OT: bandmaster ext. spkr

[marc]

apologies for OT subject, but i've searched everywhere and figured i'd ask here as a last resort. i have a '63 bandmaster head and i'd like try an extension speaker cab for a little extra air movement. however, i haven't a clue about impedance matching. does using the ext. output jack mean i have to use two cabs that would total 4 ohms? or is 4 ohms each fine?
thanks for the help.

[Lonestarjohnny]

Correct on the 4 Ohm's, if it see's anything lower, say 2 ohm's it could pull to many amp's from your amp's transformer's and  do damage to them.
JD

[petemoore]

The smaller the speaker ohm load # [total speaker ohm] the greater the load on the amp.
 I believe these amps are 'ohms Minimum' types, probably set at 4ohms minimum, [should say right over the output jacks???], as long as you don't go lower than that you should be ok.
 Do you know how to compute ohmage of two parallel 8 ohm loads [4ohms]? and compute two seriesed 8 ohm loads= 16ohms.
 Any number of speakers can be used as long as the ohmage 'adds up' and doesn't fall below the min ohms rating.
 "adds up= multiplying or dividing...depending on series or parallel connectings, if that makes sense.

[marc]

thanks peter, jd.  i think my question wasn't clear. i know that the main speaker jack needs to be connected to a 4ohm cabinet. if i were to connect my 4ohm speaker cab to the main speaker jack,  is the recomended extension speaker cabinbet impedance also 4ohms? or is the total impedance for both speaker jacks 4 ohms? lordy, i can't seem to be able to articulate the question. in other words, say i only have the head, and i want to connect TWO cabinets, using both the main AND extension speaker outputs. what is the impedance of each speaker cabinet. it's more a question of how this amp is designed. and no, that info isn't anywhere on the back of the amp.

[petemoore]

THose speaker jacks on the back of the amp are paralleled.
 Ie if you have a 4 ohm load plugged into one jack, and then plug another 4ohm load into the other jack, your amp would be running at 2 ohm...don't try this at home.
 parallel equation is divide by 2 ...two 4ohm loads paralleled is 2ohm
 Series equation is multiply by two....two 4ohmers seriesed is 8ohm
 Mixing different ohm speakers or figureing three speaker configurations...your on your own...measure and compare resistance readings with the DMM for further info.  
 Using both jacks on the back of the amp....the same as or is: parallel connection.
 This is expecially true for mono amps.

[jplaudio]

apologies for OT subject, but i've searched everywhere and figured i'd ask here as a last resort. i have a '63 bandmaster head and i'd like try an extension speaker cab for a little extra air movement. however, i haven't a clue about impedance matching. does using the ext. output jack mean i have to use two cabs that would total 4 ohms? or is 4 ohms each fine?
thanks for the help.

You don't need to worry about  mismatched loads with your Bandmaster. Almost all tube amps can tolerate mismatched loads with no problems. There is no "minimum" load for a tube amp like there might be with a solidstate amp. You can't draw too much current through the transformer and damage it. You should, however avoid very high impedance loads or an open circuit. What you do need to consider is the power distribution between the two cabs.
If you use a 8 ohm cabinet with the stock 4 ohm fender cab 2/3 of the power will go to the 4 ohm cab with 1/3 going to the 8 ohm resulting in lower out put from the 8 ohm cab, everything else being equal. Fender did not specify an impedance for the ext cabinet because it is not critical.
from the London Power website:
"The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors."
The actual impedance of any speaker will also vary with frequency. The 4 ohm rating is nominal.

[petemoore]

But because of warnings not to do so, I dont recommend not heeding the amplifier manufacturers recomendations or requirements.
 Mismatched loads in the realm of 1/2 ...ie an 8ohm load in one cab paralleled with a 4 ohm load in the other cab, for normal use isn't worth the trouble of connecting it, unless they're in different rooms, the 4ohm cab is so much louder it's hard to tell if the 8 ohm is on unless you get your ear in the speaker.
 I like speakers that are closely matched. even small msimatches caused by component tolerance in speakers can be heard, even in the same model speaker.
 I had an old VOX 4 X12 cab, very old, one speaker was 7.9ohm the other was 7.3, the 7,9 was noticable louder, and as coulud be expected, was the first to exibit unwanted overload tones.

[marc]

 Using both jacks on the back of the amp....the same as or is: parallel connection.

Thank you petermoore!!!!

[Hal]

add 3 more speekers!

one in series with the first, then 2 in series with eachother...and those 2 series in paralel.  Total impedance = 4 ohms

:-D