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Topic: Ohms question -- Speaker jacks on Fender Bassman head |
Tommy Boswell
From:
Virginia, USA
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Posted 2 Feb 2012 4:55 am |
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My Fender Bassman 70 head has 2 speaker jacks labeled "Speaker" and "Ext Speaker". Then next line says "Minimum Loads". And the bottom line says "4 Ohm" under the Speaker jack, and also "4 Ohm" under the Ext Speaker jack.
So the label is inviting the poor uninformed user (that's me) to plug a 4 ohm speaker into each jack. But that would be a 2 ohm load if they are wired parallel. I'm afraid to try that. Is my amp really OK with a 2 ohm load? |
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Leland Ogle
From:
Baxter Springs, Kansas, USA
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Posted 2 Feb 2012 6:45 pm
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Tommy, I have a blackface and a silverface bassman. I use a 4ohm cab for each head. My tech guy said to use two 8 ohm cabs if you are going to use both jacks. This has worked well for me.
Lee |
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Tommy Boswell
From:
Virginia, USA
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Posted 2 Feb 2012 7:14 pm
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I agree, it makes sense to use 2 8-ohm cabinets.
What doesn't make sense is the labeling on the back of the amp. It says 4 ohms under each jack. I just think it's a little strange for Fender to label it that way, and then expect the owner to plug in 2 8-ohm cabinets and not 2 4-ohm cabinets.
Last edited by Tommy Boswell on 2 Feb 2012 7:18 pm; edited 1 time in total |
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Tim Marcus
From:
San Francisco, CA
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Posted 2 Feb 2012 7:16 pm
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It's not great, but fender amps have survived decades with people doing that. It's fine but if you can avoid it you should by using two 8ohm speakers.
_________________
Milkmansound.com |
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Leland Ogle
From:
Baxter Springs, Kansas, USA
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Posted 3 Feb 2012 2:01 pm
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I have a rack rig, but I like to use a bassman head with a profex and one justice cab with an eminence neo speaker. I don't need alot of power and this is lightweight.
Lee |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 3 Feb 2012 2:36 pm
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| Two 4-ohm cabinets is the design load for the output transformer, just as it is marked. With a typical solid state amp this would mean two cabinets in parallel for a net two-ohm load with a near doubling of total power, but with a tube output section such as yours the transformer usually has separate windings for each output jack, and the total power output remains the same regardless. |
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Tim Marcus
From:
San Francisco, CA
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Posted 3 Feb 2012 3:06 pm
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Fender never brought out separate windings for the Ext. Speaker Jack - adding a second cabinet drops the load.
_________________
Milkmansound.com |
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Chris Reesor
From:
British Columbia, Canada
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Posted 3 Feb 2012 6:48 pm Bassman 70
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I just checked the schematic for the Bassman 70 in Aspen Pittman's Tube Amp Book Vol. 3.
The output transformer secondary is tapped, and switching jacks select which OT tap is used.
So I would say, don't use cabs of less than 4 ohms, and you should be fine. Whether 8 ohm cabs or 4 ohm cabs would optimize your power output, is anyone's guess. A test with dummy loads, a signal generator, and a scope would answer that question.
Interesting circuit; ultralinear (screen taps} and 500V B+.... better have good 6L6's in that one!
Cheers, CR.
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Excel Superb U12, MIJ Squier tele, modified Deluxe Reverb RI, Cube 80XL, self built acoustics & mandolins |
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Tim Marcus
From:
San Francisco, CA
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Posted 4 Feb 2012 12:42 am
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Oops. Ignore my previous post. I missed the "70" there
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Milkmansound.com |
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Bob Carlucci
From:
Candor, New York, USA
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Posted 4 Feb 2012 4:37 am Re: Bassman 70
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Interesting circuit; ultralinear (screen taps} and 500V B+.... better have good 6L6's in that one!
[/quote]
Yes they do tend to "eat up" modern 6L6 tubes. Nice sounding amp though, and quite good for steel as well. I used one for several years, and sold it for double what I paid for it.. Used it for pedal steel, guitar and bass, and it did very well for them all... bob
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I'm over the hill and hittin'rocks on the way down!
no gear list for me.. you don't have the time...... |
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