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Post subject: Fender Bassman 4ohm going into 8ohm cab, safe?
Posted: Mon Apr 26, 2010 5:09 am
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Hi all,

I recently acquired a original 1965 bassman, which only runs off 4ohms. Trouble is my one and only cab is for my Marshall JCM 800, which only has a 8ohm input (2 16ohm speakers). My main question is: Is it safe?. The bassmans a thing of history, and I love the tone I get out of it and I don't really wanna put any unecessary pressure. I was told by my local amp tech who did a check up on the amp, that it would be fine to put it through a 8ohm cab and would not put any pressure on the transformers at all, and in fact it would put less pressure (where putting a 8ohm head into a 4ohm cab would put alot of pressure, or as he said).

On the same side, I recently spent alot on guitar stuff, and it will be difficult to buy a 4ohm cab for now, but I would consider it if I was seriously putting the bassman under stress.

Cheers for the help!


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Posted: Mon Apr 26, 2010 5:45 am
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no problem using a higher impedance cab 8)

what you can't do is go under the impedance, that hurst the output transformer, but going higher will only make your amp sound not as loud and a tad more polite.

I've used an 8 Ohm cab with a '68 bassman SF for year and had no problems whatsoever. Now I'm using a 2x12" with two 8 Ohm speakers so I can switch between Fenders and Marshalls (in parallel, with a switch for 4 and 16 Ohm) or I simply remove two screws from the back panel, jiggle the wires around and use one of the speakers rated 8 OHm for other stuff ;)

if you find someone with a 4 Ohm cab make sure you have a go, you will want to spend that extra cash for a cab with a 4 Ohm!!! (brings out the real power in an old Fender)

:D


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Posted: Mon Apr 26, 2010 5:53 am
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Thanks for the reply so far, glad to know its ok to do it (at least until i get a proper fender ab for it).

Second question is, if it is safe, is there something cheap to switch between my marshal and Fender so each can use the cab, without changing the speaker leads from each amp every time. I ask this as my amps against a wall, and quite hard to reach when its got the jcm and bassman stacked. I know theres the tonebone headbone or w/e, but it seems like its used when both amps are on and to switch on the go (so that the current from both amps dont damage the other or ones not left without a output). However usually for me when ones on, the others off by the plug, so only one is powered, anything cheap out there to get rid of the hassle of switching?


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Posted: Mon Apr 26, 2010 5:57 am
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the only thing I've found that's reliable is the Weber two head amp switcher (not cheap) - puts a dummy load on the amp head that's not being used, i.e., both heads are actually "connected" to a speaker, one directly and one virtually.

it's quite a hassle because of the amount of cables you have to use.... (I think it was 6 or 7...) but I haven't tried anything else.

just be VERY careful with what you use because using a head with no load can seriously damage the amp... anything that switches between heads and leaves the amp with no load, even if it's for a few seconds, can eventually damage the amp :!: :!: :!:


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Posted: Tue Apr 27, 2010 4:14 am
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Just a update, my amp tech said he can build me a custom switch pedal with all the necessary input sockets and components to handle the current of a amp head, where there are 3 inputs. One from the pedal to the speaker cab, and 2 inputs for both my bassman and jcm800. He said he will install a switch, which I'll need to use depending on which amp Im playing, and switch it correctly (or my amp gets it). He did not say anything about a dummy load or anything as he said whichever amp Im not playing it will need to be OFF completely, which is fine as I dont switch amps on a frequent basis, just a hassle to do so when I want to. Now he did state this is perfectly safe as long as I have the switch in the right position (to match which amp Im playing). Said he can do it for around £35.

My question, what do you guys think of this. I'd rather get a wider response, opinion.


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Posted: Tue Apr 27, 2010 4:21 am
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if you're prepared to pay 35 squid for that.... :?:

I'd rather plug and unplug the speaker cable myself, imagine you leave an amp turned on by mistake or press the switch by mistake?

that's a mechanical solution that's fine as long as you're not of the forgetful type, i.e., myself :P


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Posted: Tue Apr 27, 2010 4:31 am
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No problem to plug your 4 ohms amp into 8 ohms load


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Posted: Tue May 04, 2010 4:41 pm
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You guys must be amp repair techs looking for business. Driving a higher impedance load (16 ohms) with a lower rated impedance amp (4 ohm) will fry your output transformer and tubes eventually. Old Fender amps could handle alot, but anything else, doubtful.


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Posted: Wed May 05, 2010 3:12 am
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rjake: The question is to drive into a 8 ohms load ( 2 X16 ohms speakers) not in 16 ohms.

This is safe


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Posted: Wed May 05, 2010 3:14 am
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rjake wrote:
You guys must be amp repair techs looking for business. Driving a higher impedance load (16 ohms) with a lower rated impedance amp (4 ohm) will fry your output transformer and tubes eventually. Old Fender amps could handle alot, but anything else, doubtful.


I've used a bandmaster and a bassman with an 8 Ohm cab for donkey's years and I've never had a problem....

I would'nt use a 4 Ohm cab with a head rated 8 Ohm impedance but the other way there's no problem.... using a higher impedance only reduces power and the strain on the transformers is minor (no one's playing a 40w amp full blast 100% of the time) :D


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Posted: Wed May 05, 2010 2:45 pm
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You are actually backwards on that. An 8ohm tube amp into a 4ohm cab is better, though more wear on the tubes. More impedance is bad, that's why you don't want to turn a tube amp on with no load. If you are talking solid state, then you would be correct. You can turn on a solid state with no load and it won't care. Tube amps can survive an output short, solid state most likely won't. Old Fender amps are robust, therefore, you can get away with the mismatch. At higher volume, you are overworking your OT on a higher impedance load.

As far as the 4 ohm amp to 16 ohm load, I was using that as an example. I saw that the OP was using the 8 ohm load, which would be less hazardous than the 16 ohm load. But this is a '65 Bassman, I wouldn't advise a mismatch at all.


Last edited by rjake on Mon May 10, 2010 5:57 pm, edited 1 time in total.

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Posted: Wed May 05, 2010 9:32 pm
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Azar, I agree with Rjake, I wouldn't advise any mismatch with a '65 Bassman. Although, my problem with the 8ohm impedance would be the "fly back" voltage generated by the speakers and the switching method between the two amps and the speaker cab. The "fly back" voltage could be dealt with by the use of stringed diodes off the power tube socket plate pins to ground, and can be removed with only a small solder footprint on existing points. The OT would be suppling less current to the 8ohm load, so overheating isn't the problem. The arcing that can occur at the tube sockets is a problem, your tech will know what I'm talking about(I use this in my old Fenders, some call it spike suppression, it doesn't affect the tone). IMO The amp head switching is a more critical issue, than the impedance issue. Although I've built a number of switching boxes for players with good success, I've never made one for two amps and one speaker cab. The main concern would be making it "fail safe" if both amps are on with some input signal. That's why a dummy load was mentioned. But if you are definately turning amps off and using one at a time, then it becomes a matter of isolation. I'd be interested in your tech's solution. I drew up a couple of boxes for this purpose the other night, but the care of my grand son took priority over that effort. I'll revisit that and see if I can find a simple yet elegant design. Art

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