Using highest speaker load for best tone?

[Cygnus X1]

16 ohm VHT 4/12 cab for my left channel is plugged into the 16 ohm output of the amp... 2ea Crate 2/12 cabs at 6.8 ohms on a digital meter
are plugged into the "B" side of the VHT power amp, with the 4 ohm setting chosen. Keep the amp set lower than the actual impedence of
the cab...

That 6.8 impedance is actually much closer to an active 8 ohm load.

But you knew that.
It's a frequency thingy.

[Spivonious]

What does the impedance switch do, exactly?

And how do you measure impedance of a speaker? It's very dependent and frequency, since at 500Hz the speaker could be 6 ohms, and at 1000 Hz the speaker could be 20 ohms.

[markD]

I understand.
Running a 1/2 load usually isn't that big of a deal.
A good OT will take it, but mixing and mussing loads is usually counterproductive at best and risky at worst.

You just have to cross your fingers and hope the designer of the amp figured that mismatch in.

Just as many will take it fine as will give up magic smoke.

Can't say much for others than classic Fender or Marshall, but matching loads is usually the best way to get the most out of the amp.
I know I "design" that way.
(That is a very loose term in the amp world).
I just know what sort of signal it is pushing, what the output tubes like to see, and what OT I put in to accommodate.
The output transformer's very existence is to buffer what the amp puts out to what the load is after all is said and done.

paul rivera designed those old heads i am using to withstand a nuclear war. i asked him about it once when i called about buying (of all things) a new impedance selector switch. he said it would be fine.

it's pretty funny, because on my s120 - which has 2x60 watt power amps in it - i run one of the amps in pentode at 8 ohms with a mesa 2x12, the other amp runs in triode at 4 ohms with (2) rivera 1x12 cabs. it makes for a really neat sound. then i run the whole amp at either full or half power, depending on the room.

complicated but interesting, in the way those things effect your tone.

[Cygnus X1]

What does the impedance switch do, exactly?

And how do you measure impedance of a speaker? It's very dependent and frequency, since at 500Hz the speaker could be 6 ohms, and at 1000 Hz the speaker could be 20 ohms.

Just set your multimeter to ohms (0-200 range) and measure from the tip to the shield (pos/neg) on your speaker wire. Use a relatively short wire of appropriate gauge.
8 ohms will measure about 7.X, 4 will measure 3.X...etc.

One oddity I have seen several times but I have not seen elsewhere-sometimes a speaker or cab will measure way off.
Anywhere from close to zero or double what is expected.
If that happens I "pop" the speaker or cab quickly with a 9V battery and recheck. Usually it will come in.

I always thought it could be a voice coil hanging but I'm not sure since these speakers usually end up performing just fine.
Other times it is a voice coil about to go out.

[marcus]

Just set your multimeter to ohms (0-200 range) and measure from the tip to the shield (pos/neg) on your speaker wire. Use a relatively short wire of appropriate gauge.
8 ohms will measure about 7.X, 4 will measure 3.X...etc.

This will give you the DC resistance. It is generally close to the impedance, but not exactly. There is no standard way that I know of to specify the impedance of a particular speaker. It is generally some kind of average of the impedances at all of the audible frequencies. To measure impedance at a particular frequency, you need a function generator. Set it to produce a sine wave at the frequency you are interested in, and set the voltage to a particular value. By measuring the current, you can calculate the impedance.

As for what the impedance selector switch does, it switches between different taps off of the secondary winding on the transformer. Different numbers of windings of wire around the secondary of the transformer will produce different voltages going to the speaker. Essentially, the purpose is to keep the load seen by the output tubes constant. This also causes the power output to be exactly the same if you have matched the speaker impedance to the appropriate tap on the output transformer.

[toejam]

I understand.
Running a 1/2 load usually isn't that big of a deal.
A good OT will take it, but mixing and mussing loads is usually counterproductive at best and risky at worst.

You just have to cross your fingers and hope the designer of the amp figured that mismatch in.

Ever play a THD amp? They have great transformers. Their amps have a switch for 8/16 or 2/4 ohms, a full power or attenuator switch (like a Hot Plate built in), a switch for hi voltage or low voltage depending on what kind of power tube you wish to use, and they have their own built-in load in case you want to use it for recording without having it hooked up to a speaker cab.

[Spivonious]

This will give you the DC resistance. It is generally close to the impedance, but not exactly. There is no standard way that I know of to specify the impedance of a particular speaker. It is generally some kind of average of the impedances at all of the audible frequencies. To measure impedance at a particular frequency, you need a function generator. Set it to produce a sine wave at the frequency you are interested in, and set the voltage to a particular value. By measuring the current, you can calculate the impedance.

As for what the impedance selector switch does, it switches between different taps off of the secondary winding on the transformer. Different numbers of windings of wire around the secondary of the transformer will produce different voltages going to the speaker. Essentially, the purpose is to keep the load seen by the output tubes constant. This also causes the power output to be exactly the same if you have matched the speaker impedance to the appropriate tap on the output transformer.

Thanks for that post. So you're saying that if the amp is designed properly, and the impedance switch matches the speaker load, that the amp will always be delivering the same power to the speakers? So by setting the switch to 16 and running an 8 ohm load you'll run the amp hotter, therefore driving the tubes harder?

[marcus]

Thanks for that post. So you're saying that if the amp is designed properly, and the impedance switch matches the speaker load, that the amp will always be delivering the same power to the speakers? So by setting the switch to 16 and running an 8 ohm load you'll run the amp hotter, therefore driving the tubes harder?

This is right. Power = V^2/R, so halving the impedance while maintaining a constant voltage will give you twice the power.

[Dreamland_Rebel]

some heads can handle that but some cannot. you risk buring up a screen resitor or cap or even blowing a tube.
you are safer using a lower tap and higher cab than the other way around.

[Cygnus X1]

Ever play a THD amp? They have great transformers. Their amps have a switch for 8/16 or 2/4 ohms, a full power or attenuator switch (like a Hot Plate built in), a switch for hi voltage or low voltage depending on what kind of power tube you wish to use, and they have their own built-in load in case you want to use it for recording without having it hooked up to a speaker cab.

Thank you Joe (and marcus).
I've not played the THD, but have looked at the designs.
As I mentioned, I build mine with a multi tap, actally 2, 4, 8, and 16 are available on the taps but I go with 4, 8, and 16.
I dance around the "line out option" because I'm afraid someone will just go ahead and not put a load on the transformer, thus melting the output transformer and worse.

As you mention, The THD has an internal artificial load that takes up for that.
Good stuff!
But it can get pricey for a home builder.

Although I play with options, I still want to keep my amps in the sub- 500 dollar category so I go with what works without running up the cost or robbing the tone.


Another edit-I simply assume people are going to go up or down on their cab/speaker matching.
So I always "play it safe" and recommend just going with a match.

[Rupe]

I think the point of the article is being missed here. It's not about ohm matching, its about tone in relation to the output tranny being tapped. The writer is claiming that by using anything other than the full load setting (16 ohm) that only a portion of the tranny is being used because the tranny needs to be tapped to match impedence to an 8 or 4 ohm cab...alluding that this is not as good as using all of the windings in the tranny (kinda like tapping a pickup).

I have seen this point argued both ways...I have no idea if it holds water or not. I planned to test this with my 2x12 Chieftain by wiring the 8 ohm speakers to 16 ohms and comparing it to the stock wiring of 4 ohms but I haven't done it yet. Seems to me that a boutique builder like Mark Sampson/Matchless wouldn't wire the speakers at 4 ohms if it wasn't going to provide the optimal tone???

[marcus]

I think the point of the article is being missed here. It's not about ohm matching, its about tone in relation to the output tranny being tapped. The writer is claiming that by using anything other than the full load setting (16 ohm) that only a portion of the tranny is being used because the tranny needs to be tapped to match impedence to an 8 or 4 ohm cab...alluding that this is not as good as using all of the windings in the tranny (kinda like tapping a pickup).

I have seen this point argued both ways...I have no idea if it holds water or not. I planned to test this with my 2x12 Chieftain by wiring the 8 ohm speakers to 16 ohms and comparing it to the stock wiring of 4 ohms but I haven't done it yet. Seems to me that a boutique builder like Mark Sampson/Matchless wouldn't wire the speakers at 4 ohms if it wasn't going to provide the optimal tone???

This thread has been hijacked somewhat, but this effect has been discussed. My take is that with a good transformer, the effect of using more windings on the secondary of the transformer, with everything else being equal, is going to be minimal at best. Especially with a good transformer, which I am sure your chieftain has.

[MetalMedal II]

I think the point of the article is being missed here. It's not about ohm matching, its about tone in relation to the output tranny being tapped. The writer is claiming that by using anything other than the full load setting (16 ohm) that only a portion of the tranny is being used because the tranny needs to be tapped to match impedence to an 8 or 4 ohm cab...alluding that this is not as good as using all of the windings in the tranny (kinda like tapping a pickup).

I have seen this point argued both ways...I have no idea if it holds water or not. I planned to test this with my 2x12 Chieftain by wiring the 8 ohm speakers to 16 ohms and comparing it to the stock wiring of 4 ohms but I haven't done it yet. Seems to me that a boutique builder like Mark Sampson/Matchless wouldn't wire the speakers at 4 ohms if it wasn't going to provide the optimal tone???

Cool, a real world experiment! Can't wait for your input. BTW, this is the *only* type of discussion, where I feel comfortable using the term "tranny"!