Ohm mismatches....

[jpfamps]

Basically, with a high mismatch (eg 8-ohm amp into 16-ohm cab), there is more of a risk of flyback voltages causing arcing in either the power valves or output transformer... the latter is catastrophic and usually permanent, and expensive. Valves sometime survive it if the arc occurs there. With a low mismatch (eg 16-ohm amp into 8-ohm cab), more current is drawn which will stress the power valves more; this can sometimes lead to valve failure and in a worst case can also then damage the amp, but is less likely to fry the output transformer... usually screen resistors or similar cheaper parts.

A high mismatch will also cause screen grid current to rise significantly, which also promotes valve failure.

[jpfamps]

Not sure if this is mentioned somewhere else, but can I hook up the 16ohm internal speakers in my combo AND my external 8ohm 2x12? if so would I be best to use 2 4ohm sockets?

You can hook up a 16 ohm and 8 ohm cab to an amp and present the correct impedance, however you will need to have access to seperate 8 ohm and 4 ohm taps.

The 16 ohm cab should be hooked up to the 8 ohm tap and the 8 ohm cab to the 4 ohm tap.

I usually don't advise doing this due to potential for confusion!

[Viz]

Thanks you guys for such informative answers. god this forum can be good

[Professor SourTone]

Can't recall if I posted this before, but this is a very useful bit of kit. I believe its just a multi-tapped transformer.

https://taweber.powweb.com/store/zmatch.htm

I use it to run a Dual Showman (4ohms) into an old Marshall 2x12 (8ohms)

It has balanced and unbalanced line outs as well, which could be handy. And another output for running two cabs.

[ecc83]

Can't recall if I posted this before, but this is a very useful bit of kit. I believe its just a multi-tapped transformer.

https://taweber.powweb.com/store/zmatch.htm

I use it to run a Dual Showman (4ohms) into an old Marshall 2x12 (8ohms)

It has balanced and unbalanced line outs as well, which could be handy. And another output for running two cabs.

Whilst no half decent valve amplifier will be bothered by the above device do not ever connect it to a solid state power amp!

It just might be safe on a very old design with an output capacitor but not on anything DC coupled!

Dave.

[dilbert]

This has probably been asked in another form so excuse me for appearing to be an idle sod and not having read the entire thread but I got a headache by Page 3 .

Anyway, here is the query: I have a Peavey Classic 30 which, I'm sure you are aware uses a 16 ohm internal speaker, there is the facility to plug in an external speaker, also requiring a 16 ohm impedance which will give a load of 8 ohms, that much I get.

I also understand that I can:

1. plug an 8 ohm speaker into the external socket provided I pull the leads from the internal speaker and tape them.
2. Fit an 8 ohm speaker internally provided I fit a dummy plug into the external socket.

But........ practically speaking, how much damage would occur if I were to run an 8 ohm external speaker with the internal speaker still connected ?. Whilst idly surfing I have read many answers to this most of the recent ones claiming that unless the amp is kept dimed, it should be fine. A couple have actually quoted Peavey service reps as confirming it on the Q.T.

I'd like to try out my Classic 30 with my Laney 1x12 cab at low volume as an experiment but I really don't want to have to replace the OT as a result !.

Thanks for your' help.

dilbert.

[ecc83]

You will do no harm running a 16 and an 8Ohm speaker on the 8 Ohm jack of that amp.

The combined impedance is 5.33 Ohms and near enough to 8 as to make no difference. IF you were to drive the bllx off the amp for nights on end you would, in theory, shorten the life of the op valves but in practice you would never tell!

There would be slightly more of a concern using an 8 R tap into a 16R load but again, in practice it hardly matters.

Manufacturer's lawyers insist on these arse saving instructions.

N very B you should NEVER run a solid state amp into less than the rated impedance, usually 4Ohms but sometime s 2R for bigass bass and PA jobs. Also NEVER connect transformer devices to sstate amps. Valves don't give a toss.

Dave.

[dilbert]

You will do no harm running a 16 and an 8Ohm speaker on the 8 Ohm jack of that amp.

The combined impedance is 5.33 Ohms and near enough to 8 as to make no difference. IF you were to drive the bllx off the amp for nights on end you would, in theory, shorten the life of the op valves but in practice you would never tell!

There would be slightly more of a concern using an 8 R tap into a 16R load but again, in practice it hardly matters.

Manufacturer's lawyers insist on these arse saving instructions.

N very B you should NEVER run a solid state amp into less than the rated impedance, usually 4Ohms but sometime s 2R for bigass bass and PA jobs. Also NEVER connect transformer devices to sstate amps. Valves don't give a toss.

Dave.

Dave,

many thanks, I appreciate the benefit of your knowledge and more importantly your' experience. I'll hook up the can and give it a go and bugger the neighbours ................ well, not physically

Thanks again.

dilbert.

[smudge_lad]

hey all,

I'm leaving for rehearsal soon, and planning on running my Egnater Tweaker 40 head through one of the Marshall 4x12 cabs in the studio.

No idea of the ohm rating on the cab until I get there (and maybe not even then???) and don't really understand much about how it all works. The Marshall head is an AVT150, guessing the cab is a basic, low end 4x12.

My Egnater has a switch for 16/8/4 ohm. It's currently set to 16, which runs into one of the sockets on the back of my Egnater 1x12 cab - also already 16ohm.

If the cab is 16 ohm, do I just leave the amp on the 16ohm setting? Or since there are 4 speakers, do I need to change anything???

[ecc83]

hey all,

I'm leaving for rehearsal soon, and planning on running my Egnater Tweaker 40 head through one of the Marshall 4x12 cabs in the studio.

No idea of the ohm rating on the cab until I get there (and maybe not even then???) and don't really understand much about how it all works. The Marshall head is an AVT150, guessing the cab is a basic, low end 4x12.

My Egnater has a switch for 16/8/4 ohm. It's currently set to 16, which runs into one of the sockets on the back of my Egnater 1x12 cab - also already 16ohm.

If the cab is 16 ohm, do I just leave the amp on the 16ohm setting? Or since there are 4 speakers, do I need to change anything???

The cab is going to be either just 16Ohms, just 4 Ohms (very unlikely) or switchable/pluggable to 16/4 and 2x 8R stereo. So the Egnator can be set for any eventuality!

Dave.