impedence and attenuators

[SSII x 2]

I realize that you can use any spkr load with higher impedence

than the output impedence of your amp safely, e.g. okay to use

a 16 Ω cab with a 8 Ω output. May not sound ideal, but no

danger of damaging the amp. How does using an attenuator

change the situation, if at all? I've got an amp with a 4 Ω

output, but no 4 Ω cabs, and I'd like to use an attenuator with

it. Is it safe to go from the 4 Ω output, to the attenuator, and then

to an 8 Ω cab? I've got 2 attenuators, one a Dr. Z Brake Light,

which has no impedence settings, i.e. use the same Ω on both

ends, no problem. Also have a Rivera Rock Crusher with 8 Ω

and 16 Ω settings. How would you guys proceed so as not to

damage my amp?

[tbonesullivan]

there actually is some danger to harming the amp if the amp is not designed for 8/16 ohm operation. I know that Marshall and MB often have a 4 ohm tap, and an 8/16 ohm tap off the output transformer.

Running into a higher impedance load can cause "flyback voltage" which is NOT good for the amp. voltage builds up and can potentially arc at the power transformer or tubes. when you have NO load connected, that is pretty much INFINITE impedance, which quickly builds up voltage and your amp fries.

What amp do you have that only has a 4 ohm output?

[SSII x 2]

there actually is some danger to harming the amp if the amp is not designed for 8/16 ohm operation. I know that Marshall and MB often have a 4 ohm tap, and an 8/16 ohm tap off the output transformer.

Running into a higher impedance load can cause "flyback voltage" which is NOT good for the amp. voltage builds up and can potentially arc at the power transformer or tubes. when you have NO load connected, that is pretty much INFINITE impedance, which quickly builds up voltage and your amp fries.

What amp do you have that only has a 4 ohm output?

It's a Fender Champion 600, recent issue. I can hear the howls of

derision over wanting to attenuate a 5-watt amp, but I like to practice

at whisper quiet volume sometimes so as not to disturb others. I also

want to experiment with other cabs as the onboard 6" is okay, but not

great. Cool amp, BTW, esp. after replacing the tubes with an RCA

NOS 12ax7 and a Sylvania 6v6 from the 60s!

[Jakeboy]

All but one of my amps is NMV so attenuators are my friends. Fender amps wil tolerate an impedance mismatch in either direction...Marshall's, not so much. You are always better off to match your speaker cab impedance to what the OT is looking for.

All attenuators have an impedance associated with them. My Badcat Unleash and my Weber Mass Lite are 8 ohm loads so my amps see 8 ohms when I use them....you can plug any impedance cab into the other end of ***most**** attenuators and your amp will still see whatever impedance the attenuators rated at. This is cool as it lets you experiment with different cabs without worrying about impedance on the cabs.

Not all attenuators are the same though, so read the manual and do what it says. If in doubt, contact the builder with specific questions. OTs are expensive, as is the labor in getting your precious amp repaired.

I get it on the Champion. I have a 5 watt tweed Champ that screams. If I want it dimed with wifey home, I use an attenuator too.

To sum up: if possible, match the amp output to the attenuators impedance. If not possible, be sure your amp can take the mismatch...

Enjoy!

[JES1680]

I've had lots of attenuators. I've blown up a Reeves 50 watt using an Ultimate which I found out later presents a 32 ohm load on my 4 ohm amp.

Nothing a power tube socket and capacitor couldn't fix but it made an impression.

Currently I have 2 Aracoms which allow you to set the input impedance from 2-16 ohms and the output from 2-16 ohms. This allows you to match what the amp wants to see and what the cab is rated for. No worries.

I also have an Alex (now Scumback) which is rated for 16 ohms. According to what I've read, for this one what the amp sees is important to match, but the cab rating can be 8,4,16.

Used the Alex can be had for about $300 - $350

the Aracom is more expensive but a very nice unit if you have several amp and cabs.

Typically when you run an attenuator you push the amp hard. You want to match the impedance when doing this.

[Steve Haynie]

For just matching cabs, no attenuation, look at a Weber Z-Matcher.

[gtone]

Check out the Phaez Brink attenuator. It's designed to use a passive load and a calculated intentional impedance mismatch to carry the groceries in terms of the amount of attenuation. The bad - it only operates at a fixed level of attenuation, although that level might be exactly what you need (he makes a 4 into 8 ohm model that gives about 3-4 db of attenuation and a 4 into 16 ohm that trims about 5-6 db IIRC). The good - it sounds fantastic, needs no external power, is butt simple, is small/light and the price is very low (like $79 plus shipping).

With my Marshall JMP (1987), it absolutely blew the doors off a BadCat Leash and a HotPlate, both of which sounded pretty choked at the same levels of attenuation.