Metropoulos Forum

I have a Weber MASS 150. I want to run it IN PARALLEL with my 4x12 cab so that the amp thinks it's seeing 2 cabs. I need to make the cab match the impedance of the attenuator.

The problem is that it is unclear on Weber's site what the impedance of the unit is.

https://taweber.powweb.com/weber/mass150.htm" onclick="window.open(this.href);return false;
"MASS with 150 watt rating. 8 ohms. May be used on 4, 8, and 16 ohm amplifiers." Additional info on this I found says this:

http://www.mylespaul.com/forums/1533453-post15.html" onclick="window.open(this.href);return false;
"Well, technically it is "impedance swamped" which allows you to use it with really any impedance cab and speaker. It is a resistor network Ted devised..."

I(found on forums at freestompboxes.org):

"At different levels of attenuation, you will get different impedances depending on the ratio of the impedance of the attenuator in tandem with the impedance of the speaker plugged into it. This sort of attenuator will never give you an exact impedance match. They say that you can use it with 4, 8, or 16 ohm amps because most tube amps can tolerate impedance mismatches as long as they're not too far off. Using a 16 ohm speaker with the Mass Lite could potentially get you a range of impedances anywhere from 8 to 24 ohms, depending on the level of attenuation and the setting of the "Range" switch. This should be okay for a 16 ohm amp, but you have to be aware that mismatching the impedance will change the tone and response of the amp from its "normal" sound."

SO.......
Because of the confusion, I would like to simply measure the impedance of the unit myself with a multi-meter.

Here is what I did:
I plugged a speaker cable into the INPUT of the attenuator. I measured the ohms across the tip and the sleeve of the cable. It reads 27 ohms. Is that the impedance the amp will see if I set the attenuator to FULL LOAD?

Is there more to it than that?

FYI - the 27 ohm reading is NOT affected by changing the level of attenuation on the attenuator. Don't know if that helps explain what's going on or not...

Thanks for your help on this - looking forward to some insight... (READ: chomping at the bit to try hooking it up this way!!)

-Rob

[crickets]

Plug your ohm meter into the input jack of the attenuator. If you're going to use it without a speaker hooked up don't plug anything into the speaker jack. In the typical attenuator you need to turn the volume to zero for load use but if you are running it in the second speaker jack it will absorb half the available wattage in any case. It is still probably best to turn the volume all the way down so that you use the units maximum heat dissipation abilities. Expect it to read low say 5 to 6.5 ohms for 8 ohm use and 12 to 14 ohm for 16 ohm use.

Thanks for the info!

Expect it to read low say 5 to 6.5 ohms for 8 ohm use and 12 to 14 ohm for 16 ohm use.

Plugging into the input of the attenuator and no cab it measures 27 ohms. It stays at 27 ohms and the reading is not affected by turning the attenuator knobs from 0 to 10 - it always reads 27 ohms.

I'm a little concerned that the measurement I am getting is so much higher than what you expected... It makes me wonder if I am getting an accurate reading. The weber uses a motor to soak up the juice according to what I've read on the site - don't know if that has any effect on how to determine the impedance.

Do you think it would make sense that the unit has an impedance of 27 ohms?

Meanwhile, I just got this from Weber...

the nominal impedance of the Mass150 is 8 ohm. So if you wire the cab for 8 ohm, and run your amp at 4 ohm, that will work for you.

The thing about the Mass150 is that it also works at 16 ohm, so you can wire the cab for 16 ohm and run the amp at 8 ohm.

just be sure you have both knobs turned all the way down on the Mass150 when you are using it like this.

I'll ask them about the reading I got - and see if there's an explanation.

I don't see how it can be other than what you measure. If you measure 27 ohms from a cable end that will be going to its destination, that destination will see that 27 ohm resistance. unlike politicians, physics doesn't lie. but might better wait and see what weber says.

Ok - evidently with the method I am using - I am only measuring the DC resistance of the input section of the unit - which is different from impedance. There are capacitors and other components in there that keep the meter from reading the full DC resistance of the unit, which is different from impedance.

http://www.ebay.com/itm/30-Ohm-100W-Wat ... 3a7413dd8a" onclick="window.open(this.href);return false;

or the offer still stands if you want my extra. just cover ship cost

awangotango wrote:http://www.ebay.com/itm/30-Ohm-100W-Wat ... 3a7413dd8a

wow ! that's pretty much perfect, ay?

According to weber it looks like the final answer is that an 8 ohm cab in spkr 1, mass150 in spkr 2. that is a match. you set the amp on 4.

So, if i put 2 16 ohm speakers in parallel in the cab, that's a matched set up - should work - ay?

so the weber load is 8? if so, won't work.
load needs to be 30 or so if you use 2 16 ohm speakers series to 32 amp at 8
load needs to be 16 if using 2 8 ohm speakers series to 16 amp at 4
these are the only two scenarios because once you pull two tubes, you have to knock down the imp selector a notch. so it leaves only these two options available.

oh that's right i forgot about the adjustment needed b/c only 2 tubes... i'll have to get a resistor like the one you posted from ebay.

off topic but while we're on the subject...

i have a 1x12 cab w/ a weber pre-rola (greenback clone) - a single 8 ohm speaker - what if i pulled ANOTHER tube - and ran that one speaker off the one tube - to 1.) remove the attenuator and 2.) hammer the speaker a little. running the amp at 4 ohms would be a mismatch by one "level" shouldn't "hurt" anything...

When I measure the resistance of my attenuator set to load (it should be 16ohm, no cab connected) via speaker cable and mm it's really close to 16ohm, much closer to 16ohm than my cab (around 14ohms). I'd be freaked out if it were 27