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Hmm, side question: could this be used to make speaker mismatch safer? For example, my cab is currently 8 ohms, but I'd prefer to run the amp at 16 ohms.

JMPGuitars wrote: ↑

Thu 04/30/20 8:38 pm

Hmm, side question: could this be used to make speaker mismatch safer? For example, my cab is currently 8 ohms, but I'd prefer to run the amp at 16 ohms.

FWIW, I updated the impedance plot in my post on the previous page to show the "0" position where the attenuator is off. The impedances shown are what the amp "sees". The mismatch is pretty significant, but it is a "one-size-fits-all" solution, rather than a specific impedance. Changing the 25R shunt resistor will not correct the mismatch much. The attenuator is better suited for use at 8 or 16R.

You could use it to do a mismatch. Here is a quote from an earlier version of the Dr. Z Airbrake manual:

"Helpful Hints:
When using the Airbrake with older vintage amps, mismatching the load will be a safer way to operate. This will eliminate any overheating of the output transformer. For example, a Vox AC30 or vintage 100 watt Marshall should be run at 8 ohms through the Z Airbrake with a 16 ohm speaker load.

A useful technique for live applications is to use the [Z Airbrake] in parallel with another speaker cab. If your amp is equipped with dual speaker out jacks, connect the [Z Airbrake] as you would normally to one speaker cab. Connect the other cab directly to your amp. This will soften the attenuation effect and allow more control in live settings."

colossal wrote: ↑

Fri 05/01/20 7:13 am

"Helpful Hints:
When using the Airbrake with older vintage amps, mismatching the load will be a safer way to operate. This will eliminate any overheating of the output transformer. For example, a Vox AC30 or vintage 100 watt Marshall should be run at 8 ohms through the Z Airbrake with a 16 ohm speaker load."

A lot of people generally say it's okay to mismatch as long as the cabinet is higher than the amp (within 2:1). Do you agree with that? That thought fits in line with the helpful hint. What about going the other way though? I would think having an 8 ohm load with amp set at 16 would at the very least reduce the amount of attenuation? Possibly cause other problems given the variation in impedance depending on the airbrake's settings?

JMPGuitars wrote: ↑

Fri 05/01/20 7:55 am

A lot of people generally say it's okay to mismatch as long as the cabinet is higher than the amp (within 2:1). Do you agree with that? That thought fits in line with the helpful hint. What about going the other way though? I would think having an 8 ohm load with amp set at 16 would at the very least reduce the amount of attenuation? Possibly cause other problems given the variation in impedance depending on the airbrake's settings?

An 8ohm speaker plugged into the 16ohm tap halves the reflected impedance back to the power tubes. So with an 18W amp running an 8k primary, with an 8ohm speaker on the 16ohm secondary tap, the power tubes now "see" a 4k primary. This rotates the loadline in favor of more power; not necessarily a good thing, depending on where the amp is biased under "normal" as-designed conditions and load. But speakers are inductive/reactive loads so their impedance is not constant and rises significantly at the resonant peak (usually in the 75-100Hz region). This plot is very similar to a V30:

Here's what mine looks like. I was going to engrave it with a rotary tool, but that looked terrible. So I used a paint marker instead.

Very nice! What enclosure did you use? Is that a Hammond? I like the black powder coat.

colossal wrote: ↑

Fri 05/01/20 9:02 am

Very nice! What enclosure did you use? Is that a Hammond? I like the black powder coat.

Thanks! Yes, it's a Hammond. Any enclosure they offer, you can usually find a powder coated version.

Here's the exact parts list I setup on Mouser: That includes everything I used except all the 18AWG wire.

Thanks,
Josh

Here's the demo of mine: https://youtu.be/3oSZVg2STXw

Most of the demo is okay, but when I turned it down too low, my camera automatically increased the sound level. Oops.

Good work! Despite the camera increasing the volume, you can hear the strings once you are down into bedroom level, so you know it is definitely is doing its thing. How did you find the transparency/tone retention at higher levels of attenuation?

colossal wrote: ↑

Tue 05/05/20 10:30 am

Good work! Despite the camera increasing the volume, you can hear the strings once you are down into bedroom level, so you know it is definitely is doing its thing. How did you find the transparency/tone retention at higher levels of attenuation?

That's a good point.

I haven't played enough with it to form a solid opinion on the tone retention. There's maybe a drop in trebles, but not bad at all. And that's with the EQ on the amp flat, so it seemed like it would be easy to compensate for. What I really want to do is compare it to my MiniMass and see which one I like better. My first, and biased opinion, is that I like this shiny new one better.

In the short time I have played with it, it sounds pretty transparent to me otherwise. The perception of the change in tone is hard to measure accurately. So I will focus on which unit I think sounds better, and update this eventually.

Good work Josh. Thanks for the report back. Looking forward to seeing if you still like it after the honeymoon period.

I shared the doc I made: files/JMPGuitars_Break_Wind_Attenuator.pdf

It's also in the Miscellaneous Stuff page: viewtopic.php?f=25&t=24414&p=235766#p235766

I simplified it. The other layouts I've seen weren't very logical.

OK, great! I want to read up on this thread! I want an economical power soak and dummy load! And I'm on a budget, so a DIY attenuator, sounds just what I was hoping for!

Great info.

\not sure if I missed in in the threads but is this for a 16 ohm load and how much attenuation will it give.
Jim

JimbpGaff wrote: ↑

Tue 05/12/20 2:58 am

Great info.

\not sure if I missed in in the threads but is this for a 16 ohm load and how much attenuation will it give.
Jim

I posted a demo above. When the attenuation is increased, you can hear the guitar strings in the video. That's pretty significant.

16 ohm is supposedly ideal, but it also works with 8 and 4 ohm loads.

JimbpGaff wrote: ↑

Tue 05/12/20 2:58 am

Great info.

not sure if I missed it in the threads but is this for a 16 ohm load and how much attenuation will it give.
Jim

As Josh said, the attenuator will "work" for 4 and 8 ohm loads, but is best suited for 16. See the first page of this thread for a plot of impedance vs. attenuation.

Can I use a different value Rheostat ?
The 150 ohm 50 watt is hard to come by at the moment in the UK as they all seem to be on back order until the end of the year.
Jim

JimbpGaff wrote: ↑

Wed 05/20/20 12:36 am

Can I use a different value Rheostat ?
The 150 ohm 50 watt is hard to come by at the moment in the UK as they all seem to be on back order until the end of the year.
Jim

You can. Lower values will limit attenuation. Higher values will increase it at the expense of increasing impedance.

colossal wrote: ↑

Wed 05/20/20 8:05 am

JimbpGaff wrote: ↑

Wed 05/20/20 12:36 am

Can I use a different value Rheostat ?
The 150 ohm 50 watt is hard to come by at the moment in the UK as they all seem to be on back order until the end of the year.
Jim

You can. Lower values will limit attenuation. Higher values will increase it at the expense of increasing impedance.

Limiting attenuation isn't a bad thing, since there will be less tone sucking that way. BUT there's limited options in that price range. I looked at alternatives, and they get pretty expensive. The RJS125E for example is 125 ohms, and $80. Some other variations were hundreds of dollars.

Personally I got a couple of the RJS150Es on eBay for $40 a piece. There is a seller in the UK selling the 150 also, but not as cheap. The 125 is available on there too, but I dunno about in the UK.

Thanks,
Josh

Thanks Josh

Just found the one on ebay for $77.00.

A lot of money in these difficult times .

I will have to do some thinking .

The pure resistor attenuator is a cheaper option, But.....
Jim