, rsadec--(at)--tech.com says...
>
>>I have a Blackface Super Reverb (64-65) and a Blackface Vibrolux
>Reverb (64-65). Both of these amplifiers have extension speaker
>jacks. I have a Laney 4x12 speaker cab (16 ohms). I was wondering if
>I will hurt my amplifiers buy using this speaker cab. with them.
>>And... does anyone know what Fender recommended for extension cab
>impedance
>>for each one of these amplifiers.
>>
>>Thanks
>>Rick
Super Reverb amps had 2-ohm output transformers to accomodate the four
8-ohm speakers wired in parallel. The recommended total impedance is 2
ohms, which doesn't let you plug any extension speaker in without
mismatching the recommended load. Vibrolux Reverb amps had 4-ohm output
transformers to accomodate the two 8-ohm speakers wired in parallel.
Here, the recommended total impedance is 4 ohms, again leaving no
allowance for extension speakers.
One word of warning: Be very careful about using the extension speaker
jack. Impedance mismatches can damage your amp. Also, Fender amps use
shorting jacks on the main speaker output. This means that if you plug
a cabinet into the extension jack with nothing in the main jack the
output transformer will be shorted. While this is better than no load
connected (or a sharp stick in the eye), it may trick you into turning
the volume up all the way and driving the amp hard into a short - not
good. I have also seen these shorting jacks get "sprung", where the
shorting contact gets bent and the output is left open when you unplug
the speakers. If you only use one jack, be sure to use the main
speaker out.
Now, to your question about the Laney 4x12 cabinet: You have several
choices here.
First the Super Reverb: There is no good way to use this cabinet as an
extension with your Super Reverb's internal speakers and properly match
impedances. There are two possible ways that will cause only a small
impedance mismatch: One is two just plug it directly in to the
extension jack, which would give you a total load of 2 ohms in parallel
with 16 ohms, which is about 1.8 ohms. The other would be to rewire the
Laney as a 4-ohm cabinet (all 4 speakers in parallel, assuming each
speaker is a 16 ohm speaker) and rewire your Super Reverb speakers for
an 8-ohm load (two speakers in series in parallel with the other two
speakers in series). This would give you a total impedance of 8 ohms in
parallel with 4 ohms, which equals 2.7 ohms.
I doubt that these small impedance mismatches will harm your amp, in
fact, in his book "A Desktop Reference of Hip Vintage Guitar Amps",
Gerald Weber claims Fender amps are very tolerant to impedance
mismatches as long as they aren't more than 100% off! He's probably
seen a hell of a lot more Fender amps than I have, so.....
In either case, the cabinets will not be equal volumes due to the
impedance difference as well as the speaker efficiency differences. In
the case of the second method, you would have to always run both
cabinets to maintain the proper load. You could not just unplug the
extension and use the Super by itself for practice or smaller clubs.
A better approach would be to leave the Super Reverb speaker wiring
alone, and use TWO external 4x12 cabinets, each wired for 4 ohms.
Unplug the Super's speakers and plug one 4x12 cabinet into the main
jack and the other into the extension jack. This will give you the
desired 2-ohm load, and allow you to unplug both and plug the internal
speakers back in if desired. (Not to mention the added benefit of two
very cool-looking 4x12 cabinets on stage with you! :)
Next the Vibrolux Reverb: Same problem, different impedances. Here if
you plug the Laney into the extension speaker jack you will be loading
the amp with 4 ohms in parallel with 16 ohms, which is 3.2 ohms.
Although this may not be too hard on the amp, again there will be a
large volume difference between the internal speakers and the external
cabinet. Since the Vibrolux wants to see a 4 ohm load, you could
rewire the Laney as mentioned above for a 4 ohm load and unplug the
internal speakers and use just the Laney cabinet. Remember to plug it
only into the main speaker jack, and be sure to plug the internal
speakers back in when you unplug the Laney, in case you forget and turn
the amp on with no speakers connected (don't laugh - it happens!).
I hope this helps you. Sure would have been nice if Fender had provided
an impedance selector like the one on back of a Marshall....!
Randy
reaike--(at)--x.netcom.com
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