2 heads into 1 cab at the same time?

[L3stat]

Hi all, got some noob questions on running 2 heads. I got a Univalve running into a 2x12 8ohm cab. The 2x12 cab has 2 input jacks, and is labelled "parallel", which seems to imply 2x16ohm speakers wired in parallel to give 8ohms (why 2 input jacks?).

Am thinking of swapping my other combo amp into the head version, to use with this same cab (2 inputs). I know some basics, like amps need to be operated with a load, or damage results.

1. Can I plug the outputs of both heads into the 2 input jacks of that cab?
2. With 1. above, can I play both heads simultaneously, like if a friend or teacher comes by? (Internet literature seems to imply this is a huge no)
3. With 1. above, will there be issues if I plug both, but play only 1 head at a time?
4. As with 3., any issues if I play 1 head, but turn the other on (but not play it, like using an A/B/Y box)?

Where I'm coming from, I'm trying to preserve space by converting a combo into a head, and using an existing cab to run 2 heads. I've read about the Tonebone and similar options, but am trying to avoid spending more cash if I can do the same just by being careful (keeping one head off while the other is on).

 

[KissMyAxe]

Nice title you got there bro

 

[L3stat]

LOL. Hopefully it doesn't conjure anything sexual to anyone. :mrgreen:

 

[lightice]

If you have a multimeter, you can test the impedance of each speaker if it is not written down.

You can utilise both jacks, by disconnecting both speakers, and wiring each to an individual jack. I'm assuming the speakers can take the power output and there are no impedance mismatch.

Worst case scenario if you want to run 2 heads into 1 cab, is to change speakers to those that can handle the individual amp's power and get the correct impedance.

Hope this helps.

To answer some of your questions directly:

1. Can I plug the outputs of both heads into the 2 input jacks of that cab?
No, without disconnecting the 2 speakers and wiring them individually to each jack.

2. With 1. above, can I play both heads simultaneously, like if a friend or teacher comes by? (Internet literature seems to imply this is a huge no)
No

3. With 1. above, will there be issues if I plug both, but play only 1 head at a time?
Yes, switching the amp on and not playing will still require a load; dummy or speaker

4. As with 3., any issues if I play 1 head, but turn the other on (but not play it, like using an A/B/Y box)?
I won't use A/B/Y, only A/B, and the A/B box must have a switching dummy load. Assuming you switch to A, the B signal gets pushed to a dummy load, and vise versa.

 

[L3stat]

Thanks for the info. Can you explain the theory behind not sharing of (speaker) loads for 2 amps? I get the part about having a load, just don't understand how things change when 2 amps get plugged to 1 cab with 2 inputs. Impedance wise I have to check, should be fine.

3. With 1. above, will there be issues if I plug both, but play only 1 head at a time?
Yes, switching the amp on and not playing will still require a load; dummy or speaker

Refining the question a little, any issues if I connect both heads, but at any one point, only one amp is played, the other is off / not powered?

4. As with 3., any issues if I play 1 head, but turn the other on (but not play it, like using an A/B/Y box)?
I won't use A/B/Y, only A/B, and the A/B box must have a switching dummy load. Assuming you switch to A, the B signal gets pushed to a dummy load, and vise versa.

For this, you mean the A/B is between the heads and the cab right?

 

[lightice]

If you are sharing 1 cab with 2 amps, the 2 amp's are connected in parallel.
The impedance will be equal to (1/A + 1/B)^-1 , while the speakers are rated as is.

In practical terms, if your amp's impedance is 16ohms each.

( 1/16 + 1/16 )^-1 = 8 ohms

Assuming your cab is 16ohms , that is a mismatch. Also, there is a high chance you will overload and burn the voice coil on the speakers if it's power handling cannot match 2 amps. Not to mention the strain on the OT of the amp.

Also tube amps are rated RMS, not peak output. So a 50W amp can actually push out 90W of power when cranked. My personal rule of thumb is to get speakers rated at least 50% extra of the amp's RMS power.

I don't suggest connecting 2 amps into 1 cab, whether its on at the same time or 1 on, 1 off. You don't want to fry the output transformers on the amps. You'll need something to keep the 2 amp's circuits separate.

I suggest getting a weber 2 head amp switch for this application.

Yes the A/B is between heads and 1 cab. No Y, because Y = A+B combined signal.

I'm not sure if what I mentioned made any sense to you. But I think randolf can help explain things better if he does pop by this thread.

Oh btw, I forgot to add that the speaker actually completes the amp's circuit, so it doesn't just end at the "Output" of the amp.

 

[gsonique]

You can wire your cab with two sererate inputs where one master (activates 2 speakers wired to choice) and a slave (cuts connection and renders both inputs as single dedicated to one speaker).

You take it futher by putting a switch for series/parellel or two.......ok it gets a bit confusing from here on

 

[L3stat]

[=lightice

Thanks for all the advice, it's clearer now. It wasn't obvious to me that even when the inactive head is not powered, the connection with its innards screws with the load for the other one. Am I right in that interpretation?

Dang! Gotta spend money again!

 

[L3stat]

[=gsonique

Thanks goose, I'll look into that when I'm brave enough to explore the idea.

 

[Ratboy]

i think the parallel plug is for wiring a cable out to another cab

 

[lightice]

Somewhat right, you don't want to send speaker line power (X watts) into another amp's output transformer.

 

[L3stat]

Thanks fellas!