Mark IV 8-4-4 speaker jack load with multiple cabinets

I am buying a V30 for my Mark IV combo and was curious about impedance.

I plan on running the built in speaker for practice and a cabinet for shows.

The manual says if you connect a 4 ohm cabinet in each 4 ohm jack then the amp treats them as parallel jacks making the amp output 2 ohm for each jack. So I would want to plug in 2 8 ohm cabinets to make the output jacks 4-4.

The manual does not address the 8 ohm jack. With a single 8 ohm speaker plugged into the 8 ohm jack plus a 4 ohm cabinet in a 4, will this lower the output impedance on the 8 ohm like it would if you ran 4x4 in parallel?

So ..
8 ohm jack/speaker = 8 ohm load
4 ohm jack/cabinet = 4 ohm load
2 x 4 ohm jack with 2 x 4 ohm cabinet = 2 ohm total load "this is bad"
8 ohm jack/speaker + 4 ohm jack/speaker cabinet = ??

Would it needs to be ..
16 ohm jack/speaker + 8 ohm jack/speaker cabinet = ??

In the end I guess the question I am asking is does the Mark IV drop the impedance by half, like in the 4+4 usage, if the 8 and 4 ohm jacks are used at the same time with matching 8/4 ohm speaker setup?

Also can I unplug the combo speaker and plug an 8 ohm speaker cabinet into the 8 ohm jack, leaving the built in speaker silent?

Also - Also .. Can I use all output jacks at the same time? And if I do should the configuration be
8 ohm speaker, 8 ohm cabinet, 8 ohm cabinet?
Or 16 ohm speaker, 8 ohm cabinet, 8 ohm cabinet?

Hope this was not terribly confusing,
Donnie

donniecrump said:

I am buying a V30 for my Mark IV combo and was curious about impedance.

I plan on running the built in speaker for practice and a cabinet for shows.

The manual says if you connect a 4 ohm cabinet in each 4 ohm jack then the amp treats them as parallel jacks making the amp output 2 ohm for each jack. So I would want to plug in 2 8 ohm cabinets to make the output jacks 4-4.

Click to expand...

Yes, but it "doesn't make the jacks 4-4". Two 8 ohm cabs in parallel equals 4 ohms [(8 x 8 ) / (8 + 8 ) = 4]. A single 4 ohm load should be plugged into the 4 ohm tap; two 8 ohm loads in parallel equals 4 ohms and should be plugged into the 4 ohm taps.

donniecrump said:

The manual does not address the 8 ohm jack. With a single 8 ohm speaker plugged into the 8 ohm jack plus a 4 ohm cabinet in a 4, will this lower the output impedance on the 8 ohm like it would if you ran 4x4 in parallel?

Click to expand...

The impedance of the output taps doesn't change when the load changes; determine what load the amp is seeing and select the proper taps to best match the load. These jacks are drawn from different points on the output tranny winding. See also this recent thread: http://forum.grailtone.com/viewtopic.php?f=3&t=68422

donniecrump said:

So ..
8 ohm jack/speaker = 8 ohm load
4 ohm jack/cabinet = 4 ohm load
2 x 4 ohm jack with 2 x 4 ohm cabinet = 2 ohm total load "this is bad"
8 ohm jack/speaker + 4 ohm jack/speaker cabinet = ??

Click to expand...

An 8 ohm and 4 ohm load in parallel results in a 2.67 ohm load: (8 x 4) / (8 + 4). You would plug these into the 4 ohm taps, but it's bordering on an "unsafe mismatch"

donniecrump said:

Would it needs to be ..
16 ohm jack/speaker + 8 ohm jack/speaker cabinet = ??

Click to expand...

16 + 8 ohm loads in parallel = 5.33 ohms (128/24), and represents a safe mismatch when plugged into the 4 ohm taps.

donniecrump said:

In the end I guess the question I am asking is does the Mark IV drop the impedance by half, like in the 4+4 usage, if the 8 and 4 ohm jacks are used at the same time with matching 8/4 ohm speaker setup?

Click to expand...

Calculate the load and determine which output taps line-up best. Again, you're not changing the impedance of the taps but selecting which taps to use based on the load the amp sees

donniecrump said:

Also can I unplug the combo speaker and plug an 8 ohm speaker cabinet into the 8 ohm jack, leaving the built in speaker silent?

Click to expand...

Yes

donniecrump said:

Also - Also .. Can I use all output jacks at the same time? And if I do should the configuration be
8 ohm speaker, 8 ohm cabinet, 8 ohm cabinet?
Or 16 ohm speaker, 8 ohm cabinet, 8 ohm cabinet?

Hope this was not terribly confusing,
Donnie

Click to expand...

Not sure, but I believe three 8 ohm loads in parallel is also a 2.67 ohm load, which would be an unsafe mismatch (see the impedance calculator below). You could run two 8 ohm cabs into a series box that would result in 16 ohms, which when paired with the other 8 ohm speaker results in 5.33 load, and should be plugged into the two 4 ohm taps. If the speaker cabs have series chaining jacks, you would not need the serial box. Here's an impedance calculator:

http://colomar.com/Shavano/impedance_proc.php

Read some of the narratives links below the calculator as well.

You've got to be careful when mixing loads,i.e using an 8ohm and 4ohm speaker in parallel to get 2.6ohms.In this scenario the wattage wont be distributed evenly,if you had 2x25 watt speakers and expected to cover 50 watts output power,one speaker will get twice the power of the other,or 2/3 of the 50 watts which will exceed 25 its rated for.

stokes said:

You've got to be careful when mixing loads,i.e using an 8ohm and 4ohm speaker in parallel to get 2.6ohms.In this scenario the wattage wont be distributed evenly,if you had 2x25 watt speakers and expected to cover 50 watts output power,one speaker will get twice the power of the other,or 2/3 of the 50 watts which will exceed 25 its rated for.

Click to expand...

...And in this scenario (8 and 4 ohms paralleled) the 4 ohm speaker will be louder.
You can't fool Mother Nature.

MrMarkIII said:

stokes said:

You've got to be careful when mixing loads,i.e using an 8ohm and 4ohm speaker in parallel to get 2.6ohms.In this scenario the wattage wont be distributed evenly,if you had 2x25 watt speakers and expected to cover 50 watts output power,one speaker will get twice the power of the other,or 2/3 of the 50 watts which will exceed 25 its rated for.

Click to expand...

...And in this scenario (8 and 4 ohms paralleled) the 4 ohm speaker will be louder.
You can't fool Mother Nature.

Click to expand...

Obviously,if it is getting twice the wattage.