Dummy load 2, 4, 8 ohms

[elnaldo]

What if you fit one fan blowing inside, and the other one extracting?

 

[carlinos]

I considered it, but that way you just circulate the hot air throughout the entire case. I've already drilled two holes at the top near the front, and now it's ventilating well. The problem is that if I put in a grille like the ones on the rear fans, something could get inside the case and damage the connections if it's a metal object. I've ordered some aluminum grilles, which I hope will solve the problem.

 

[carlinos]

My 200W 1% thick film resistors just arrived. They're very small for the power they handle.

 

[whoaru99]

Sure. The size is small because they rely on having enough heat sink to facilitate rated power. There would be a steep power derate with no or too small heat sink.

 

[carlinos]

I hope my 4.4kg ventilated radiator is enough to handle 1600W without exceeding 85ºC, which is when these resistors start to lose performance.

 

[whoaru99]

If you have temperature monitoring/display, I'd expect you'll know pretty quickly at 800wpc / 1600W total, if the heat sink and airflow are enough or not enough. Of course, duration of power applied will have some forbearance too.

 

[carlinos]

We'll see what happens. The most powerful amplifier I have is a Yamaha MX-1000, but I don't think I'll start by testing with that. I have some fairly powerful professional amplifiers that I'm not entirely sure about, and I think I'll start with those. At the moment, the 10A relay sockets have very small connections, and quite a few jumpers need to be made between them. I'm used to assembling electrical panels from my work in electrical maintenance before I retired, but I still have some doubts because the cable cross-section needs to be larger than 2 mm², and the sockets have very small connection contacts.

 

[carlinos]

We marked and drilled. Later we tapped and milled the holes, and after applying thermal paste we placed the resistors, leaving the screw that will hold the type K thermocouple in the middle.

 

[Hipocrates]

Very nice project! thanks for share!

 

[carlinos]

I'm glad you like my project. Today I connected the first cables. I used 4 mm² cable since the only closed terminals I had with M4 holes were for that gauge. I'm waiting for an order of ceramic terminal blocks that I was planning to put on the radiator, but I'm still not entirely sure.

 

[TwoCat]

Keep one thing in mind; if you are working on tube equipment, any load interruption at high power levels will (usually) kill an output transformer.

My own home-brew load bank does use high current switches for impedance switching, but is designed such that there is always a hard wired resistor (actually, two in parallel) presented to the load, so that the possibility of a switch or resistor failure will not result in total load loss.

I was thinking about the same...but a sudden impedance change can also be a problem.
A trick designers use in RF amps, to avoid blowing the finals, is to use 'make-before-break' relays. Note these are not cheap, but then neither are the AF amps you are working with.
k

 

[carlinos]

It's important to select the desired impedance first and not touch anything during the test. I think it's unlikely that a 200W resistor will fail when testing tube amplifiers, which aren't usually very powerful, or that there will be a power outage in the case of my dummy load. I think it's much more likely that one of the input cables will come loose from a sudden pull or something similar, although in the electrical world, anything can happen.