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Load Box/Line Out - 300W~16 ohm 
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If all the resistors in this network are rated at 150W, then what is the wattage rating for the circuit?

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Thanks! Trying to layout a load box for a 100W tube amp.

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Last edited by Albert Ross on Tue Dec 20, 2016 6:23 pm, edited 1 time in total.



Mon Dec 12, 2016 7:30 pm
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Where the green crosses the green - is that a connection?
It doesn't look right to me, not that I am a wiring whiz.

I really don't know - but will be interested in an answer from someone who knows more then me.

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Mon Dec 12, 2016 7:37 pm
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mamberg wrote:
Where the green crosses the green - is that a connection?


Yes, should have put a dot there to show that.

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Mon Dec 12, 2016 7:51 pm
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I think 112.5w. But let me ask someone and I'll get back to you.


Mon Dec 12, 2016 8:21 pm
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Ok - had to do some reading, and it is a bit confusing to me, but here is my take.

I think you have the resistance calculated correctly.

The power dissipated by the two 30 ohm in parallel is max 300 watts. Putting them in series with the 16 ohm resister knocks it back down to 150 watts.

The 33 ohm resistor in parallel with that circuit gets you back up to 300 watt total max dissipation.

All that said,
1.) I don't know if I would trust myself and my amp with those calcs
and
2.) A load box of just pure resistance affects an am much differently than a true reactive speaker-type load - you may not like the results. I guess it may depend where you are pulling your signal from, but in general it will kill your highs, and take a lot of dynamics out of the amp.

Again, only my 2¢ - there are a lot of people here who know much more than me.

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Mon Dec 12, 2016 8:21 pm
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From the top, if the 2nd and 3rd are in parallel then they sum to 75w. And if those are in series with the bottom one then that sums to 225w, and if all of that is in parallel to the top one then that makes 187.5w in total. So maybe that's the answer... this is a good exercise.


Mon Dec 12, 2016 8:32 pm
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mamberg wrote:
Ok - had to do some reading, and it is a bit confusing to me, but here is my take.

I think you have the resistance calculated correctly.

The power dissipated by the two 30 ohm in parallel is max 300 watts. Putting them in series with the 16 ohm resister knocks it back down to 150 watts.

The 33 ohm resistor in parallel with that circuit gets you back up to 300 watt total max dissipation.

All that said,
1.) I don't know if I would trust myself and my amp with those calcs
and
2.) A load box of just pure resistance affects an am much differently than a true reactive speaker-type load - you may not like the results. I guess it may depend where you are pulling your signal from, but in general it will kill your highs, and take a lot of dynamics out of the amp.

Again, only my 2¢ - there are a lot of people here who know much more than me.


parallel knocks the number down, series adds them up.


You got it backwards, no wonder you voted the way you did. It's not your fault ;)


Mon Dec 12, 2016 8:36 pm
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My reading told me in series, there is only one path through them, so each had to handle the full power - there is no gain. In parallel, there are two paths - so the power handling doubled.

In a speaker cabinet though, if you have two 150 watt speakers, it doesn't matter which way you wire them (series or parallel), you still have a 300 watt cab.

Will be watching for the definitive answer.......

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Mon Dec 12, 2016 8:37 pm
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mortatone wrote:
parallel knocks the number down, series adds them up.


That's for resistance though, not power handling.

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Mon Dec 12, 2016 8:39 pm
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ah you're right!

So maybe I voted backwards :D


Mon Dec 12, 2016 8:41 pm
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perhaps the 30, 30 and 16 will sum to 150W, and in parallel with teh top one at 150w alone will sum to a total of 300w...


Mon Dec 12, 2016 8:59 pm
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I'm going with mam's theory on this one - if you put things in parallel, they share equal access to the current path.

What I'm wondering is why not use 4 16-ohm resistors in series-parallel (or parallel-series) and call it good? :idk:

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Mon Dec 12, 2016 9:28 pm
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What are you trying to achieve by using different values?


Tue Dec 13, 2016 3:35 am
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GRIMESPACE wrote:
What I'm wondering is why not use 4 16-ohm resistors in series-parallel (or parallel-series) and call it good? :idk:


The power resistors I have on hand are:

One 33 Ohm, 150W
two 30 Ohm, 150W
and two 16 Ohm, 150W.

I live in a rural area, and it takes about two weeks to get anything shipped/delivered here. :bang:

I tried running the amp into one 16 Ohm, 150W resistor. This worked, but after about ten minutes of playing the amp wide open into this dummy load, the heat sink the resistor was bolted to was hot enough to vaporize an egg.

Image

This was v2.0 load box using two power resistors.
Image

Was trying to come up with a 16 Ohm load with a 300W power rating using only the parts I have here.

I have more power resistors ordered from Mouser, so I'll prolly build a more symmetrical resistor network when they arrive.

I had someone else point out that this asymmetrical arrangement (in the original post) creates a certain amount of inductance which may add some harmonic content to the signal - while applying undue stress to the 33 Ohm power resistor... :eek:

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Tue Dec 13, 2016 4:58 am
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Mamberg was right. 300w.


Maybe you need a bigger heat sink? Or a fan? I haven't messed around with dummy loads at all...


Tue Dec 13, 2016 8:05 am
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mamberg wrote:
The power dissipated by the two 30 ohm in parallel is max 300 watts. Putting them in series with the 16 ohm resister knocks it back down to 150 watts.


So, in other words, I don't gain anything with regard to wattage rating by adding more power dissipation in series? In fact, I lose some.

With all power resistors rated at 150W; a circuit like this would be 600W... 400W... 300W ?
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Tue Dec 13, 2016 9:28 am
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I think you might be trying to get less heat by doing it the long way. There are calculations for power, resistance and how big of a heat sink you need. The single 16 ohm resistor will work just fine but you just need to deal with the heat. What are the specs on your heat sink?


Tue Dec 13, 2016 9:33 am
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Does the fan motor produces resistance as well?

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Tue Dec 13, 2016 9:38 am
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mortatone wrote:
I think you might be trying to get less heat by doing it the long way. There are calculations for power, resistance and how big of a heat sink you need. The single 16 ohm resistor will work just fine but you just need to deal with the heat. What are the specs on your heat sink?


I think you're right. The last layout I posted (w the 6 power resistors) might not need a cooling fan at all, just a larger heat sink than I was previously using for the 2 power resistors.

Here's as far as I think I understand the power distribution:
Image

But what kind of wattage are those two 16 Ohm, 150W resistors seeing? Mathematically I could believe it'd be 150W each, at peak.

But how much power is lost going thru the 4 parallel 30(ish) ohm resistors? Are the 16 Ohm resistors seeing full power?

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Last edited by Albert Ross on Tue Dec 13, 2016 11:07 am, edited 4 times in total.



Tue Dec 13, 2016 9:55 am
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ajaxlepinski wrote:
Does the fan motor produces resistance as well?


The cooling fan I was using was powered by a separate wall-wart power supply. There was a barrel-type connector on the back side of that enclosure to connect fan power.

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Tue Dec 13, 2016 9:57 am
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mamberg wrote:
...A load box of just pure resistance affects an am much differently than a true reactive speaker-type load - you may not like the results. I guess it may depend where you are pulling your signal from, but in general it will kill your highs, and take a lot of dynamics out of the amp.


This is very true. I've read some good forum threads elsewhere on building a resistive/reactive load for tube amps. I definitely wanna get into one of those type builds someday.

A reactive load is mos def what you need for direct recording, but that's not what I'm doing.

I'm looking for a way to run a 100W tube amp wide open while I'm in a 10x10 room with it - without being rendered unconscious, and to be able to tap a line-level signal off the amp's output to run into another solid state power amp.

Image

Notice the audio transformer's primaries are tapped right across the amp's output. You're getting a pretty clean signal from whatever the amp really sounds like, with some nice warm compression from the dummy load being in the circuit. But it's not like the power resistors are between the amp and the audio transformer, squashing the life out of the sound.

I've used this type of device for years, and it's sounds great going into a clean solid state amp that has all your time-based effects running between the line-out and the SS amp.

Difference was, I only had one 16-ohm 150W resistor as a load, and I was running it in parallel w a 16-ohm 4x12 cabinet. Running like this, I never had an overheating issue. The load drained off appox 1/3 the volume, and gave me a line out for a plexi clone.

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Wed Dec 14, 2016 4:04 am
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FWIW, I sorted the power distribution with help from a booteek amp builder who is very cool with answering my dumbass questions (going on 10 years now).

I need to send him a ManCrate* or something...

Image

This puts about 25% power rating on the four 30'ish power resistors, and about 50% power rating on the two 16 ohm'ers.









*http://www.mancrates.com

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Thu Dec 15, 2016 7:37 am
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ok - I was a bit scared to click that mancrates link - but that is some cool stuff!!

think I will be picking up the whiskey crate for my boss.........

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Thu Dec 15, 2016 7:51 am
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mamberg wrote:
ok - I was a bit scared to click that mancrates link - but that is some cool stuff!!


Buddy of mine (who is in a wheelchair) got sepsis from a cut on his leg. Was laid up in the hospital for months on an IV antibiotics drip. Was really down, physically and emotionally.

I searched around for something to send him. A fuckin' Hallmark card would've been like a poke in the eye.

That's when I stumbled upon the mancrates. Sent him the ammo-can-beef-jerky crate.

Sumbitch sounded better next time I spoke to him on da phone! He got a real kick outta that thing.

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Thu Dec 15, 2016 8:02 am
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Still waiting on parts to complete my DIY load box/line out version 3.0

I think I'm pretty settled on the design and layout. Don't wanna be the guy who tries to "reinvent the wheel" so to speak, so I started searching around for load boxes others had built to get some ideas. Maybe avoid some pitfalls/mistakes.


I got a boner when I saw this.

Image


This one just made me feel dumb. Here I am wasting all this time and money on power resistors when it's really this simple:

Image


This one has a kinda "Christmas tree" vibe to it:

Image


An example of an Aiken's reactive load DIY box. I think I hear angels singing.

Image

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Sat Dec 17, 2016 5:15 am
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So this geographical oddity where I live, that's two weeks from everywhere, has finally started to receive a trickle of parts/components from Mouser, Amazon, DigiKey, etc.

Image

I got the Hammond aluminum enclosure, and finally have all 6 power resistors for the project.

Two 33 ohm, two 30 ohm, and two 16 ohm - all 150 watt.

The extruded aluminum heat sinks are ridiculously expensive for the size I need. The footprint for the 6 power resistors is about 4" x 10".

I finally found one on heatsinkusa.com for $35. Should be here next week. Not sure how it's all going to physically come together. Thinking of attaching heatsink to top of enclosure and bolting resistors to underside of enclosure lid, but that may change.

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Sat Dec 17, 2016 5:42 pm
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There are some real, Mad Scientists out there! Cool shit!!! :thu:

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Mon Dec 19, 2016 2:21 pm
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test fit power resistors on heatsink.

Image

Image

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Tue Dec 20, 2016 6:26 am
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Hi.
First post n all. I build tube amps for a living.
What caught my eye was your sketch of your,what i suppose,output transformer and surrounding components.
Are you running that setup together with the powertubes? Ie;does the components of your line out see DC voltage,or what gives?

In turn,have to ask,why all this bother instead of a powerscaling setup or a well designed PPIMV? If the point of the excercise is to be able to let the amp breathe,only.

FWIW any line out setup often benefits from an elevated ground. Ie; a resistor of minor value between chassis ground,if active chassis,and output.


Mon Feb 20, 2017 2:41 am
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Racing wrote:
Are you running that setup together with the powertubes?

Yes.


Racing wrote:
In turn,have to ask,why all this bother instead of a powerscaling setup or a well designed PPIMV?

This sounds much, much better than a PPIMV when you're looking for distorted tones. Ran a PPIMV in this amp for many years. Try it yourself.


Racing wrote:
FWIW any line out setup often benefits from an elevated ground. Ie; a resistor of minor value between chassis ground,if active chassis,and output.

Ground loop elimination is achieved by de-coupling the audio output from ground.
Image

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Tue Feb 21, 2017 7:00 am
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Ah!
Now i get it...That entire setup is within your load box. For a sec there i thought the transformer was you active OT....Sry.

Much better than a PPIMV..yeah well,opinions vary. PPIMV´s as well as powerscaling setups can be very very versatile IMO. But granted,i keep an attenuator at the shop too. A TAD one..works,what can i say,and is a valuable tool when diagnosing some amps. No argument there.

Then again. Have to say that i have a hard time understanding running a 100w amp balls to the wall for a whisper at the other end. But that´s just me i guess.


Tue Feb 21, 2017 4:09 pm
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Racing wrote:
Much better than a PPIMV..yeah well,opinions vary.

Larry, is that you?

:lol:

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Wed Feb 22, 2017 3:42 pm
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