Do Ohms have to be right?

tomhanzo · #1 04-17-2011, 02:01 PM

I've looked through the guides here and on the web and I can't seem to find the answer to this.

If I have a amp head and don't know the ohm rating, can i put the wrong cab with it?

Its an old Lafayette tube head and it doesn't say anything about ohms on it. I can't find the head anywhere on the web. Would it be safe to plug an 8 ohm cab into it?

I figure it takes a 4 ohm cab, so can't i put an 8 ohm cab on it?

selftitled907 · #2 04-17-2011, 02:06 PM

your "probally" safe to put an 8ohm cab to it but dont take just my word on this as i have no experience on that particular amp. and i say your probally safe because well, the standard seems to be 4ohm min. for bass heads. which would make that safe....but again....might be best to wait till someone with some exp. on that particular amp chimes in.

if you do find its got a 4ohm min load on it....then plug and play my friend! and even add another 8ohm cab if you want!

-Gabe

BTW.
do you know which model it is? cause as i look im finding many search results on google for those amps.

tomhanzo · #3 04-17-2011, 02:11 PM

It has nothing on it other than its a lafayette. I have a serial number thats it. I don't even know if it is for sure a bass amp. It just has bass, high and low inputs.

So if its a 4 ohm, I can plug an 8 in? I thought that, but wasn't sure.

dukeisdog · #4 04-17-2011, 02:12 PM

Quote:

Originally Posted by tomhanzo

It has nothing on it other than its a lafayette. I have a serial number thats it. I don't even know if it is for sure a bass amp. It just has bass, high and low inputs.

So if its a 4 ohm, I can plug an 8 in? I thought that, but wasn't sure.

Usually that's the case, but not with other all tube heads such as Ampegs SVT.

JimmyM · #5 04-17-2011, 02:14 PM

more than likely it can handle 8 ohms. but i'm sure there's someone on here with the same amp that will let you know for sure.

groooooove · #6 04-17-2011, 02:17 PM

Quote:

Originally Posted by JimmyM

more than likely it can handle 8 ohms. but i'm sure there's someone on here with the same amp that will let you know for sure.

yup.

almost every bass amp i've ever used was a 4 ohm minimum, and i can think of maybe 2 models that can go down to 2 ohms (the iamps and i think a carvin model?)

i cant recall ever seeing a head that coulden't handle 8ohms.

iiipopes · #7 04-17-2011, 02:21 PM

If you know the amp takes a 4-ohm load, yes, you can use an 8-ohm load. You will have decreased efficiency, so if you push it to the same volume level, you might wear out the output tubes a little quicker.

But if you're not sure, I would search the web and enthusiast groups to find out for sure, as to little a load can burn out an output transformer and cause other damage.

Rocker47 · #8 04-17-2011, 02:22 PM

If you can find a schematic drawing either on the web or stuck to the inside of the amp or in a manual, that would help greatly. That should show the resistance that the amp likes to see and the entire circuit of course. If you do have one of them and you have trouble with it I can try to help. I'm not an amp tech< but being an electrician helps to read the drawings. Let me know. Hope you find out more!

JimmyM · #9 04-17-2011, 02:32 PM

Quote:

Originally Posted by groooooove

yup.

almost every bass amp i've ever used was a 4 ohm minimum, and i can think of maybe 2 models that can go down to 2 ohms (the iamps and i think a carvin model?)

i cant recall ever seeing a head that coulden't handle 8ohms.

the tube svt is only rated for 4 or 2 ohms. it can handle an 8 ohm load but it can put stress on the amp to do so for extended periods of time. doesn't always, but isn't recommended. and impedance matching is more mission critical in tube amps than ss amps. as long as you don't go under the minimum in an ss amp, you're golden. but tube amps like to actually see the impedance the output transformer is rated for or close to it.

HolmeBass · #10 04-17-2011, 02:41 PM

Most of the idea that "if it takes 4 Ohms then 8 Ohms is ok" comes from the world of solid state amps. Tube amps need to be matched in impedance to their load pretty closely. Now that is also confusing- when I say "tube amp" I mean that the power amp section uses tubes. Tubes in the pre-amp are common in many amps that still use a solid-state power amp section (mosfet transistors are a common solid-state power amp configuration). So the first question is: is it *really* a tube amp? Does it have a large number of bigger tubes, like KT-88's, 6550's, etc. If it's just 12ax7's and such then it is probably actually a hybrid amp, with a tube pre-amp and a solid state power amp section, and the 8 Ohm cabinet will be just fine. Another way would be to pop the top (while it is unplugged and powered down!!) and see how many transformers are in there. If there is only one, it is probably a power transformer and you have a solid-state head. If there are two transformers, and/or one that is clearly in-line with your output jacks, then you do have a tube power section. Hopefully something will be printed on your output transformer to let you know what kind of taps it has (8 Ohm, 4 Ohm, 16 Ohm, etc).

tomhanzo · #11 04-17-2011, 03:41 PM

Here are some pictures. I really can't find anything that matches this. The guy told me it was a Lafayette. It doesn't say it on the amp.

JimmyM · #12 04-17-2011, 03:43 PM

nice! a little cleanup and that'll be a hot looking amp!

selftitled907 · #13 04-17-2011, 03:45 PM

cant read the ohm rating next to those tubes....however....those tubes look to be for the main power section which suggests not a solid state? might want to match your ohm ratings to be safe.

RickenBoogie · #14 04-17-2011, 03:51 PM

"Do Not Exceed Rating", what does it say under that? And +1, a tube amp needs the load to match the OP Transformer very closely. I'd research further before risking damage.

Munjibunga · #15 04-17-2011, 03:56 PM

He said it's a tube amp, so I would advise him to ignore this post.

Quote:

Originally Posted by selftitled907

your "probally" safe to put an 8ohm cab to it but dont take just my word on this as i have no experience on that particular amp. and i say your probally safe because well, the standard seems to be 4ohm min. for bass heads. which would make that safe....but again....might be best to wait till someone with some exp. on that particular amp chimes in.

if you do find its got a 4ohm min load on it....then plug and play my friend! and even add another 8ohm cab if you want!

-Gabe

BTW.
do you know which model it is? cause as i look im finding many search results on google for those amps.

selftitled907 · #16 04-17-2011, 04:01 PM

Quote:

Originally Posted by Munjibunga

He said it's a tube amp, so I would advise him to ignore this post.

which is why i said that he should wait for others to speak up on it(says it right in the post that you quoted.....there was to many variables that were unaccounted for...not only that it was NOT made clear that it was a tube amp at the time of my post....but thanks for taking time to read everything.

tomhanzo · #17 04-17-2011, 05:17 PM

lol It just says
~" Do Not Exceed Rating
117 volts, 50/60 cycles, 200 watts
fuse 2amps"

Its actually not my amp. It belongs to a friend of a friend. They let me borrow it for a show. I was gonna use it, but he doesn't know the ohm rating for it. They plug it into a cab that is old and would fall apart if transported. It didn't say the ohm rating, so I told him i would just look it up and let him know for future reference.

selftitled907 · #18 04-17-2011, 05:29 PM

Quote:

Originally Posted by tomhanzo

They plug it into a cab that is old and would fall apart if transported. It didn't say the ohm rating, so I told him i would just look it up and let him know for future reference.

what is the rating on the cab "they" plug into? or did the cab also not have an ohm rating? and maybe its time to have a tech look into it?

Foz · #19 04-17-2011, 06:29 PM

Measure the unmovable cab with an ohm meter - if it sez about 6 ohms use an 8 ohm load - if it sez about 3 ohms use a 4 ohm load. You will do no worse than the guys you borrowed it from and given the amp worked long enough for the old cab to rot in place you probably got it right.

T-Bird · #20 04-17-2011, 10:24 PM

Hi.

90% of the people who can say that a tube amp can safely be operated at any given impedance without even having a clue about the circuit, wouldn't do that if that was their own amp=their own money. They are quick to give it a thumbs up for someone else to try it over the 'net though, for **** & giggles if for nothing else.

Some amp designs can live with drastic mismatch, some give up the ghost with a difficult load even if the nominal impedance was correct.

OP, if You want the amp to survive, and possibly save some vintage tubes from being destroyed, take the amp to a tech. It most likely needs anew set of capacitors anyway, and determining the correct load impedance is a straight forward measurement any thech should be able to do. He/she can also verify the circuit and most probably tell what the amp really is.

Regards
Sam