G12-65H: 16 Ohm or 8 Ohm, any difference?

[CHIP]

So.... A 16 ohm speaker will work with either the 8 ohm or 16 ohm OT tap with out any problems.

[Max]

So.... A 16 ohm speaker will work with either the 8 ohm or 16 ohm OT tap with out any problems.

Hi Chip,

That is at least how I understood the advice of the Electrovoice tech, that - from a technical point of view - there is no valid reason not to use an ODS 100 Watt Combo with a 4 Ohms OT with just one (built in) 8 Ohms EVM 12 L, if I like the sound of this combination and don't need urgently the maximum loudness (if you measure say an output of 100 watts with two 8 Ohms speakers in parallel, he told me, you will get around 70 watts if you use just one 8 Ohms speaker in the 4 Ohms output.

If this drop in wattage results in an audiable drop in loudness depends on things like placing of the extension cab (phase cancellation, reflections from walls), dimensions of the extention cab, kind of enclosure of the extension cab (open back, closed back, Thiele...). If things add up in a unfortunate way, you can even end up with less loudness in the last row of the club if you use an extension cab).

As I understood this EV-tech in the end it is just a matter of personal taste if you use two 8 Ohms speakers in parallel in an 4 Ohms OT output or just one 8 Ohms speaker (as long as this ones coil can take the output wattage of course). Both setups are O. K. from a technical point of view and will do no harm to the speakers or the amp. That is at least, what he told me about this

All the best

Max

[Max]

double

[jelle]

So.... A 16 ohm speaker will work with either the 8 ohm or 16 ohm OT tap with out any problems.

This holds true for a amp with old fender iron like the 70's ODS. In a Marshall you'll end up blowing up your output stage at high levels.

Have fun!

Jelle

[Max]

So.... A 16 ohm speaker will work with either the 8 ohm or 16 ohm OT tap with out any problems.

This holds true for a amp with old fender iron like the 70's ODS. In a Marshall you'll end up blowing up your output stage at high levels.

Have fun!

Jelle

Hi Jelle,

Yes, this of course may be (I don't know). But the advice of the Electrovoice tech was obviously meant by him as a general one (This is at least how I understood it) and not only for the 4 Ohms Fender OTs in many of the ODS (BTW: These you find also in many "blackface" ODS from the early up to mid eighties, like e.g. #124).

I can't judge this advice anyway, but I thought, telling it here, could perhaps be of interest to some. In my talk with this tech about this topic I had the impression, that he was serious about what he said to me and knew his stuff (was just a personal impression, so of course it may be wrong).

Have fun too

Max

[Fischerman]

Hi Fisherman,

if you are that certain about this, how do you explain, that HAD (Or do you think he isn't someone "that really knows his stuff"?) delivered most (if not all) of his "silverface" ODS Combos (50W and 100W) with 4 Ohms OT (Bassman, Dual Showman) and 8 Ohms speaker (Altec, EVM).

I have never heard of someone using these early amps without a switchable (8 Ohms/4Ohms) version of OT to complain about blowing transformers or blowing tubes because of a speaker mismatch.

Obviously the opinion of Mr. Dumble in this matter was: If you prefer to use it as a Combo with just the built in 8 Ohms speaker (and this is the reason why there are combo-amps), feel fine and play some good tunes; if you prefer to use it with an extension cab (to get a bit more loudness or a different sound or whatsoever), feel fine too.

Why do you think HAD did wrong in using 4 Ohms OT in combination with 8 Ohms speaker? And what is from your point of view wrong with the arguments of the Electrovoice tech I asked about this (see my last post in this thread)? Could you (or someone else) explain this to me?

Hi Max,
You can usually get away with running the amp 'one notch' up or down...I do it all the time. Running the amp at a different impedance than what it 'expects' will change the tone...no question about it. For example, running one of those old SF ODS amps through an 8 ohm speaker will sound different than running it through the same model 4 ohm speaker (even if the speakers theoretically sounded identical). Running at the 'incorrect' impedance will lose some bandwidth in addition to losing volume and punch. Maybe HAD liked the way they sounded better with an 8 ohm load...I dunno. I'm not speculating on why he did what he did. But if you run the amp 'two notches' from what it expects (like a 4 ohm OT into a 16 ohm load or a 16 ohm OT into a 4 ohm load) then you're getting out of the safe zone and the question was...which is worse? And that was when the debate ensued...same debate I remember having on Ampage in the 90s (and having Ken Gilbert or Randall Aiken come on to straighten us all out ).

As far as HAD 'knowing his stuff'...I really have no idea. Guys like Aiken or Ken Gilbert have written enough to demonstrate their knowledge but all I've ever heard from HAD is stuff about 'fragile harmonics' etc. so I have no idea the depth of his knowledge. He designed/built some great, unique amps...that's about all I got to go on.

[butwhatif]

Mr Dumble is a common sense guy, and knows that if you're going to hook up a second speaker to one of his amps, it means most likely that you're looking for more oomph, and need to have the output power curve working for you, not against. If you ever put an amp on the bench and measured the output power vs the impedance mismatch, you'll understand. The tone is in your hands, not the impedance mismatch, check it out.

[butwhatif]

2x

[Fischerman]

If you ever put an amp on the bench and measured the output power vs the impedance mismatch, you'll understand. The tone is in your hands, not the impedance mismatch, check it out.

What do you mean by this...I'm not sure I understand you. Are you saying that an impedance mismatch sounds exactly the same as when it is matched? It doesn't.

I just mistakenly did that this weekend! I was playing my Rocket and then measuring/scoping the amp. Playing into an 8ohm cabinet but scoping/measuring into a 25ohm load. I forgot to switch the impedance selector and scoped/measured the amp set at 8ohms instead of the usual 16ohm setting. It measured different...even the clipped wave on the scope looked different. It was actually the difference in the clipped wave on the scope that first tipped me off that something was amiss.

[butwhatif]

Of course it will, you have a whole different set of physical circumstances. What is the clean available measured power output at proper setting, and at mismatch?

[Fischerman]

Can't remember the mismatch number...I just noticed it was lower than expected and immediately realized why.

I just didn't understand your; 'The tone is in your hands, not the impedance mismatch' comment. It sounded like you were saying it sounds the same.

[thyx]

Therefor, too small a load and the OT will fry.

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

This will protect it since the amp will blow a fuse first, Flyback spikes of no load is what kills a transformer. Quickest way to blow a transformer is with no load, not to be confused with a higher load impedance. At least that is my experience. I have replaced many OT for old Marshalls and the most common cause was , there was no load on the amp.

You place a lot of faith in the ability of the fuse to protect the OT. More modern amps this would be true...but not an old BF Fender or plexi. Those old amps the one fuse protects against high current-draw by the PT. I'm not sure when they started adding the second fuse to protect the OT.

[thyx]

As far as HAD 'knowing his stuff'...I really have no idea. Guys like Aiken or Ken Gilbert have written enough to demonstrate their knowledge but all I've ever heard from HAD is stuff about 'fragile harmonics' etc. so I have no idea the depth of his knowledge. He designed/built some great, unique amps...that's about all I got to go on.

I kind of go the other way on this. HAD built some amps that demonstrated his knowledge...how do amps by Aiken, for example, compare? Just because you write a lot doesn't mean you know a lot. Sometimes it just means you like to write...or perhaps for others to read what you wrote.

[drz400]

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

This will protect it since the amp will blow a fuse first, Flyback spikes of no load is what kills a transformer. Quickest way to blow a transformer is with no load, not to be confused with a higher load impedance. At least that is my experience. I have replaced many OT for old Marshalls and the most common cause was , there was no load on the amp.

You place a lot of faith in the ability of the fuse to protect the OT. More modern amps this would be true...but not an old BF Fender or plexi. Those old amps the one fuse protects against high current-draw by the PT. I'm not sure when they started adding the second fuse to protect the OT.

Well I'm not saying either is good but no load the amp goes crazy, look at it on a scope. I have toasted a few Marshalls myself when people kicked out the speaker cable on stage, and they had HT fuses. If you have a HT fuse and a short to ground you will pop the fuse. If you have no load you will not. There is a reason the Fenders were set up to short to ground with no speaker load. It was safer than no load.

[drz400]

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

This will protect it since the amp will blow a fuse first, Flyback spikes of no load is what kills a transformer. Quickest way to blow a transformer is with no load, not to be confused with a higher load impedance. At least that is my experience. I have replaced many OT for old Marshalls and the most common cause was , there was no load on the amp.

You place a lot of faith in the ability of the fuse to protect the OT. More modern amps this would be true...but not an old BF Fender or plexi. Those old amps the one fuse protects against high current-draw by the PT. I'm not sure when they started adding the second fuse to protect the OT.

Well I'm not saying either is good but no load the amp goes crazy, look at it on a scope. I have toasted a few Marshalls myself when people kicked out the speaker cable on stage, and they all had HT fuses. If you have a HT fuse and a short to ground you will pop the fuse. If you have no load you will not. There is a reason the Fenders were set up to short to ground with no speaker load. It was safer than no load.