to the load or smoke will follow. If your amp doesn't match your cab, buy another cab. A 4x12 1960 cab can be had for $300 in good shape. A new OT and labor is about the same...and you won't have your amp for a month. And you'll have a replaced OT along with whatever collateral damage to the components happened.
Ken
it depends how those 8 ohm speakers are wired...
mickeydg5
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If the two 8 ohm speakers are wired in parallel for a 4 ohm cabinet then selecting 4 ohm impedance at the amplifier output is correct.
Ken
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Good point. What's the impedance of the cabinet? If you don't know, invest $30 in a cheap meter and find out. 8 ohms will read about 6 and 16 ohms will read about 13-14.
Ken
jross68052
Active Member
The other thing you might do is open the cab and look at the wiring. If the wiring is in series, it will go from the jack to the positive lead of one speaker, to the negative lead of the other speaker, then from the positive lead back to the jack. This will make the cab 16 ohms. If it's wired in series, the two positive leads on the two speakers will be connected and the two negative leads will be connected. This would be a 4 ohm set up.
chuckharmonjr
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Well....then there is always the old reliable smoke test
Ken
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Yes! Light up some smoke and pretend understanding impedance matching doesn't matter. If the smokey taste is piney you're fine, but if it tastes like burning wires you probably have a problem.
Ken
chuckharmonjr
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but its such a pretty blue-white
chuckharmonjr
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Simply put it all boils down to one of three things. Invest 20 bucks in a cheap radio shack ohmeter, invest 10 minutes and a philips head screwdriver for a visual or do nothing and risk a $350+ transformer job. Your choice.
NO you're not... 2x 8 ohm in series will become 16 ohm...
royslead
Well-Known Member
Two 8 Ohm speakers in series is a 16 Ohm load. You are using the 4 Ohm setting. It is not matched, but will not do long term damage because the actual load is greater. Conversely, if you had been using a 16 Ohm setting and the speakers were wired in parallel, giving a 4 Ohm load; damage could occur. If you have the ability to math the impedences, do so.
jack daniels
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Fender cabs w/2x12"s of that era are typically wired to four ohms, so it sounds like someone wired it for a British head @ 16 Ohms.
jross68052
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Correct.Thanks
Penguinchit
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Sorry for the transparent background...
