impedence of a blackface twin reverb

mlynn02

Silver Supporting Member
Messages
1,178
i've got on old blackface twin that's about to get some new speakers. but, i don't know what the impedence of the amp is. can someone help? it's a 2x12, of course, so what ohm speakers should i buy? also, how many ohms should the ext. out be?

thanks,
matt
 

mlynn02

Silver Supporting Member
Messages
1,178
so each speaker should be 8 ohm, because the line out from the amp and the ext. out are each 4 ohm?

did i understand that right?

thanks
 

KCblues

Member
Messages
217
That's correct, 8 ohms each in parallel for 4 ohms total is standard for a Twin Reverb.
 

hasserl

Member
Messages
4,708
Originally posted by mlynn02
so each speaker should be 8 ohm, because the line out from the amp and the ext. out are each 4 ohm?

did i understand that right?

thanks
Not quite, each of the internal speakers are 8 ohms each for a 4 ohm load at the internal speaker jack.

But if you want to plug into the ext jack you need to juggle the speakers so there is still a 4 ohm oad on the amp. The output transformer only has one tap, and it wants to see a 4 ohm oad. So whatever combination of speakers you use should add up to 4 ohms.

If you put a 4 ohm load into both the internal speaker jack and the ext speaker jack at the same time that would make for a 2 ohm load total, not the best arrangement, but you can probably get away with it.
 

John Phillips

Member
Messages
13,038
You want two 8-ohm speakers for the amp, wired in parallel for 4 ohms and connected to the Main jack.

You can then run any 4, 8 or 16-ohm cabinet from the Ext jack, at the same time.

All those Fender amps are designed to handle a low mismatch down to half the matching impedance with no problem at all.

The only thing not to do is run a 16-ohm cab as the only load (ie with the internal speakers disconnected). Even then you probably won't kill it, but it's a bad idea.


One major reason old Fenders are so reliable is because Leo understood that musicians didn't pay too much attention to correct matching, so he did it for them by making the internal speakers (how the amp was normally expected to be used) the right load, and then making sure the amp would handle any normal mismatch too. There's even a shorting switch in the Main jack so if you forget to plug the speakers back in, the amp doesn't run with no load.
 

mlynn02

Silver Supporting Member
Messages
1,178
here's to leo! glad to learn it's so easy afterall. thanks for the info guys!

-matt
 

AdamGian

Member
Messages
488
Originally posted by John Phillips
You want two 8-ohm speakers for the amp, wired in parallel for 4 ohms and connected to the Main jack.

You can then run any 4, 8 or 16-ohm cabinet from the Ext jack, at the same time.

All those Fender amps are designed to handle a low mismatch down to half the matching impedance with no problem at all.

The only thing not to do is run a 16-ohm cab as the only load (ie with the internal speakers disconnected). Even then you probably won't kill it, but it's a bad idea.


One major reason old Fenders are so reliable is because Leo understood that musicians didn't pay too much attention to correct matching, so he did it for them by making the internal speakers (how the amp was normally expected to be used) the right load, and then making sure the amp would handle any normal mismatch too. There's even a shorting switch in the Main jack so if you forget to plug the speakers back in, the amp doesn't run with no load.
I wish I had a nickle for every time John has provided this answer to this question - I'd have two bucks :D
 

TheAmpNerd

Member
Messages
1,056
Originally posted by HassanBinSober
I wish I had a nickle for every time John has provided this answer to this question - I'd have two bucks :D

Adam,

Good thing you aren't greedy 'cause a C-note will yield ya
four grand!
 




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