Running both speaker outs into 1 cab (Y cable)?

[gregsk777]

Quick question. I heard that running both speaker outputs in a head (say a Marshall JCM 800 or Fender head) makes the amp sound better? I know that two separate cabs can be used to push more air and get a stereo effect, but can both speaker outputs be run into a single cab using a Y adapter?

I found this link (Power Y Adapter III w/2 male legs) and it got me interested.

If this would work, how would this effect impedance?

Thanks, Greg

[Hardtailed]

The speaker jacks (assuming same impedance tap) are just jumpered together internally, there's absolutely no reason that would change anything.

As for your link, this is a cable to run two speakers from the same amp, assuming the amp only has one speaker out and the cabs don't have a parallel "through" output

[gregsk777]

I figured as much. It was just something I heard and was considering to try as an experiment but it sounds like it would not have any effect.

Thanks

[black_labb]

if you had multiple output taps on a valve amp you would have a short across a section of the output transformer which wouldnt be good.

[J M Fahey]

*Very* short answer: forget it.
As was stated above:
*if* both outputs are in parallel, you gain nothing.
*if* they are not, as in multiple taps in a power transformer or, what I'd like to add, outputs from *different* power amps, as, say, in a stereo chorus amp or stereo power amp, it's a quick and sure one-way trip to Heaven for some very expensive components.

[Kagliostro]

Assuming that the amp has only one output connection

if you want to connect two separate speaker you must build an adaptor

you can plan to connect the speaker in parallel

or you can plan to connect the speaker in series

assuming the speaker have 8 ohm of impedance

if parallel you have 4 ohm

if serial you have 16 ohm

if you want to to connect it to the amp you must select the proper output impedance in the impedance selector of the amp (if it has one)

otherwise it is not a good thing to do

if you MUST do it is better to connect to he amp a higher impedence speaker than a lower impedance speaker

as for example in a 8 ohm output (if you MUST do it) is better to put a 16 ohm speaker instead of a 4 ohm speaker

if you put a 4 ohm speaker in a 8 ohm output you half the reflected impedance seen by the tubes and this rises their current, and that is a BAD thing and you can blow tubes and OT (also if this isn't sure to happen)

Kagliostro

[pdf64]

"if you MUST do it is better to connect to he amp a higher impedence speaker than a lower impedance speaker

as for example in a 8 ohm output (if you MUST do it) is better to put a 16 ohm speaker instead of a 4 ohm speaker"

Kagliostro, you've got it the wrong way around. The reasoning you have described is good for solid state amps, ie that a load impedance above rated is ok, but working into a load below rated impedance might cause damaging currents to flow.
But tube amps are most frequently damaged by too high a voltage appearing across the output transformer primary, due to flyback emf, usually from overdriving the amp into too high of a load impedance. This can cause insulation damage in the output transformer, tube sockets etc, the worst case being an open load circuit.
Also the screen grid currents / power dissipation are increased when the load impedance is above the rated value. As the screen grids are run way over their max ratings in regular guitar amp when overdriven, pushing them even harder by driving into an above rated load can cause the tubes to wear out very quickly.
Test this for yourself by measuring the voltage (ac and dc) across the screen grid resistors on a cranked amp with different loadings.
Whereas a tube amp will tolerate a below rated load impedance very well, the cathode and screen grid currents aren't significantly higher.
To summarise, the worst case scenario for a tube amp is an open circuit for a load, whereas the worst case for a solid state amp is a short circuit for a load. With a tube amp, it is better to connect a load impedance below the rated value, rather than above the rated value
That's why many tube amps have shorting jack sockets for speaker outputs, so that if the user forgets to plug the speaker in, the amp is driving into a short circuit rather than an open circuit. And other amps have a resistor (about 470ohms) wired permanently across the speaker output, so as to avoid an open load circuit in all circumstances.
Most tube amps will be fine working into a 1:2 or 2:1 load discrepancy (ie an 8 ohm rated amp will cope with a load range 4 - 16 ohms), but don't go outside those limits when there's any chance of the amp working hard.

[Kagliostro]

Maybe pdf64 is right and I'm wrong


but if I'm wrong why Mesa Boogie in their manual say to put a 8ohm speaker in the 4ohm output and a 16ohm speaker in a 8ohm output saying that is a safe mismach ??

see page 19 of this Mesa Boogie Manual

http://www.mesaboogie.com/manuals/2-100.pdf

and page 31 of this

http://www.mesaboogie.com/manuals/3chRecto.pdf

and so on in many others ??

please can you explain me why they don't say to put a 4ohm speaker in a 8ohm output or a 16ohm speaker in a 8ohm output as you say ?

That's why many tube amps have shorting jack sockets for speaker outputs, so that if the user forgets to plug the speaker in, the amp is driving into a short circuit rather than an open circuit. And other amps have a resistor (about 470ohms) wired permanently across the speaker output, so as to avoid an open load circuit in all circumstances.

please can you give me examples of amps that have shorting jack sockets for speaker outputs

Thanks

and also thanks for any further explanation, i'm just a beginner and is easy to see things backward

Kagliostro

[Gaz]

Uh, I think you guys are saying the same thing. But to clarify: connecting a larger speaker load into a lower output tap is a 'safer' mismatch.

i.e.:

8 ohm tap/16 ohm speaker

or

4 ohm tap/8 ohm speaker


This would increase the reflected primary impedance to double (i.e. 5k to 10k). The inverse would reduce the reflected primary impedance by half (i.e. 5k to 2.5k).

The general consensus is that a 50% mismatch, i.e.:

8 ohm tap/16 ohm load

or

8 ohm tap/4 ohm load

are okay if the transformer is well-built. Most modern transformers are, and mismatching is not recommened (especially a smaller load into a 'larger' tap) with most vintage amps.

Some companies are so confident in their transformer quality that they use the impedance selector as a tone switch.

Of course the mismatch is 'worse' if the primary impedance is low for a tube set to begin with, but still I've only damaged tubes accidentally running a 16 ohm load to a 4 ohm tap. I'm sure the other way round would have been much worse.

[hasserl]

No they're not. pdf64 is correct in saying that the safer mismatch is with a lower impedance load than optimum. I.e. a 4 ohm speaker w/ an 8 ohm output. And that is due to the reflected voltage as he described above. He spelled it out very well, and it is all correct. High reflected voltages and increased screen grid current are the result of applying a too high load on the amp.

So what is too high? Good question, that depends on the amp I suppose, as some tend to be less tolerant of this than others. But commonly double the rated load seems to be safe, and is commonly done. Which explains Mesa Boogie's recommendation mentioned by Kagliostro above. As for what amps use shorting jack sockets for speaker output's, Fender for one. Take a look at any vintage Fender amp schematic at the speaker output jacks, you will see a switching type jack with the switch connected to ground, in case no speaker cord is inserted there will be a path to ground, and no harm is done to the amp.

[J M Fahey]

Some very short answers:
1)

why Mesa Boogie in their manual say to put a 8ohm speaker in the 4ohm output and a 16ohm speaker in a 8ohm output saying that is a safe mismach ??

Because it *is* a mismatch, and it *is* safe (duh)
2)

please can you explain me why they don't say to put a 4ohm speaker in a 8ohm output or a 16ohm speaker in a 8ohm output as you say ?

I don't know what goes on in somebody else's mind, yet putting a 4 ohms speaker into an 8 ohm output *is* also a "safe mismatch".
3)

please can you give me examples of amps that have shorting jack sockets for speaker outputs

For one, all classic Fenders, and many others.
Surprisingly, classic Marshalls did not.
I have replaced and rewound *many* Marshall output trannies, and some Mesas, very few Fenders, and those are usually traced to shorted tubes coupled to grossly oversized line fuses, including some pulled from a car/motorcycle or neatly rolled aluminum "cigarette pack paper".
Statistics tend to crushingly favor Fender's approach (if in doubt, go down in impedance) rather than the opposite way.
If I may add a very simplified "truth", quite open to discussion and dissent, I have a general rule: higher impedance is going towards open circuit, which kills transformers: very expensive, difficult to even impossible to get, in *many* (most) cases nobody knows the exact specs , and so on; while lower impedance goes towards shorted output, which doesn't brutally kill output tubes, certainly wears them faster, no doubt about that, but they are *much* cheaper, easily available, "universal" (only very few types are actually used), and, best of all, you don't even need to solder, because they are "plug-in" .
PS: the line

Never run below 4 ohms in a tube amplifier unless you are absolutely certain that the system can handle it properly; this can cause damage to the Output transformer.

in the Recto Manual isn't exact. *Maybe * a few extra mA will run through its windings, not that much more, because tubes are naturally "current limited"
It will definitely stress output *tubes*.

[Gaz]

Oh dear. It's so easy to get this muddled. Please excuse the misinformation. In fact, just reverse what I said towards the beginning about a "safer" mismatch. It's funny because my personal anecdote at the end goes against what i stated.

[Kagliostro]

Thanks to all for the useful explanation

I got also this explanation and as it explains the same thing with different words and this helped me to understand better, I post it here hoping it can help some other

If you decrease the load on the output tubes, the loadline cuts above the maximum dissipation for that tube, and could result in overheating. It depends on how far over maximum dissipation the tubes run and how long they stay there during each cycle. In class AB, you might get redplating but not usually. In class A, the power dissipated by the tube is lower at max output, so you'd really have to run over max dissipation to make the tubes redplate. It is always a good idea when mismatching loads to keep an eye on the output tubes and see if they tend to overheat in your particular situation.

For a higher-than-spec load, you might have issues with screen dissipation, because the loadline will be such that you cut well below the knee of the plate curves and in this area plate current drops dramatically but screen current can skyrocket. Screen damage is only likely if you do not have screen resistors and the power supply node feeding the screens has low series resistance. In general, screen resistors will cause a voltage drop with increased current that keeps screen dissipation in check. And since the speaker's impedance is not constant (often rising significantly at the bass resonant frequency and at highs), part of the time you're playing with a higher load than you think anyway.

so, at the end, Mesa can say that because they know their particular amp situation

a different amp = a different draw = a different situation = a different solution

Kagliostro

[J M Fahey]

You're quoting a great truth here.
Some usually equivalent mismatches may not be so in certain cases, if the designer chose, for particular reasons , to deviate from "normal" or "accepted" values into something else.
As I said, I have no way to know what was Randall Smith trying to achieve in that particular amp. Some love it, some hate it.
Happy 2010.

[wizard333]

I would think it depends on the plate impedance the tubes are going to see and the tube type.

Good example is a DR OT. ~6500 plate impedance with 8 ohm load. Lots of folks put 6L6s in them. 6500 is on the high side for 6L6s but ok; if you ran a 4 ohm load you'd be on the low side but ok, sort of one end of the range or the other. You wouldnt want to run a 16 ohm load, 13k is more than 6L6s want to see. If you were running the stock 6V6s; they should be just fine with a 16 ohm (13k at the plate) load, especially at the voltages in a lot of DRs, where I've routinely seen 465v at the plates (and more). They probably would not do well with a 4 ohm load (3250 plate impedance).