50-Watt Attenuator On A 50-Watt Amp?

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stevieb

Just another guy, really.
I have read that tube guitar amps actually put out quite a bit more power at "10" than they are usually rated- the amp rating is at a clean output, and diming the volume goes well beyond that, both in "dirt" and wattage.

I have used an Altair PW-5 50-watt attenuator to very good effect on a 22-watt Deluxe Reverb. I would like to try/use it on a 50-watt Ampeg Reverberocket 212 re-issue, but I love, love, LOVE my PW-5, and don't want to hurt him. Conventional wisdom holds that a 50-watt attenuator is no match for a 50-watt amplifier.

My questions:
Is the conventional wisdom true? I suspect it is, so next question:

Can I prevent attenuator burn-out by not over-attenuating the signal, and still get good results? I am sure I can prevent burn-out by not dimeing the volume, but how much is not too much? 6? 8? Is there any test, either quantitative (using a meter, for instance) or qualitative (using my ears) that I can run, to make sure I don't have a very tearful attenuator burial in my back yard?
 
I've never pushed it, exactly, but yes, you do want to leave yourself a fair bit of, um, let's call it wattage headroom. In your shoes, I'd just grab a second attenuator with quite a bit more wattage rather than chancing it.
 
stevieb said:
I have read that tube guitar amps actually put out quite a bit more power at "10" than they are usually rated- the amp rating is at a clean output, and diming the volume goes well beyond that, both in "dirt" and wattage.

I have used an Altair PW-5 50-watt attenuator to very good effect on a 22-watt Deluxe Reverb. I would like to try/use it on a 50-watt Ampeg Reverberocket 212 re-issue, but I love, love, LOVE my PW-5, and don't want to hurt him. Conventional wisdom holds that a 50-watt attenuator is no match for a 50-watt amplifier.

My questions:
Is the conventional wisdom true? I suspect it is, so next question:

Can I prevent attenuator burn-out by not over-attenuating the signal, and still get good results? I am sure I can prevent burn-out by not dimeing the volume, but how much is not too much? 6? 8? Is there any test, either quantitative (using a meter, for instance) or qualitative (using my ears) that I can run, to make sure I don't have a very tearful attenuator burial in my back yard?
Click to expand...
well ...... first off, it's not always true that tube amps put out much more than their rating ...... they just have a higher 'percieved' volume. There's a lot of reasons for this which would be an entire thread by itself. But suffice it to say that a tube amp putting out exactly 50 watts will usually have a louder 'percieved' volume than a SS putting out exactly 50 watss.

Also, I have one of those reissues ..... as I remember it ..... it's a 40 watt amp.
oh ..... I see, mine is a single 12" so it's only putting out 40 watts into the 8 ohm load. I imagine the 2-12's do give you 50 watts into 4 ohms.

What's the impedance of your Altair? If it's a 8 ohm then you're likely gonna be getting more like 40 watts out of that thing.
And at least in theory, a 50 watt attenuator should be able to handle 50 watts. And you're mostly not gonna be getting 50 watts out of that Ampeg unless you're gonna be diming it.
 
You might as well be plugging your amp into a toaster. Those Altair attenuators are notorious for killing output transformers.
 
Weber recommends always going higher on the attenuator rating comapred to the amp's top wattage.

I use 50W Weber MiniMass attenuators on my 30W amps and 25W attenuators on my 12W-20W amps.

I'm actually going to get some 50W MiniMass for my 20W amps, as I find the 25W Minis are too "dramatic" with its attenuation on the 20W amps.
The 25W Minis are perfect for my 12W amps.

I have 4 Weber MiniMass attenuators. I really like them with their reactive speaker motor + resistor design...rather than the all-resistor types.
 
You can certainly measure the power dissipation in the attenuator--feed a test tone--let's use 200Hz--into the amp, crank it to the desired level, measure the voltage across the attenuator. Now, disconnect the attenuator and put the meter in series set on current--use the higher amp tap on the meter for that. Multiply volts and amps, and you have watts.

That Altair uses a filament--that will operate somewhat as a *very* slow compressor. Filaments have increasing resistance with temperature, so you can't measure resistance out of circuit . . . if they do indeed kill output transformers, that could be the reason as a very high power amp would cause increasing temperature and therefore resistance.

A good attenuator would provide a constant-impedance load to the amp . . .
 
I have an Altair PW-5. It's pretty much bulletproof; I used it for years with a dimed 50w Marshall and it never complained. It never hurt the output tranny of the amp, either.

I have also used it for a stage monitor volume control and to drive headphones from a speaker line. It's a handy little box.
 
I have a 50 watt Marshall power brake - when I use it with my 50 watt Marshall, it sounds like something bad is happening - can't fully describe it, but there seems to be some very high frequency screaming and a general you-shouldn't-be-doing-this sound and feeling - I stopped using it with the Marshall after a couple of experiments. The power brake works great with my 5 watt Epi Valve Jr. - I've resolved to keep it at that. I've never used or seen the Altair one
 
Lt. Bob, nowhere on the PW-5 does it say anything about impedence, and nothing on the web, apparently- so I have no idea. Could I check DC resistance, like I would on a speaker, and get a close reading?

Odd that a MARSHALL attenuator would make a MARSHALL amp sound like crap- you'd think they would addressed that, eh? My PW-5 made my 15-watt Super Champ XD sound WAY over attenuated, even at lower settings, I am surprised you got good results with a 50-watt attenuator and a 5-watt amp.

I think the Altair too often gets a bad rap- I suspect that it's driving the amp hard that actually causes transformers and power tubes to give up the ghost, not the attenuator- they would have fared as poorly if driving speakers as hard, but nobody can stand that SPL for long, so the amp gets turned down and lives to see another day...
 
Anybody know why I can't see anything but the first post in this thread?
 
Anybody know why I can't see anything but the first post in this thread?

Never mind.
 
stevieb said:
Lt. Bob, nowhere on the PW-5 does it say anything about impedence, and nothing on the web, apparently- so I have no idea. Could I check DC resistance, like I would on a speaker, and get a close reading?
Click to expand...

No, because the resistance of a cold filament is very low. It will be very different while driven.

I am surprised you got good results with a 50-watt attenuator and a 5-watt amp.
Click to expand...

The wattage rating is for power handling, not attenuation. Let's make a really simple 6dB nominal attenuator: an 8 ohm series resistor (this would not be a good attenuator because it changes the total load impedance, but play along). Whether you drive that at 16V (16V / 16R = 1A --> 16W, 8W in the resistor) or 64V (64V / 16R = 4A --> 256W, 128W in the resistor), the attenuation is the same: 6dB (8W / 32W, for example). The problem in the latter case is if you don't have a 200W rated resistor you might start a fire. There is no problem using a 200W rated resistor and only forcing it to dissipate 8W.
 
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