Name: Steve Kennedy
Email: steve@pacair.com
Subject: series or parallel?
Thread: 57
Time: Wed, 25 Apr 2001 03:17:24 GMT
It isn't that straight forward, but I see you have had a lot of good input so far!
My MM 410-65 amps have 4 10" speakers. The speakers are 8 ohms each. Each pair is wired in parallel for a total impedance of 4 ohms for each PAIR. The pairs are then wired in series to give you 8 ohms. The amp is then run in the 8-ohm setting.
If an external cabinet (preferably 8 ohm) is connected, this goes in parallel with what you already have, so an 8-ohm external cabinet in parallel with the 8-ohm internal speakers gives you a grand total of 4 ohms, so you throw the switch.
Two things you need to understand... tube amps are usually very forgiving of impedance mismatches as compared to amps with solid-state output stages. This is due primarily to the output transformers.
Most well-designed tube amps (Music Man included) can handle a 100% mismatch with no problem. The outpout transformer will get hotter, but speaker impedance changes with frequency so it isn't simple mathematical calculation.
A 100% mismatch (+ and -) in the 8-ohm switch setting would allow a load range from 4 ohm to 16 ohm. It is actually BETTER to be on the low side! In the 4-ohm switch setting, the load range is 2 ohms to 8 ohms.
I personally would never exceed these ranges and would err on the low side if you have a choice.
Steve