SMF Head

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SMF Head

Postby midlife on Sun Jan 26, 2014 1:11 pm

Good Morning! This is my first posting.
I have a 1974 SMF tour head. I am familiar with ohms rating and fully understand it. But, with the SMF head it requires one cabinet per output group in order to work.You can load up to four cabinets on this head. I know they have created an open in the circuit to protect the head from imbalance. My question is: does the 4, 8 and 16 ohm settings represent the total load for the two cabinets (minimum) required or is the ohm output setting per group? i.e. with two 4 ohm cabinets can I set the plug at 4 ohms and not hurt the output transformer? My tech is not quite sure, because this head was designed for touring and it might have been laid out so that the ohms rating was on the conservative side. (in case a roadie plugged in the wrong cabinets) No offense to roadies, I was one.
I have a pair of 215 cabinets, each rated at 4 ohms.
I am looking for a valid answer, not speculation. I can can do my own speculation.
Thank you in advance to ANYONE who can answer this.
John (midlife crisis)
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Re: SMF Head

Postby Papa Dog on Wed Jan 29, 2014 7:09 pm

midlife wrote:Good Morning! This is my first posting.
I have a 1974 SMF tour head. I am familiar with ohms rating and fully understand it. But, with the SMF head it requires one cabinet per output group in order to work.You can load up to four cabinets on this head. I know they have created an open in the circuit to protect the head from imbalance. My question is: does the 4, 8 and 16 ohm settings represent the total load for the two cabinets (minimum) required or is the ohm output setting per group? i.e. with two 4 ohm cabinets can I set the plug at 4 ohms and not hurt the output transformer? My tech is not quite sure, because this head was designed for touring and it might have been laid out so that the ohms rating was on the conservative side. (in case a roadie plugged in the wrong cabinets) No offense to roadies, I was one.
I have a pair of 215 cabinets, each rated at 4 ohms.
I am looking for a valid answer, not speculation. I can can do my own speculation.
Thank you in advance to ANYONE who can answer this.
John (midlife crisis)


Hello Midlife,

Welcome to the forum.

The ohms rating on that head is for the combined of all cabs plugged in.
To run your cabs you either have to run just one or else re-wire them in series (internally) to 16 ohms each and then run them both in parralel. The set the impedence switch to 8 ohms.
That head does not create anbd open, bvut rather a short on both groups.. Plugging into either jack in a group breaks the short.
There is an easy mod to "fix" that. JUst have your tech remove the jumpers on the jacks and then all four outputs are in parrallel. But, I would not recomend running it with only one cab unless you have a 300 watt or better cab.
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Re: SMF Head

Postby midlife on Thu Jan 30, 2014 12:38 am

Thanks Papa Dog, I appreciate this. Also, thank you for the open vs. short comment. I'm always "open" to learn more about amps. haha
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Re: SMF Head

Postby superdrag29 on Wed Mar 05, 2014 3:37 pm

Hello there!

I purchased the SMF cabs from our friend from the post above yours. I saw that you mentioned selling your SMF head? I'm interested! Please give me a shout. Thanks, Jake
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