
dragonfly66 June 15th, 2011, 12:34 PM I'm a little paranoid so I just wanted someone else who knows this stuff better than me to look at this and tell me if what I want to do is OK and why it is OK.
I have a Custom Vibrolux Reverb. It has two 10" 8 Ohm speakers wired in parallel which makes the load 4 Ohms. The manual says this about the external cab jack:
EXTERNAL SPEAKER  Connection for an
external speaker. This jack is wired in parallel
with the MAIN SPEAKER JACK, and must only
be used with a speaker plugged into the MAIN
SPEAKER jack. The amplifier is optimized for a
4 ohm speaker load, and the speakers in the
cabinet are wired for 4 ohms. Should a total
load of more or less than 4 ohms be used, the
amplifier will not put out its maximum power
output before distortion occurs and tube life
span will be reduced.
I have a 1x15 16 Ohm cab that I want to use with the Vibrolux. Because the external speaker jack is wired in parallel with the main speaker adding this 16 Ohm cab would make the load 3.2 Ohms. The amp itself says the minimum load is 2 Ohms.
So my question is: How bad is 3.2 Ohms in an amp optimized for 4 Ohms. Will this take a significant toll on the amp or tubes if the cab is used all the time or no toll if used every now and then.
jh45gun June 15th, 2011, 12:38 PM 3.2 ohms is close enough to 4 ohms as a lot of amps back in the old days the rating on the speaker was 3.2 ohms but they were 4 ohm speakers. 3.2 is pretty much what the meter read. My opinion is don't bother as the speakers in the amp will be more efficient then the 16 ohm speaker and be louder then the 16 ohm speaker so you will not have a equal match between the cab and the amp.
daddyopapa June 15th, 2011, 12:39 PM Won't hurt it at all. Tube amps are good for a 2:1 impedance mismatch so 4 ohm amp can safely drive 3.2 ohms.
piece of ash June 15th, 2011, 12:47 PM Yep... the 16 Ohm speaker will only receive 20% of the amps output anyway. Won't hurt the amp unless you're near diming it, but you won't get the low end you're looking for anyway.
dragonfly66 June 15th, 2011, 01:07 PM My opinion is don't bother as the speakers in the amp will be more efficient then the 16 ohm speaker and be louder then the 16 ohm speaker so you will not have a equal match between the cab and the amp.
Yep... the 16 Ohm speaker will only receive 20% of the amps output anyway. Won't hurt the amp unless you're near diming it, but you won't get the low end you're looking for anyway.
I was intending to fill out the sound with the 15" cab, but both of you are saying that it won't do that. Why is it that the cab will only get 20% of the power and not be as loud as the amp speakers? Would this be the same if I plugged in a 2x12 4 Ohm cab or any cab?
piece of ash June 15th, 2011, 01:14 PM Power = Voltage squared over R (impedance)
All the speakers see the same voltage... so just pick a number... 50
(1) 8 Ohm speaker: (50*50)/8 = 312.5 Watts
(1) the other 8 Ohm = 312.5 Watts
(1) 16 Ohm speaker  156.25 Watt.
Total Watts = 781.25
Percentage to 16 Ohm = (156.25/781.25) * 100% = 20%
The power is divi'ed up based on the impedance... the more impedance... the less power.
Now you could rewire your 8 Ohm speakers in series... and then add the 16 Ohm in parallel. This would route half the power to 16 OHm speaker. But it would cost you half your total power to do so... and that may not be an issue. Losing half your output power does NOT equate to losing half your volume.
Twice the volume requires 10 times the power. So... half the power means 85% of the perceived volume. That might be your solution.
6stringelectric June 15th, 2011, 01:30 PM You could wire the existing speakers in the amp in series (thus 16 ohms there too) and then all speakers will get an even share of the output and the total will be 8 ohms.  if you plan to use the extension cab all the time.
If you want to sometimes run just the internal speakers alone, you could wire in a switch so you could change them from parellel to serial as needed.
jefrs June 15th, 2011, 01:38 PM Won't hurt it at all. Tube amps are good for a 2:1 impedance mismatch so 4 ohm amp can safely drive 3.2 ohms.
No, not all of them are, only old Fenders ( an old "4ohm" Fender may dive a 2ohm speaker).
Most manufacturers actually mean 4ohm minimum impedance when they say 4ohm minimum impedance.
A 4ohm impedance speaker has a DC resistance of ~3ohm.
An 8ohm impedance speaker has a DC resistance of ~6ohm.
I'm not aware of anyone making a 3.2ohm speaker but 3ohm were once common, few still made.
If you are measuring 3.2ohm with an ohmmeter then you have a 4ohm speaker there.
RockerDuck June 15th, 2011, 01:40 PM A while back, Fender made a "solid state" speaker cabinet with 3 speakers. 8 ohm times 3 was 3.2, so one 8 ohm and one 16 ohm is the same and works fine.
teleamp June 15th, 2011, 01:55 PM No, not all of them are, only old Fenders ( an old "4ohm" Fender may dive a 2ohm speaker).
Most manufacturers actually mean 4ohm minimum impedance when they say 4ohm minimum impedance.
A 4ohm impedance speaker has a DC resistance of ~3ohm.
An 8ohm impedance speaker has a DC resistance of ~6ohm.
I'm not aware of anyone making a 3.2ohm speaker but 3ohm were once common, few still made.
If you are measuring 3.2ohm with an ohmmeter then you have a 4ohm speaker there.
This is correct!
jh45gun June 15th, 2011, 01:55 PM A while back, Fender made a "solid state" speaker cabinet with 3 speakers. 8 ohm times 3 was 3.2, so one 8 ohm and one 16 ohm is the same and works fine.
That is not what he has he has two 8 ohm speakers equaling 4 ohms in parallel and wants to put a 16 ohm speaker in parallel with it. The 16 ohm is only gonna put out what was stated about 20 percent. I did not know the math part of it but messing with speakers and amps since 1968 I do know that the higher impedance speaker will not be as loud. What your saying is three equal impedance speakers so all three would get the same voltage as all three are the same impedance. In this case the 16 ohm speaker would not be as efficient.
SnorkelMonkey June 15th, 2011, 01:57 PM Nope
jefrs June 15th, 2011, 02:54 PM That is not what he has he has two 8 ohm speakers equaling 4 ohms in parallel and wants to put a 16 ohm speaker in parallel with it. The 16 ohm is only gonna put out what was stated about 20 percent. I did not know the math part of it but messing with speakers and amps since 1968 I do know that the higher impedance speaker will not be as loud. What your saying is three equal impedance speakers so all three would get the same voltage as all three are the same impedance. In this case the 16 ohm speaker would not be as efficient.
Put the two 8ohm in series with the 16ohm in parallel to make total load 8ohm.
An 8ohm speaker in parallel with a 16ohm series pair makes for 5.3ohm load, and this /is/ acceptable onto a 4ohm load outlet (Laney say so too). You can go up from the recommended load a little ways, but not down, certainly not below minimum spec.
However we read above that the OP's amp will accept minimum load of 2ohm.
The amp is a Vibrolux and minimum load is 2ohm. The dire warning printed on the amp is correct, it won't sound as nice and it will burn the bottles up by making them work harder. So a total load of 3.2ohm is ok.
Logic, theory and the maths say you should get more power into 3ohm load than 4ohm load, but you won't because of the mismatch, neither the transformer nor the bottles will be running efficiently. To my ears, underrunning the load sound flabby. Using a slightly higher load is a bit tighter. Select speaker with ears.
The best sound from a valve amp will be obtained when its load is matched and on its longest secondary winding i.e. if you have 4ohm, 8ohm and 16ohm taps then use 16ohm on the 16ohm because it is the most efficient.
dragonfly66 June 15th, 2011, 03:06 PM Power = Voltage squared over R (impedance)
All the speakers see the same voltage... so just pick a number... 50
(1) 8 Ohm speaker: (50*50)/8 = 312.5 Watts
(1) the other 8 Ohm = 312.5 Watts
(1) 16 Ohm speaker  156.25 Watt.
Total Watts = 781.25
Percentage to 16 Ohm = (156.25/781.25) * 100% = 20%
So from what you are telling me my 4 Ohm 2x12 cab would work better in terms of balancing the sound. These things are good to know.
What about those devices that change impedance so you can use a 16 Ohm cab with an amp that wants to see 4 or 8 Ohms? Would using one of those devices set at 4 Ohms result in the same volume disparity?
piece of ash June 15th, 2011, 03:09 PM I'm not following your question...
DOGMA Dunn June 15th, 2011, 03:39 PM Colose enough for Public School Work.
dragonfly66 June 15th, 2011, 04:07 PM I'm not following your question...
If you use some thing this:
https://taweber.powweb.com/store/zmlogo.jpghttps://taweber.powweb.com/store/zmrear.jpg
https://taweber.powweb.com/store/zmatch.htm
Would the 16 Ohm still only get 20% power.
piece of ash June 15th, 2011, 04:18 PM Yes... you could use that to convert the 16 Ohm speaker to a 4 Ohm... then it share 50/50 with your 212's.
That would put the load on your amp at 2 ohms... perhaps a bit much.
Also for the price of that convertor... you could buy an 8 Ohm speaker... and them wire it IN PLACE of one your existing 12's
I think your best shot is post 6 and 7... we had the same idea... overall you lose 15% of your volume... How often do you play beyond 8 on your amp?
telex76 June 16th, 2011, 10:20 AM Short answer, no.
fezz parka June 16th, 2011, 10:45 AM Won't hurt it at all. Tube amps are good for a 2:1 impedance mismatch so 4 ohm amp can safely drive 3.2 ohms.
IMO, modern import transformers like Fender uses are not up to the strain of a mismatch like Schumachers and Triads were. The 100% mismatch rule doesn't apply to Modern Fenders, FWIW.
jefrs June 16th, 2011, 06:26 PM So from what you are telling me my 4 Ohm 2x12 cab would work better in terms of balancing the sound. These things are good to know.
What about those devices that change impedance so you can use a 16 Ohm cab with an amp that wants to see 4 or 8 Ohms? Would using one of those devices set at 4 Ohms result in the same volume disparity?
This is the output transformer. Some have more taps than others.
A good output transformer should be half the cost of the amp.
One of the easiest ways of changing the overall sound of an amp is to swap the speaker for another model. Perhaps the most expensive way of changing the overall sound of an amp is to swap the output transformer for another model.
You could probably drop in the output transformer from an Epi BC30 (a Bassman 2x5881 clone, or 6L6GC or EL34) with its 4ohm, 8ohm and 16ohm taps. It's the same transformer as in the SoCal50 so it is good for 50 watts or more.

