Getting a 1965 Bassman Head, Speaker Cab questions..

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hvrock13
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Getting a 1965 Bassman Head, Speaker Cab questions..

Post by hvrock13 » April 10th, 2010, 11:23 pm

First off, I'd like to say that I'm new here. So hi! But about my question, I've got the awesome opportunity to buy a 1965 Fender Blackface Bassman head. Retubed and everything. Now, I understand this amp is 4 ohms. I'm not an expert or even close to one when it comes to electronics or impedance, just started researching it today but I've had no luck finding an answer to my problem. You see, I have a Behringer 4x12 cabinet, each speaker is 8 ohms, and its total impedance is 8 ohms. (By the way, correct me if I'm using the wrong terminology, I could use some more knowledge on this stuff). I don't want to risk damaging the amp if I do get it, and I've read mismatching the impedance of the amp and the cabinet is a no-go for tube amps. But I've also heard that the Bassman is a tough amp, and it won't have a problem with the 8 ohm speaker cab. I've been running a Marshall Valvestate 8100 at 4 ohms into the same speaker cabinet for 3 years, and the previous owner was too, but it recently stopped working, so I'm not sure if its because of the mismatch or not. So I'm a bit skeptical. In short though, I'm wondering if I could rewire the cabinet to be 4 ohms, or if disabling 2 speakers and running the other 2 in parallel to proved 4 ohms, or if I should just sell the cab. Bad thing is, the cabinet isn't worth much, and the cabinet they offer with the head (optionally) is a '67 Bandmaster cabinet. But I've looked that up, and it seems to be at 8 ohms too, and its a 2x12 cabinet. So I'm extremely confused, and just want to keep it as cheap as possible. I'd prefer modding my current cabinet, if possible. I also thought it might be important to note it has a stereo/mono switch on the back, not sure if that would cut the resistance or not? Like I said, I'm new to the technical stuff with amps. Any help would be GREAT. Thanks ahead of time.

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Ricochet
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Re: Getting a 1965 Bassman Head, Speaker Cab questions..

Post by Ricochet » April 11th, 2010, 9:17 am

Don't blame me if your amp has a problem, but I'd just run the 8 ohm cab. It's highly unlikely that a 2:1 mismatch in impedance in either direction will harm a tube amp. I've done it with my amps, but I don't have an old Bassman. With amps having pentodes or beam power tubes in their output stage (= all modern guitar amps), the chosen load impedance is rather arbitrary and is near the point of minimum output stage distortion. Changing the load will increase distortion a bit and it may sound different. It will affect the output power very little, unlike an amp with power transistors or triode tubes in the output. Increasing the impedance of the speaker load will increase the range of voltage swing in the output transformer, but its insulation ought to have enough extra rating to tolerate a 2:1 change.

As for very large changes in the output load, a tube amp will run for a while with a dead short on the output without harm. Running it unloaded, which is infinite output impedance, the output energy goes into producing very high voltage in the secondary winding of the output transformer, much like the spark voltage in your car's ignition coil. That will hurt something. Often it arcs through the insulation, leaving carbon tracks that will short the coil sooner or later, and it may also arc over at the tube sockets leaving carbon tracks. Even an instant of running without a speaker load can cause permanent damage.
"A cheerful heart is good medicine."

hvrock13
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Re: Getting a 1965 Bassman Head, Speaker Cab questions..

Post by hvrock13 » April 11th, 2010, 1:20 pm

Thanks for your reply and all that information. I learned quite a bit from reading that. Unfortunately, Guitar Center lied to me when I spoke to them over the phone about the amp, they said it was in perfect condition and been in the shop for 4 months. I went there today, and, well, they pulled a Guitar Center on me. They lied completely and it turns out the bassman was in pieces and is not being sold as a unit, just for parts. Apparently UPS shipping dropped it or something 4 months ago, when they received it. Last time I trust what anyone at Guitar Center says. So, I'm stuck with no amp for the time being.. At least I didn't spend a lot of money. Thanks again for your information though!

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Ricochet
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Re: Getting a 1965 Bassman Head, Speaker Cab questions..

Post by Ricochet » April 11th, 2010, 5:05 pm

Ugh.
"A cheerful heart is good medicine."

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