DIY Weber Tru-Load Dummy Load Box, Questions Herein

[mabinogeon]

Hi all,

I found a schematic for Weber's Tru-Load dummy box, and I'd like to knock one together for myself since it appears they don't sell them anymore:

wtl

• mabinogeon
• Oct 5, 2020

Two (or more) questions:

1) I’d like to double-up some 100W wirewound resistors for the sake of heat dissipation. Instead of two 16Ω 100W resistor in parallel to get 8Ω 200W, say, could you just as well use two 4Ω 100W resistors in series to get that same 8Ω 200w rating? I know the math is there, but is there any reason parallel is preferable to series in a situation like this?

I don't have any speaker motors laying around the shop, but I do have a working 8Ω 100W neodymium speaker that I could mount in there.

I'm having a hard time visualizing this for some reason, and way over thinking it! Wouldn't an 8Ω speaker in parallel with an 8Ω resistor present a 4Ω load to the amp? Would you need two sets of resistors in there, one for when the speaker is in the circuit and one for when it's out? That doesn't seem right. Does using just the speaker motor somehow react differently than using a complete speaker? Clearly, I don't know a ton about speakers.

Alternately, I have an old 8Ω 50W Becker speaker that I could use for parts, but then how does one hook up just the motor portion of the speaker without having the leads attached?

Bonus question: Mouser has both inductive and non-inductive wirewound resistors. If one uses inductive resistors, could the speaker motor in this scenario be eliminated all together? The non-inductive are twice the price of the inductive resistors. I'm assuming that's one of those things that matters in the hifi world, but not in the guitar amp world. I also see these low-inductance dummy load resistors which claim to made for just our purpose.

I think I've made it abundantly clear that I have no idea what I'm talking about in this scenario. Apologies.

 

[wabashslim]

1) I’d like to double-up some 100W wirewound resistors for the sake of heat dissipation. Instead of two 16Ω 100W resistor in parallel to get 8Ω 200W, say, could you just as well use two 4Ω 100W resistors in series to get that same 8Ω 200w rating? I know the math is there, but is there any reason parallel is preferable to series in a situation like this?

NO! In series the max is 100 watts or whatever the lowest-rated component is. In parallel the load is split & shared, in series both carry the full current.

I think I've made it abundantly clear that I have no idea what I'm talking about in this scenario. Apologies.

Lotsa questions here, some of which indicate you need more instruction than would be appropriate for a forum IMO.

At this stage in your journey, you should just buy something before you blow something up. Call or write Weber & see if they'll still build you one.

 

[tubegeek]

NO! In series the max is 100 watts or whatever the lowest-rated component is. In parallel the load is split & shared, in series both carry the full current.

Power is a function of both amperage and voltage (P= I × V.)

The series resistors would see half of the total voltage drop across each one, and as you say, they would each pass the same current.

So two 100W resistors would safely dissipate 200W in a series connection.

 

[Tom Kamphuys]

NO! In series the max is 100 watts or whatever the lowest-rated component is. In parallel the load is split & shared, in series both carry the full current.

NO! Resistors in parallel split the current. Resistors in series split the voltage.

Lotsa questions here, some of which indicate you need more instruction than would be appropriate for a forum IMO.

I don't find the questions weird at all.

 

[tubegeek]

re: the Weber schematic: the 2.2uF cap is in the circuit because why exactly?

re: why use parallel 16's instead of series 4's?

With the parallel connection you could make the load box switchable between 8/16 very conveniently.

 

[mabinogeon]

NO! In series the max is 100 watts or whatever the lowest-rated component is. In parallel the load is split & shared, in series both carry the full current.

It's been a while since I've taken a physics class, but a string of N identical resistors in series each rated for P watts can dissipate NP watts.

Series or parallel will increase the wattage/power dissipation of the network. For instance, if you have a 10KΩ requirement at 2W, you can use two 5KΩ resistors at 1W in series to get 10KΩ @ 2W. Similarly, if you need 10KΩ at 2W in parallel, you can use two 20KΩ resistors at 1W in parallel and achieve 2W and 10KΩ resistance.

Lotsa questions here, some of which indicate you need more instruction

Hence my question about how to hook up a speaker motor. I build tube amps, not speakers. I've never disassembled a speaker.

 

[tubegeek]

It's been a while since I've taken a physics class, but a string of N identical resistors in series each rated for P watts can dissipate NP watts.

They change classical physics pretty often, you might want to do a review.

Not.

You are correct there.

 

[mabinogeon]

re: the Weber schematic: the 2.2uF cap is in the circuit because why exactly?

Haven't the least idea. It isn't part of a crossover network or anything.

re: why use parallel 16's instead of series 4's?

Idle curiosity, mostly. In my mind, parallel would be slightly preferable in this scenario simply because they'd be sharing an identical load simultaneously, not first it hits one resistor, then travels though a wire to the other resistor. I mean, it clearly doesn't matter in the slightest, it was just a thought that occurred to me.

 

[tubegeek]

Haven't the least idea. It isn't part of a crossover network or anything.


Idle curiosity, mostly. In my mind, parallel would be slightly preferable in this scenario simply because they'd be sharing an identical load simultaneously, not first it hits one resistor, then travels though a wire to the other resistor. I mean, it clearly doesn't matter in the slightest, it was just a thought that occurred to me.

The second part of my post was a possible answer to whether parallel or series might be preferable. (I understood why you asked the question.)

The first part only makes sense if you have DC on your speaker feed, and if you do, you've got some problems!

 

[wabashslim]

Power is a function of both amperage and voltage (P= I × V.)

The series resistors would see half of the total voltage drop across each one, and as you say, they would each pass the same current.

So two 100W resistors would safely dissipate 200W in a series connection.

Yeah, oops on my part. Realized that after I sent it.

 

[wabashslim]

In my mind, parallel would be slightly preferable in this scenario simply because they'd be sharing an identical load simultaneously, not first it hits one resistor, then travels though a wire to the other resistor. I mean, it clearly doesn't matter in the slightest, it was just a thought that occurred to me.

Parallel is safer, in case a resistor or solder joint fails you still have a load on the tranny. A full open could be disastrous.

 

[wabashslim]

I don't find the questions weird at all.

I never said "weird".

So...where are your answers?

 

[Tom Kamphuys]

I never said I had the answers, but I'll give it a try:

1) I’d like to double-up some 100W wirewound resistors for the sake of heat dissipation. Instead of two 16Ω 100W resistor in parallel to get 8Ω 200W, say, could you just as well use two 4Ω 100W resistors in series to get that same 8Ω 200w rating?

I implicitly answered this question in post #4: yes

I know the math is there, but is there any reason parallel is preferable to series in a situation like this?

I agree with the answer of @wabashslim: Parallel seems to be more failsafe.

Wouldn't an 8Ω speaker in parallel with an 8Ω resistor present a 4Ω load to the amp?

I would say yes.

Would you need two sets of resistors in there, one for when the speaker is in the circuit and one for when it's out?

I would say yes.

Does using just the speaker motor somehow react differently than using a complete speaker?

I would say yes. This post shows different impedance for open and closed back cabinet: https://www.tdpri.com/threads/dual-se-vs-push-pull.1050779/#post-10151291

@mabinogeon 's questions tell me that although he probably does not have a degree in EE, he has clearly thought about it and came up with good questions actually. I feel he is perfectly capable of building this with the advice of the forum members.

 

[wabashslim]

Weber's site used to have a good explanation of speaker motors as opposed to resistors in their attentuators. Don't know if it's still there, they made changes to their site years ago and seemingly removed things, but I have a 50-watt Mini Mass (seldom used) that is certainly better than the Scholz and Marshall (I think) attens I had before.

 

[robrob]

I found a schematic for Weber's Tru-Load dummy box, and I'd like to knock one together for myself since it appears they don't sell them anymore:

There is a reason they stopped making them. I'm not a fan of this design.

 

[tubegeek]

There is a reason they stopped making them. I'm not a fan of this design.

Any idea what the capacitor was there for?

 

[wabashslim]

Any idea what the capacitor was there for?

Blocks low frequencies that the speaker motor can't handle.

 

[tubegeek]

Blocks low frequencies that the speaker motor can't handle.

:/

Mmmkay.... is that necessary because it's a coneless motor and the lack of a cone means it needs special treatment? Because it's not exactly standard practice to high-pass filter a guitar speaker at the jack. I can imagine though that a motor might need special handling possibly.

 

[wabashslim]

I've never looked at a motor but I'd guess it lacks the VC travel restriction a cone provides.

 

[johnDH]

hi @mabinogeon
You are right to ask those questions!

I've been designing attenuators over the last couple of years, so I've read a lot about speaker loads, load boxes and commercially available attenuators, including Weber.

Weber has been making attenuators based on a speaker motor for quite a few years, being the 'Weber Mass' series. Many like them, not perfect though but good value and well made.

But I just don't 'get' this load design. Not sure what it would be good for.

Without the speaker motor, its fine as just a simple resistive load. I think that would probably be the best option for use on a test bench when building or servicing amps. I agree with your options for series or parallel combos of resistors to get the values and ratings that you need.

But, when you switch in the motor, as you have noted, it goes in parallel with resistors, via the capacitor. The capacitor blocks its effect at low frequencies, so DC ohms is still determined by the resistors.

But here's the issue I see: If you are taking trouble to create a reactive load, following a real speaker better than just some resistors, you might as well get it somewhat correct. A real speaker has a rise in impedance with frequency, from a low point of its nominal value (eg 8 or 16) at around 400 or 500 hz, rising to several times that at the high frequencies. But this load can't do that, it will always be constrained by the resistors, ie, it can't rise above the value of them. Indeed, I think the impedance will drop with frequency in the mid range with some odd dip at around 6khz due to the interaction of the cap and the motor coil

Another thing real speakers do is they have a bass resonsnce set by the cone and the cab, at around 80 to 120 hz. This comes through as a local peak in impedance at this frequency. The speaker motor cant do that because it has neither cone nor cab.

You can build a good reactive load box if you want one, using coils and resistors. Add to that a further coil and a large cap if you want to catch that bass peak. The main purpose of these units is to get a signal out to go into an impulse-response unit or cab-sim of some kind. They trick the amp into responding as if driving a real speaker.

So, in summary, I think it depends what you want it for, but whatever that may be, this is probably not it!

(if wanted, can post links to various attenuator and load box threads)