Ohm mismatches....

[Sporky_McGuffin]

I've stickied it - you should all still be able to reply and such, but it'll be easier for people to find.

[bigdawg]

thanks sporky...

DOes anyone know if its possible to measure ohms at the amp end - if I attach a meter to the + & - of the speaker itself will that give me the reading...

[keithm]

one thing that hasn't been mentioned: if you have two mismatched speakers the lower ohmage one will always take a bigger percentage of the power. So 2 x 8 ohms share the power but 1x 8 and 1 x 4 - the 4ohm will take more of the power. That's important because if you are adding an extension cab to give more bottom end and the lower ohmage speaker is in the combo - it won't really work as the extention cab won't be driven properly. What I don't get is: if the express has a 4 ohm internal speaker then it shouldn't have a socket for an external one. I suspect the internal speaker is 8 ohms and you can add a second 8 ohm speaker = 4 ohms in total.

[bigdawg]

internal speaker is 8ohm - my ext cab is running 2 8ohm wired to give 4, so I was going to use the 2 4 ohm outs (it also has an 8ohm out that the int speaker is attached to)

[digitalscream]

internal speaker is 8ohm - my ext cab is running 2 8ohm wired to give 4, so I was going to use the 2 4 ohm outs (it also has an 8ohm out that the int speaker is attached to)

Nononononono.

*Either* plug one or two 4 ohm speakers into the 4 ohm sockets, *or* use the 8 ohm out. Don't use both. I'm not sure what the exact result of doing what you propose would be, but I'm pretty sure it would be very, very bad.

If your cab is wired to give 4 ohms, you just plug it into one of the 4 ohm outputs.

Basically, you can have one or two 4 ohm loads, or one 8 ohm load. Essentially, you can only have one set of outputs. For example, my Laney has 1 x 16 ohm, 2 x 8 ohm and 2 x 4 ohm outputs. That doesn't mean that I can use all of them at the same time to power 5 cabinets, which would almost certainly fk up the output transformer and valves.

[bigdawg]

thats what I was going to do, just worded it badly...

[keithm]

I don't think that is going to work as it would take the ohmage down to about 2. You could wire the 2x12 to 16 ohms but it will take very little of the power or disconnect one speaker and you would have an 8ohm cab.

If the cab is good why not use it on it's own with the internal speaker disconnected?

[digitalscream]

thats what I was going to do, just worded it badly...

Try not to do that. It is a sticky, after all

[bigdawg]

I don't think that is going to work as it would take the ohmage down to about 2. You could wire the 2x12 to 16 ohms but it will take very little of the power or disconnect one speaker and you would have an 8ohm cab.

If the cab is good why not use it on it's own with the internal speaker disconnected?

Apologies - you are right - for some reason I keep think my cab is 8ohm... I think Im losing it..

Any ideas on measuring ohms and the speaker end??

[jpfamps]

Firstly I would like to re-iterate:

"IT IS VERY HELPFUL TO KNOWN WHAT AMP YOU ARE TALKING ABOUT WHEN ASKING FOR ADVICE"

This is especially important when talking about impedance matching because:

VALVE AND TRANSISTOR AMPS HAVE DIFFERENT IMPEDANCE MATCHING ISSUES


In valve amplifier, for maximum power transfer from the amp to the load (speaker/s) the impedance needs to be matched to the load (actually this true for transferring power between any load and source).

The impedance of a loudspeaker varies considerably with frequency, so any matching is nominal, however we do the best we can.

If you hook up too low a load to a valve amp, then the dissipation in the valves will go up, and this can cause them to be damaged, and possibly the higher current draw could damage the transformer.

If you hook too high a load up to a valve amp, then you:

1) increase the voltage swing on the plates, which can cause the interwinding insulation on the transformer to breakdown. I've even seen evidence of external arcing from an amp where the speakers had died, and subsequently presented an open circuit, ie an infinite load.

2) you increase the screen grid current particularly at clipping. This is real problem in guitar amps as the screens are usually run at the same voltage as the plate, and they are often driven into clipping. I've seen arcing in EL34s driven into too high a load without a screen current limiting resistors.

Depending on the quality of the transformer; the plate, screen voltages and bias point of the amp; and primary impedance of the output transformer you can get away with a certain degree of "mismatch", however the safest way of running a valve amp is with a matched load, and this also produces maximum power.

Incidentally I think the sound the best too.

Transistor amps aren't transformer coupled, and have very low output impedance. This means that maximum power transfer would occur at a very low load, probably around half and ohm. Of course (assuming no output protection) such a load would result in the immediate destruction of you amplifiers output transistors!

Thus, in order to protect the output devices from excessive dissipation, transistor amplifiers are run into a higher loads than gives maximum power transfer, and as there are no screen current and flyback voltage issues this is perfectly safe.

Transistor amp are specified for a minimum safe load (because this gives the biggest power measurement, as so looks better).

Running a transistor amplifier into any load higher than it's minimum rating is perfectly safe, but will result in loss of power.

If a transistor amp is marginally specified then even running it into its lowest rated load can cause failure.