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Post subject: Extension Cab
Posted: Tue May 04, 2010 9:35 pm
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I have a '68 Bandmaster with original 4 ohm cab. Now here's the question. What impedance cab is safe to plug into the "Ext. Speaker" slot while the 4 ohm cab is still plugged in? Does plugging in an additional 4 ohm cab actually drop the impedance to 2 ohms on this amp?


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Posted: Tue May 04, 2010 10:30 pm
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Your Bandmaster's output jacks are wired in parallel and connected to a single 4Ω tap on the output tranny. Adding a second 4Ω cab drops the total load that the amp sees to 2Ω -- not really recommended, especially for prolonged periods with the amp driven hard. If you add an 8Ω cab to the extension jack (assuming a standard 4Ω load connected to the main jack) that brings the impedance total down to 3Ω, which is much more preferable and forgiving than the 2Ω mismatch.

HTH

Arjay


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Posted: Wed May 05, 2010 10:54 pm
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Yeah Doowop, Arjay's advise about using the 8ohm ext cab would work just fine, for your B Master. The mismatch would be less than 100%. Actually the nominal impedance load for three 8ohm speakers in parallel would 2.67ohms. But speakers are measured by the next highest common number. In your case the OT will have to drive the speakers with more current because of the lower impedance. The primary side will try to supply this, or try to conform to the secondary demand. While the Fender OT's are very tolerant, the signal may be somewhat compressed because the transference is not as efficent. We never talk about it, but any change in the OT will cause a change in the frequency response of the power amp section. It's a matter of the output efficiently following the input signal. Art

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Posted: Thu May 06, 2010 12:18 am
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Right you are, Art. Years ago I usta push my '65 Bassman into its 2 x 12 cab along with a Showman 15 "tone-ring" enclosure (a pair of JBL D120Fs plus an EV-SRO). I beat that thing like a rented mule for over three years and it never failed, night after night. Of course those were the days when Schumacher iron was built like the proverbial Sherman tank......

:mrgreen:

Arjay


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Post subject: Re: Extension Cab
Posted: Wed May 12, 2010 7:46 am
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doowop14 wrote:
I have a '68 Bandmaster with original 4 ohm cab. Now here's the question. What impedance cab is safe to plug into the "Ext. Speaker" slot while the 4 ohm cab is still plugged in? Does plugging in an additional 4 ohm cab actually drop the impedance to 2 ohms on this amp?


One of Ted Weber's Z-Matcher devices would work fine :

https://taweber.powweb.com/store/zmatch.htm

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