Speaker Question in Silverface Fender

Daniel Haymore · From: Nashville, TN

I have a late 70's silverface bassman ten. Based on what I have read, the 4 speakers are 32 ohm, wired in parallel for a 8 ohm load. Is that correct?

I am considering building a separate head and speaker cab and utilizing just 2 10's. Will it be ok/safe to run the amp into just two of the 4 speakers?

I will be saving the old cabinet so that it can be returned to stock, should I ever wish to.

Thanks,
Daniel

Tom Wolverton · From: Carpinteria, CA

It should work just fine. I believe that up until 1976, the Bassman 10 OT was for an 8 ohm load, but in 1977, when the amp became an ultralinear amp, the OT became 4 ohms. Just what I've read. You can check this if those speakers are the originals. Look at the speaker impedience and how they are wired. Does yours have the Middle control on the Bass channel?
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Daniel Haymore · From: Nashville, TN

Yes, it does have the middle control on the bass channel.

Tom Wolverton · From: Carpinteria, CA

There is a good chance that you have a 4 ohm OT.

I'll defer to the experts here: Is there a way to determine the OT impedience from the serial number of the OT?
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Stephen Cowell · From: Round Rock, Texas, USA

You should be able to tell from the part number on the OT... go to Schematic Heaven and verify it against the different schematics.
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Tom Wolverton · From: Carpinteria, CA

Great idea, Stephen. I've got a few Showman heads I should do that for.

I find your post here always valuable and useful. Thanks Stephen.
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Steven Husting · From: Germany

Hi -

First you need to determine which bassman 10 you have. As others have said, the older model wants to see 8 ohms, the newer ultralinear model wants to see 4 ohms.

Late 70s is probably ultralinear, but look on the back panel. Under the speaker output it should say something like "Speaker 50 watts" or "Speaker 75 watts." If it is 70-75 watts, it is an ultralinear, if it is 40-50 watts (I don't remember exactly) it is the older model. The ultralinear will probably also have a line out jack. Actually, the expected impedance should be on the back panel.

Here are the schematics:

Older 8-ohm, 50 watt version:
http://el34world.com/charts/Schematics/files/fender/Fender_bassman_10_schem.pdf

Newer 4-ohm, 75 watt version:
http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_10_75w.pdf

Measuring one of the speakers will give you an idea of what the nominal impedance. It won't read '8 ohms' or '32 ohms,' but the DC resistance might be, say, 6 ohms for an 8 ohm nominal impedance speaker, or 27 ohms for a 32. It can give you an idea.

You can use any combination of speakers that has the right nominal impedance (4 or 8 ohms) and will handle the power. If as a rule of thumb you figure an amp might peak at twice rated wattage, and you aren't interested in speaker breakup (likely for a steel or bass amp), then two 75-100 watt rated speakers will do fine. You won't be able to get the right impedance just by reusing two of the existing speakers, you will have to buy new ones, so either two 16-ohm (for 8 ohm parallel) or two 8 ohm (for 4 ohm parallel).

The other possibility is that the output transformer has unused taps. This isn't indicated on the schematics but it is possible. You would have to take it to a tech to have it modified. I doubt that this is the case, though, as especially on the later amps, Fender showed the various taps on the schematic.

steven

Daniel Haymore · From: Nashville, TN

Thank you for all the responses!

I feel pretty stupid now, because it does say on the back of the chassis, 70 watt RMS and minimum speaker load is 4 ohm.

I was hoping to avoid buying new speakers, but I thought that may be the case. I may just try to sell it and rethink. I love the way it sounds but I need something a little smaller and manageable for the places i'm playing.

Thanks for the help, I'm pretty ignorant regarding amps other than plugging into them, but I will hopefully learn.

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