The "Ultimate Attenuator"

thisisguitar

@Pete24v - No worries bro, I didn't like the concept full stop. My mate made me hook it up… against my better judgement! At least it didn't kill one of us!

ICBM

To be fair, it was only *potentially* dangerous rather than actually so, even though the issues were real. As it was, the mains cable was fairly well retained, the earth connection was intact and the little point of solder poking through the heatshrink wasn't touching anything.

But that isn't good enough. The point is that those faults could add up to creating a real risk - tug the mains cable hard enough so it comes away from the panel, the earth wire pulls off and the live wire touches the chassis, and if the unit is plugged in and not connected to an earthed amp, you have a live casing and no fuse will blow. Worse, if you happen to be touching it at the same time as you then plug the speaker cable into an earthed amp, *you* will complete the current path from live to ground and it will quite possibly kill you. OK that's not likely to all happen at once, but the point of the regulations on things like this is to prevent any chance of all the possible hazards lining up, which is when accidents happen…

I've also discovered something else interesting - it *hates* the Modern High-Gain mode on my Mesa. It sounds very good with any other mode (Clean, Blues or Vintage High-Gain), but unusably bad with Modern. The difference is that the amp's negative feedback loop is turned off in Modern. Interaction with the speakers clearly plays a big part in why it sounds so good - remove that and with the Ultimate in between, the sound is either muffled and dead or harsh and spiky depending on the setting of the 'Plexi' switch. So it probably isn't going to work well with any other amp with no NFB either.

Danny1969

@ICBM A couple of things, do you think that unit was wired incorrectly from the off as 30 Ohms seems a bizarre impedance ?

What are the downfalls of an attenuator based on purely resistive loads ? I've built plenty of these and actually had my first before you could buy em in the shops but I've never compared one of my purely resistive loads to whatevers in a Power Brake ?

 

matone

Kudos to ICBM,it`s a revelation considering all the hype around these on the US forums a few years ago !

ICBM

Danny1969 said:

@ICBM A couple of things, do you think that unit was wired incorrectly from the off as 30 Ohms seems a bizarre impedance ?

No, that is its intended resistance. The designer thinks that's the right load and claims it's safer than a correct match even with a 16-ohm amp. (He's wrong.)

Danny1969 said:

What are the downfalls of an attenuator based on purely resistive loads ?

None, if the resistance is right. In fact they're less hard on the amp than anything with reactance, of either kind.

Danny1969 said:

I've built plenty of these

Same here. The funny thing is that I put away my very first one when 'proper' commercial attenuators came out, and didn't think any more about it until I found it years later, and tried it to see how bad it would sound. It didn't .

Danny1969 said:

I've never compared one of my purely resistive loads to whatevers in a Power Brake ?

The Powerbrake is a very unusual one, in fact - instead of being mostly resistance with a bit of inductance, it's the other way round. It's a complex multi-tap autotransformer used to feed a resistive load. As a result it corresponds to the true impedance curve of a real speaker much more closely than any other attenuator I know of - although that doesn't seem to be the most important factor in how good they sound, contrary to what some makes claim… in fact, purely resistive ones do often sound very good.

This does make it a bit harder on the amp than a purely resistive one too. But it also makes it the best test load for 'proofing' repaired amps, because I know that if they will survive being run at full crank into the Powerbrake, they will survive a real speaker cabinet.

Pete24v

thisisguitar;541214" said:

@Pete24v - No worries bro, I didn't like the concept full stop. My mate made me hook it up… against my better judgement! At least it didn't kill one of us!

So how is the amp now, was it just a new set of EL34's?

ToneControl

Excellent work

thisisguitar

@Pete24v - Yeah, just a pair of valves. One of them was fine though, but better changed as a pair. 

Your new one turned up now?

Pete24v

Yep, i'll do a post about it soon.

Good news about the Cornell!

thisisguitar

^ Excellent stuff mate! 

Yeah, I was quite relieved myself. Think I'm going to get London Power Scaling fitted to it, was talking to the owner of Gartone amps about it earlier and he loves it. 

ICBM

I think @jpfamps does Power Scaling installations.

On balance I prefer the sound of a (good!) attenuator, but power scaling is definitely a lot less stressful on the amp and valves.

stickyfiddle

This is pretty shoddy, but somewhat unsurprising, sadly. People have proved, time and again that they'll buy something thoroughly average for big coin if it's the current zeitgeist on "the forums".

lustycourtier

15 years ago on the Plexi Palace forum, everyone was raving about these, but all this stuff was exposed then! How are they still getting away with it?!?

Sassafras

Is this the same firm that produced something called the Tone Enhancer or am I getting confused (again)?

ToneControl

ICBM said:

I think @jpfamps does Power Scaling installations.

On balance I prefer the sound of a (good!) attenuator, but power scaling is definitely a lot less stressful on the amp and valves.

MJW definitely does them

thisisguitar

Thanks @ICBM - Would be another thing to carry for me, hence I'd rather the power scaling option ;-)

vasselmeyer

Can someone explain power scaling?

ICBM

vasselmeyer said:

Can someone explain power scaling?

It controls the internal voltages in the amp to actually change its power output. That's a bit more complicated than it sounds in a valve amp, because there are up to four separate voltages that need to be adjusted - or not - in order for it to work properly at different power outputs.

So it isn't just a case of fitting one big control to reduce the whole thing - some people do that with a 'Variac' which is an external voltage control that goes between the amp and the mains, but it can't be used for more than a small power reduction before the amp stops working properly, and can damage the valves.

vasselmeyer

So the equivalent of a master volume but taking into account the separate voltages across sections of the amp?

ICBM

vasselmeyer said:

So the equivalent of a master volume but taking into account the separate voltages across sections of the amp?

No, not really - a master volume reduces just the signal level going into the power amp. Power scaling actually reduces the power of the power amp.

I don't know if this helps, but think of a car analogy… if you have a big car with a big engine but don't want to go as fast as it's capable of.

A master volume is like the throttle - back off and you get less power from the same engine, but there will come a point where it gets difficult to get good control and just doesn't work very well. (Assuming you don't change gear!)

Power scaling is like reducing the size of the engine in the same car, so you get the same range of throttle control but less power and acceleration, and you can drive it pretty much the same way but you will go less fast.

An attenuator is like driving with the brakes on . [© Del Amitri]