# simple impendance calculation question

dymand68
April 24th, 2010, 04:09 PM
the back of my 72 DR has two jacks for speaker connections and reads "total load 8 ohms"; if i use the 8 ohm speaker in the combo and an 8 ohm extension cabinet, is my combined load 16, or 4 ohms?

Neems
April 24th, 2010, 05:31 PM
4 ohms in parallel. Here's a handy calc (http://www.bcae1.com/spkrmlti.htm).

tazzboy
April 24th, 2010, 05:37 PM
the back of my 72 DR has two jacks for speaker connections and reads "total load 8 ohms"; if i use the 8 ohm speaker in the combo and an 8 ohm extension cabinet, is my combined load 16, or 4 ohms?

it will be 4 ohms. You'll want to get 2 16 ohms speakers in order to get 8 ohms.

dymand68
April 24th, 2010, 05:51 PM
so use one or the other in this particular case...thanks
-in the case of a 16 ohm combo speaker, that would be okay run by itself?..then combined with the 8 ohm ext. cabinet the cumulative load would be....? 6 ohms? still too low for safe usage

limbe
April 24th, 2010, 05:57 PM
4 ohm

tazzboy
April 24th, 2010, 06:04 PM
wait a minute I want to make sure this correct.

No I am wrong you need to run two 8 ohms speakers which will make it 4 ohms

I just read the schematics http://www.ampwares.com/amp.asp?id=60

mr. lucky
April 24th, 2010, 06:22 PM
The 16 ohm cabinet by itself is not a good idea, there's some chance of frying the output transformer. Going higher than rated impedance is the wrong direction for tube amps (or actually any transformer coupled output amp).

If you want to calculate the parallel impedance, it's the inverse of the sum of the inverses of each speaker's impedance -- for 16||8 that's about 5.3 ohms. And that'd probably be OK, not as likely to damage the amp, but...

For the best power transfer, and the safest operation, just hook it up to the load it says on the back ;)

If you're thinking 'but I always heard you can hook up a higher impedance but not lower", that's for direct coupled solid state outputs. Don't do that with a tube amp.

dymand68
April 24th, 2010, 07:52 PM
alright mr.lucky....if i understand correctly; my 1972 Deluxe Reverb has two speaker input jacks, and it reads "total load 8 ohms/20 watts rms"
-with an 8 ohm speaker in the combo and plugged into the head, no extension cabinet can be used safely....the way to use both available speaker connections would be to...? use two 16 ohm loads?

dymand68
April 24th, 2010, 08:13 PM
@ tazzboy- it sounds like you are saying that each speaker jack can be connected to an 8 ohm load, and that's safe...yes? the link you provided shows that the factory speaker was 8 ohms, the total load is 8 ohms and there are two speaker connections...good link btw

mr. lucky
April 24th, 2010, 08:58 PM
yeah two 16 ohm loads in parallel (which is how those jacks are wired) will give you 8 ohms total load.

That's for that Deluxe -- some amps have separate taps on the output transformer optimized for different loads.

But a '72 Deluxe just has that one output on the transformer, which is connected in parallel to two jacks.

You could probably run a 4-ohm load without any damage, but the catch there is output power will be reduced. With only an 8 ohm tap on the transformer, only an 8 ohm total load will give optimum power out.

like I said, the best thing, for safety and performance, is to hook up the specified total load impedance.

The worst thing for safety and performance is to hook up a higher than specified load impedance.

The gray area for these amps is to hook up a lower than rated impedance (but no lower than half of rated) -- probably no damage, but less than optimum output power.

hope that helps

dymand68
April 24th, 2010, 11:03 PM
okay, then my question is, why did the stock on-board speaker have an impedance of 8 ohms, and they give you an extra space for an extension cab?...was their design intent to forego the combo speaker and use two extension loads; each at 16 ohms connected in parallel? that would be a peculiar design OEM

mr. lucky
April 25th, 2010, 12:26 PM
the stock on-board speaker is 8-ohms because that's the optimum load for the amp output.

The reason 8 ohms is the one and only optimum load, regardless of the two speaker jacks, is that Fender, for whatever reason, didn't choose to use an output transformer with multiple taps on the secondary.

There is no way to combine the internal speaker with an external one and still come out with 8 ohms, if that's what you're asking.

dymand68
April 25th, 2010, 02:17 PM
right, that is what i was asking...the "for whatever reason" factor is where i got hung up....they give you two jacks to either
a) replace the on-board speaker and use an extension cab
or
b) bypass the on-board speaker and use two extension cabs
and yes, you're right, i was surprised to hear you say don't use a load with more impedance i.e., (16 ohm load for 8 ohm head)...
i suddenly see my jcm800 head with adjustable impedance selector much differently! haha