Adding 1x12" to a combo - impedance?

[blackwood]

Hey!

I'm adding a 1x12" 60W speaker cabinet to a 1x12" 50W 8 ohm tube combo.
How do I decide which poweramp output to choose and what should be the impedance of the speaker I'm adding?

impedance of an extra speaker:
1) 8 ohm speaker parallel - 8/2 - 4 ohms total
2) 16 ohm speaker serial - 16+8 - 24 ohms total

If I choose 1) and get a total of 4 ohms, how many watts does the amp deliver? 100W? Into 2 speakers. Is that just 50W per speaker or what?

If I choose 2) and get a total of 24 ohms, is that 50/3 - 16W????

Here's what it looks like:

http://i6.photobucket.com/albums/y248/glicerin/spcab.jpg

So if I use an 8 ohm speaker in parallel and connect it to jack 25, I'm getting a total of 50W per speaker? Or not?

Please help, this is killing me :D
Thanks!

[Tom Phillips]

Hey. It's very helpful that you included the figure of your speaker output jacks. I assume that your combo currently has one 8 Ohm speaker and it's plugged into jack #25 or #26. The impedance of your new speaker should be 8 Ohms. To connect, move the internal speaker to jack #27 and hook the new 8 Ohm extension speaker to jack #28.

The figure from the back of your amp indicates that the output impedance of the jack 27/28 section is 16 Ohms. Those jacks are configured such that, when you use both jacks, the speakers are hooked in series. Therefore, you will have the proper 16 Ohm load with your two 8 Ohm speakers.

Regards,
Tom

[blackwood]

Yes, you're right. The combo speaker is on jack #26, I get it now, thanks for your help!!

Just one quick question. If I was to build a 50W amp and I use two 8 ohm speakers, could I use two 30W speakers? I mean, 50W amp into 2 speakers, that's 25W per speaker, right? If I'm on a budget, I could save some money buying two 30W instead of 50W speakers..

[Tom Phillips]

...If I was to build a 50W amp and I use two 8 ohm speakers, could I use two 30W speakers? I mean, 50W amp into 2 speakers, that's 25W per speaker, right? If I'm on a budget, I could save some money buying two 30W instead of 50W speakers..

Speaker wattage ratings are somewhat arbitrary because manufacturers take liberties when setting their specifications and they do not all use the same standards. However, in general, you can use two 30W speakers with a 50 Watt amp. Depending on how it is used a low powered amp can “blow” a higher wattage speaker. On the other hand, using really high wattage speakers may not give you the sound you would like. This discussion could get very involved but I think you will do fine to use either two 30 W or two 50W speakers. Price is affected more by the exact brand and type of speaker than by the power rating alone.
Tom