Dummyload for 300 Watt Crate Blue Voodoo???

[Apparition]

Hi all,
does anyone know of such a beast to deal with this high a wattage?
Many thanks, Kris

[Apparition]

BTW I put this in MRT not Guitars because I'm asking as an engineer not a guitarist (although I am)-makes sense?

[GaryM]

Could you make one out of two THD Hotplates wired together?

[Apparition]

Good question.
Electrical engineering is not my strongest area. Is it really that simple? Have I missed a trick?
Cheers for being the first responder to my first post Gary.
Any one else any thoughts?

[GaryM]

I don't know how good a solution it is, just mentioned it because you said you were posting "as an engineer". My schoolboy physics knowledge says that connecting them in series would mean each one got half the current coming out the amp (assuming each one had the same resistance) but I'd suggest speaking to someone who has experience of this kind of thing.

[Apparition]

Understood. Maybe I should have said "as an audio recording engineer with a lesser grasp of the background electrical engineering" !!!
Just don't want to take any chances. It cost a bomb! Ideally I would have got the 150 watt model only they cost a fortune to ship in and this was a chance find.
I have used Hotplates on other amps and would like to try some alternatives, such as the Motherload Elemental just mentioned elsewhere.
Thanks again, I shall look into that

[Ed_J90]

Heres the daddy

http://www.palmergear.com/pdi03.shtml

Throw what you want at it....

Or next model up is:

http://www.palmergear.com/pga04.shtml

[Ed_J90]

Sorry attention to detail... they say only 100 watts ive had a triple rec on one and thats 150 ?!?

sorry was worth a shot

[ef37a]

600*watts worth of resistors will set you back about £30.00.

You could then feed the attenuator of your choice or build a complete attenuator of your own for probably £100 tops.

If you let me know the impedance you want to load the amp with and the power reduction you seek I will crunch some numbers for you if you feel up to the DIY?

*ALWAYS overate load resistors, never put them in series to make up the value. They really are very cheap compared to op valves/transformers/tech's time!

Dave.

[Shingles]

600*watts worth of resistors will set you back about £30.00.

You could then feed the attenuator of your choice or build a complete attenuator of your own for probably £100 tops.

If you let me know the impedance you want to load the amp with and the power reduction you seek I will crunch some numbers for you if you feel up to the DIY?

*ALWAYS overate load resistors, never put them in series to make up the value. They really are very cheap compared to op valves/transformers/tech's time!

Dave.

?

Those resistors won't last very long unless they are correctly mounted on more than £30 worth of heatsink. Or you could just dunk them in a bucket of water.

Seriously - I once had the task of running an Electrovoice P3000 (3000watts into 2 ohms) at full power for several hours. I attached a 2ohm length of 'Constantan' resistance wire to it via a couple of feet of speaker cable and put all the Constantan in a trough of water. How people laughed! ...until it had been on for a few hours. I just had to drain off some of the hot water and add more cold water every hour or so.

I guess you mean never to put them in series for a dummy load in case one burns out? Can't see any other reason.

I think the OP really needs to ask why a dummy load of the full amp rating is needed here.
It's a valve amp, sure. And folks usually want to run a valve amp close to full power to get good tone.
But this is a 300 watt valve amp. That's 3 x more than what is normally considered to be 'plenty'. And correct me if I'm wrong, but I believe it takes 6L6GC or 5881 output valves? Unlike EL34 or EL84 power valves, these are fairly linear until they get close to saturation. And remember that no attenuator is perfect. They all suck a little tone.
Unless the player gets his tone by running this amp nearly flat out into four or more 4x12 cabs - in which case you might have to shout or use sign language - then you might find that bringing the Master Volume down sounds at least as good as any power attenuator. And if you remove four of the six output valves, that might help as well.

[ef37a]

Greeting Nik,
The 300W resistors in fact have a free air rating of 75W so a pair are half way there already!

The correct heatsink is 30"x30" of aluminium but such a soak is going to be a pretty chunky bit of kit anyway and a 2U case would come to that easily.

But we are talking sound signals here, not continuous 50Hz mains. The resistors are rated at 2X overload for 3 minutes, 5X for 5 seconds and 10x for one second. Since I advise a times two rating to start with I think that is a very conservative specification!

Yup, in parallel because if one pops you still have a load of sorts, unlikely in this case of course!^^^^

As far as the "tone sucking, never as good as the real loud thing, waste of time doing it" debate. Up to the OP. I just answer the questions!

Unless the OP has the means and knowledge to rebias and take account of HT shifts I would not advise removing any valves.

Dave.

[Apparition]

Hi Guys,
struggled out into the snow to catch up some work today so just catching up.
I've been a ghost in these halls of knowledge for years and have always managed to answer questions through previous threads and my own research but never have I come across any product or discussion on this subject.
This amp goes from silence into loud, and then even louder! It's powered by an Ampeg power amp I believe. I would like to DI it at home and see how it compares to my Line 6 Gear Box/Pod Farm for basic writing in my semi's lounge/studio!
Basically, I don't have the time, inclination or the BALLS for DIY'ing this one.
As ED found, look closely and all the dummyloaders I've seen have around 100 watt ratings.
On the Motherload FAQ page is a question addressing this but I think the page is under construction as there is no access to an answer. I will give them a call during office hours and see what their thoughts are and post back.
Thanks EVERYONE, I chose my username as I have been a ghost here and feel I know many of you, much appreciated.

[Steve Hill]

This is not an amp for a small domestic writing environment!

Seriously, given the money you're likely to spend on finding or building an adequate dummy load, why not get a small studio valve amp that you can drive hard for the same money?

[ef37a]

Aha! If all you want is a DI feed that is really quite easy.

You will still need the hunky resistors but then just a low power attenuator to bring the signal down to line level. 300W into 8 Ohms is 50V rms near as. A 30dB attenuator gets you down to +4dBu..ish (but fit a pot) and then a 1:1 transformer isolates the whole shooting match and gives you a balanced feed.

Let me know if you want further details.
Dave.

[Apparition]

Sorry guys, not being very concise,
I was also hoping to get a respectable volume for playing at home on this amp, as well as seeing how good hardware speaker emulation compares to the software ones I have.
Hi Steve, I know you're right. Its great live and in the (real) studio. I know I'm being a bit greedy! I have a long garage in the back of which I eventually plan to build a walk-in amp booth, room within room. I have worked on a studio construction and am sure I can tame it "just" enough to be playable without making me public enemy no 1 in Chalgrove.
I do have a Laney VH100R but that gives me organic sounds. This amp is for modern attitude, which I currently use the Line 6 emulations for.
Cheers Dave, I just don't have the time at the moment for getting technical, so thats an area I am happy paying for, although I do solder my own cables, mostly!!!
Cheers guys

[Shingles]

Greeting Nik,
The 300W resistors in fact have a free air rating of 75W so a pair are half way there already!

Fair enough.

The correct heatsink is 30"x30" of aluminium but such a soak is going to be a pretty chunky bit of kit anyway and a 2U case would come to that easily.

You think? That's 6.75 square feet of aluminium sheet. A 2u chassis gives you four square feet at most, and it's not continuous.

But we are talking sound signals here, not continuous 50Hz mains. The resistors are rated at 2X overload for 3 minutes, 5X for 5 seconds and 10x for one second. Since I advise a times two rating to start with I think that is a very conservative specification!

OK. But bear in mind that a 100 watts Masrhall will give up to 165 watts rms when fully saturated. How close it gets to 300watts continuous or more will depend very much on the musical style, and the Blue Voodoo isn't designed for clean country picking.

Yup, in parallel because if one pops you still have a load of sorts, unlikely in this case of course!^^^^

Well, that was a solid state amp, so no worries. If it had blown I wouyld just have had to start again.

Unless the OP has the means and knowledge to rebias and take account of HT shifts I would not advise removing any valves.

It's a common approach. Just need to pay attention to the change in impedance.

[ef37a]

One point five by one point five feet is 2.25 feet but I will get to the other matters the morrow. Off to me pit.

Dave.

[ef37a]

Ok.
A 2Ux 350mm does not quite cut it and they are mostly steel so that is a nuisance (but I bet in practice one would be fine with 2x300WR's). RS 490-7309 0.43dgC/W would help and you can always fit a fan powered by the signal.

I have a 400W load in a STEEL box 12X3X8" yes, with quite a chunky heatsink but that regularly gets thrashed by a 200watter and only gets moderately warm on signal and no matter how compressed/overdriven that signal the 2 watt (yes, two!) LED indicator still pulses whereas it does not for 50W sine drive.

Pulling bottles from multi valve amps might be "commonplace" but I still advise against it UNLESS the handbook says you can and gives the correct load change.

If we take that the valves in that 300W amp are pulling say 30mA each that is 180mA total current draw. Pull 4 and the HT will rise, the transformer load will be less so bias volts will rise, thus the pair that are left will draw even less current than they did so HT will be even higher!
Overvolts kills more valves generally than anything else.

To App'

So you are happy to spend out on a custom load but make your own leads!No worries mate, I might just cobble something up for my own interest and see just how hot things get!

Dave.

[Apparition]

Hi guys,
in summation then, the reason I can't buy something 'off the shelf' to do this is because of the immense heat-sink required.
That should have occurred to me when I could not find a unit rated to handle the wattage.
At the end of the day Steve hit the nail on the head and inside I already knew this was likely to be the case.
I just had not figured out why this was the case.

I'm gonna focus on my garage. With acoustics I do know what I'm doing! However that will be set back a little after today landing an £800 service bill for my landrover, which has been in the garage throughout the worst of the snow. When I needed it most. Did not get the chance to feel smug once! (or come to anyone's rescue)

Thanks for all your interest, I'm glad I raised some, and sorry I'm wimping out to you
electronics brains, I'd just rather not experiment on this particular amp. That probably sounds pretty lame!

Thanks again

[Shingles]

One point five by one point five feet is 2.25 feet but I will get to the other matters the morrow. Off to me pit.

Dave.

What?

[Hugh Robjohns]

My schoolboy physics knowledge says that connecting them in series would mean each one got half the current coming out the amp

Sorry to be pedantic, but your schoolboy physics is a little rusty.

If they are connected in series whatever current comes out of the amp MUST flow through both of them.

The voltage across each unit would be halved relative to the source total and the resistive load seen by the amplifier would be doubled.

If they were connected in parallel then the current in each unit would be half of that coming out of the amp, but the voltage across each would be the same and the resistive load seen by the amp would be halved.



hugh

[Hugh Robjohns]

Wow! This is getting confusing...

The correct heatsink is 30"x30" of aluminium...

You think? That's 6.75 square feet of aluminium sheet.

One point five by one point five feet is 2.25 feet...

Yes, Dave, 1.5ft squared is indeed 2.25 square feet... but you said the correct heatsink needed to be 30 inches square (2.5 feet each side) giving a surface area of 6.25 square feet.

Not 6.75 square feet as Shingle suggested.



hugh

[ef37a]

"The correct heatsink is 30"x30" of aluminium "

Was my first figure Hugh, did I say something different elswhere?

The biggest danger to the resistors according to the makers is "hot spots". Having multiple units reduces the chances of these so even better and less demanding heatsinkwise would be 6x 100W (47R, e.g.)but as with any engineering enterprise, cost and quality have to be balanced.

We need some more test loads for soak test duties so I might have a crack at this and get some temps' and other numbers.

Dave.

[ef37a]

Last things,
This http://www.tubeampdoctor.com/product_info.php?manufacturers_id=&products_id=385 is rated 150W so if you had the 16Ohm version all that is then needed is 150W worth of parallel 16R (I would go 300W).

430ees tho'!
Dave.