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Post subject: Showman head/bassman cab combination
Posted: Mon Jun 21, 2010 6:49 am
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Hey,

I have recently bought a '66 Fender Showman head (85 watts, 8 ohms), and am thinking of buying a 2x15 '74 bassman cab (50 watts, 4 ohms). I've found a lot on the internet about matching up cabs and heads, but I'm still unclear about the impedance and wattage issues (if there are any). Any help would be much appreciated.

Ben


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Posted: Mon Jun 21, 2010 10:16 am
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Are you certain that your output tranny is 8Ω?

The Fender part number for the 8Ω blackface output tranny is 125A29A (the 4Ω tranny is marked 125A30A).

Were this my rig, I'd look for a pair of 75-watt drivers (JBL D130Fs would be a most excellent choice) and wire them up in parallel to suit your O/T's secondary winding. If you confirm that you need 8Ω total then look for 16Ω speakers.

The Bassman cab -- if it's the large 30" x 40" x 11½" version -- will have a rectangular port in the baffle which you may wish to blank off to make the enclosure a little more "guitar-friendly". This port was added when the Bassman acquired the big box in 1967 to decrease the resonant frequency for use with bass instruments. Make no permanent changes though.

Best of luck, HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Posted: Mon Jun 21, 2010 2:38 pm
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Hi,

Thanks for that, very helpful and interesting stuff. As I'm sure you've realised - I know very little about the electronics side of things.

Had a look for the part number you mentioned, but couldn't find an exact match. Here are all the references I could find on the different components:

Power supply choke: '125C1A' and '606-6-02'

Output transformer: '022756' and '606607'

Interstage transformer: '022897' and '606610'

Circuit AB763.

Everything seems to line up to the second Showman down on this list I found - https://taweber.powweb.com/store/xfmrcr.htm so yes I'm fairly sure it is 8 ohms. So with regard to the cab - is it worth going for (safe?) or should I wait until an 8 ohm is available for a proper match?

Thanks again!


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Posted: Mon Jun 21, 2010 5:03 pm
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That's an interesting chart -- unfortunately they've transposed the P/Ns for the 4Ω and 8Ω output trannies.

And the tranny you refer to as an "interstage" is in fact the O/T. And that FMIC P/N is consistent with an 8Ω winding (it's the same as mine in fact).

You can run a 4Ω cab with this amp but I wouldn't recommend "standing on it" too hard or for too long. It really needs an 8Ω load for proper safe operation. A replacement tranny is very expensive if you blow yours up -- rewinding an original to vintage spec, doubly so.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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