#1
Hello. I bought a 2nd hand FM212R amp. The original speakers (2x 8 Ohm) had been replaced with 1x 8 Ohm and 1 x 16 Ohm.

However the 8 Ohm just blew, and now just buzzes when anything is played through it. On investigating, the speaker coils (I believe that is what they are called - voice coils maybe - thin wires going into the cone) to the 16 Ohm were cut (!) and so there was never actually any sound coming from it at all. It’s my first amp, new to the game, lesson learned buying the next one! At least it was very cheap – roughly the price of a semi-decent speaker ironically enough.
The two speakers were still hooked up in parallel though, just no sound output to the 16 Ohm. The reason for the blow on the 8 Ohm might be in there somewhere..

I will replace the speakers. But for now, I want to know if I am ok with my temporary setup which is this:

I disconnected the 8 Ohm completely (buzzing when played). I soldered the speaker coils back together and now have just a single 16 Ohm speaker hooked up to the amp. Which to me is an increased impedance? It worked ok when I hooked up and began playing.
I believe on a Solid State amp such as this, going over the impedance should be ok in terms of safety and potential damage to the amp/speaker? The only problem might be a lack of volume and different tone? I could be way off here..

I believe
•The original speakers which should have been 2 x 8 Ohm in parallel, would have given an impedance of 4 Ohms (8/2)
•The replacement 8 Ohm and a 16 Ohm speakers in parallel gives a total impedance of 5.33 Ohms (A*B)/(A+B) ???
•One 16 Ohm speaker on its own: 16 Ohms

FM212R Specs: http://support.fender.com/manuals/guitar_amplifiers/FM_212R_manual.pdf


Please please don’t reply back telling me the FM212R is rubbish, or SS Amps are rubbish, and to get a new amp! I just need to know if its ok to use the 16 Ohm on its own without damaging the amp or the speaker? Or myself! Is the problem purely a reduced volume/different tone?

Thank you!
Last edited by AK100 at Nov 11, 2014,
#3
Great. Thank you!

the speaker is an 80W 16 Ohm if that says anything.

Much appreciated. Thanks again.
#4
You now have an amp capable of producing 100w at 4 ohms. With one 16 ohm speaker it will produce approximately 25 watts. The amp will run very cool and the speaker will not be in danger of blowing up anytime soon. If this is all ok with you... enjoy.

If it were mine and I wanted to keep it, I would probably find some nice 50w 8 ohm speakers for dirt cheap. Lots of players remove perfectly good stock speakers in their quest for tonal nirvana and they are just collecting dust on a shelf. Good like-new speakers for this amp if they match your specs. Buy them.
#5
Thanks again for the response. Good to know. And yep, I definitely intend buying a couple of 8 Ohms to match what the amp was originally set up with and spec'd for. Already started the sussing out phase! This is just a short term fix. (I may even look into picking up a different amp altogether - FM212R is a heavy unit to be lugging around)

Info and advice very much appreciated. Thanks.
#6
Being out of phase won't produce no sound, just a shitty sound. As long as your resultant impedance is >4 ohm and the power rating is sufficient, no harm will be done.

Are you sure the speakers are the problem? Stick a multimeter (on resistance) across them and see what you see. It should read slightly less resistance than the impedance of the speaker. If it tests ok like that then the problem is elsewhere.
Last edited by Cathbard at Nov 11, 2014,
#7
Hi Cathbard. I think it is the speaker alright. When I bought it, the 16 Ohm speaker was not connected (connected in parallel with the 8 Ohm, but the voice coils to the cone were cut, so no sound coming through the 16 Ohm) The 8 Ohm was working fine. But then suddenly the sound basically disappeared to a very low sound, and any note played just buzzes along with the note. After disconnecting the 8 Ohm completely and doing a bit of work on the voice coils of the 16 Ohm, that 16 Ohm speaker is now working fine, with its + and - connected directly to the amp.. The + and - originally going to the 8 Ohm also work when connected to the 16 Ohm, so I don't think its the amp or wires?

I'll get a multimeter and check it out though, as you suggested, thanks.

I hope I'm right in calling those wires into the cone "voice coils"! There are + and - wires from the amp to the speaker chassis. And then "voice coil" wires from the chassis to the cone. I hope that's the correct terminology - otherwise I'm probably making little sense to anybody!

Thanks again for the support.