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Marshall impedance matching, Dr Z Airbrake

dwagar

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Apr 18, 2005
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I'm confused. My natural state I guess.

I just bought a Dr Z Airbrake to use with my JCM800/Fender cab rig.

The JBL cab is 4 ohm - but I could rewire it to 16 ohm.

I have the head set at 4 ohms. It's really loud, I need to attenuate to get my tone back. Or convince the band that it's okay to be that loud. (I was running it before though a pair of 16 ohm 2x12 cabs and an 8 ohm hotplate)

But reading the Dr Z literature, it says older Marshalls should be set 8 ohms on the head and 16 ohms on the cab. That this would keep the OT cooler.

However, I've read elsewhere that Marshall trannies do not like a mismatch, and you risk them doing this.

Am I safest to stick with 4/4, or should I change to 8/16?
 

Gold Tone

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As long as the impedance of the cab is equal to or greater than the amp you are safe.

Tonewise, if you keep the amp at lower impedance the tone is thinner.
 

dwagar

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thanks man. That's interesting, maybe I didn't need a new attenuator after all.
 

Wannatone

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As long as the impedance of the cab is equal to or greater than the amp you are safe.

Interesting. So, you're saying it is still safe if you connect 4-times, 8-times,...,... infinite impedance to the amp? :wah
 

Gold Tone

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Sure....why not, as long as you are lowering the setting on the amp accordingly....what problem do you expect by doing that? And how often are you connecting 4 or 8 cabs to an amp!?!?!? Are you Van Halen?

Each time you adda cab you are lowering impedance NOT increasing it since they are acting in parallel. If your cabs were hooked up in series then the impedance would be increasing.

If you are ever unsure test the total impedance with a multi-meter (I carry a cheap portable one in my gig bag all the time). You should also know how to calculate total impedance for series and for parallel.

Your amp doesn't like to have no load on it....the OT wants to push something....the higher the cab impedance the more it is "impeding" or pushing back at the amp....everything is happy. If you lower the cab impedance but keep the amp high it's akin to over reving your transmission and cooking your engine.....the tranny needs something to push against.
 

Wannatone

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You can't be serious. Either you're kidding or you have no clue.

I'll not be going into boring tube amp theory, but I can tell you one thing for sure: To be safe, always set the output selector to the correct speaker impedance.

You may try mismatching one step up or down AT YOUR OWN RISK. Some amps are more, some less forgiving. The amp may be running safe if you know whet you're doing.

But, NEVER run a tube amp into a load that is more than twice of the setting of your output selector. Something like 4 ohms amp into a 16 oms cabinet is definitely asking for trouble. You would risk major damage to the amp.
 

Kap'n

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But, NEVER run a tube amp into a load that is more than twice of the setting of your output selector. Something like 4 ohms amp into a 16 oms cabinet is definitely asking for trouble. You would risk major damage to the amp.

Absolutely true. Flyback can toast your output tubes, and transformer.

Everybody's an expert on the internet. :wah
 

dwagar

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You may try mismatching one step up or down AT YOUR OWN RISK.

I want to know what's safe. Not what I can try at my own risk.

Note: this is an old Canadian head - I don't have a 16 ohm option on it.

While matching is best, I've always been under the impression one step is fine. And higher on the cab is better - ie with an 8 ohm head, it's better to run a 16 ohm cab than a 4 ohm cab.

One of the great things about the internet is you can get answers. One of the bad things is the conflicting answers.

Does it make a difference if you're running an attenuator?

Dr Z states in their manual on older vintage amps it's safer to run 8 ohm head --> attenuator --> 16 ohm cab, as this will eliminate overheating of the OT.
 

Wannatone

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Connect the cab to the attenuator and an ohmmeter to the input of the attenuator, with no amp connected.
Given I have the correct schematic of the attenuator in question, you will see the following ohm readings when you switch through the attenuator settings:

1) 8 to 22 ohms with a 16 ohm cab
2) 5 to 22 ohms with an 8 ohm cab
3) 3 to 22 ohms with a 4 ohm cab

The large variation is due to the componnt saving design of the unit.

When you look closer, you'll see less by less variation the more attenuation is switched in. With the attenuator at least at '3', there is only a 2:1 variation of the measured input impedance. Also, you'll see that the higher the speaker impedance is, the less variation there is when you're looking at the full switching range. At full attenuation the cabinet just doesn't matter anymore, the measured resistance is simply around 25 ohms, and that is what your amp is seeing as a load.

Now we could start analyzing what works best for that particular amp...

As a quick and dirty answer I can just say that, regarding the matching issue, the attenuator works best at 16 ohms (what it appears to be designed for) and worst at 4 ohms. So, running a 16 ohm amp into a 16 ohm cab through this attenuator should work fine.

If you are only using the middle positions (1...4) of your attenuator, the best match for your 8 ohm amp would be a 16 ohm cab, but 4 or 16 ohms would work as well though 4 ohms only for (2...4).

Actually, when you're using a 16 ohm cab, only thru and the bedroom level position would produce a significant mismatch to the amp.

I think it would be safe running your amp at 8 ohms and a 16 ohm cab with that attenuator if you avoid playing the amp fully dimed when using the extreme attenuator positions (over extended time).
 

Gold Tone

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Hmmmm......never cooked an amp over 35 years of playing and matching amps and cabs.....so.....
 

dwagar

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Apr 18, 2005
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thanks guys. I don't use extreme attenuation anyway, I use them to knock the edge off for gigs, not for home use.

I run my Hotplate at about -12, I'm looking at the same range of attenuation with the Airbrake.

8/16 is great - I have the option of using whichever attenuator I prefer then.
 

sickboy79

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Apr 2, 2002
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As long as the impedance of the cab is equal to or greater than the amp you are safe.

Tonewise, if you keep the amp at lower impedance the tone is thinner.

Agree with you statement about the impedance. I've heard different things about the Airbrake. I've read on numerous forums that the Airbrake is "optimized" for 8 ohms - but, you can run anything into it as long as the impedance of the cab equal or the next setting higher than the amp (8 into 16). I wouldn't go 4 into 16 - that's asking for trouble.

The tone/impedance thing is debatable. I kind of think it's a myth. I personally can't tell a difference in my testing. More than likely you're hearing the difference coming from the actual wiring of the cab (series vs. parallel vs. series/paralel, etc) rather than the impedance setting of the amp. But, who knows? I've read both sides of the story. That's what my ears hear.
 
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