Basic amplifier testing: dummy loads

[kwayde]

Decided to follow what others have done. Bought two Parts Express 8 ohm 100 watt dummy loads and while waiting for them to arrive I bought a heat sink online for $10 and started prepping it. Had to remove one fin on each side of the sink to fit the loads, which fit nearly perfectly. Got the loads today and mounted them to the sink, then changed the lugs for female connectors to easily interchange a few different leads I made up with male connectors. Pulled some feet off an old cassette deck that's been waiting to go to be recycled and mounted them to the sink with 3/4" pieces of PVC.

 

[conductivity]

Ever dabble in liquid cooling the dummy load to get more capacity from it? Heathkit's "Cantenna" RF dummy load was a 50w resistor in a 1 gallon paint can full of oil. It was rated up to 1kw for very brief periods but several hundred watts sustained. I've never had the need, but I've considered it as an option if my dummy load won't get the job done and I need more power in a pinch.

Aluminum heatsink is the only reliable recommendation, thermal conductivity is paramount for preventing hot spots:
"Heat dissipation: Whilst the use of proprietary heat sinks with lower thermal resistances is acceptable, uprating is not recommended. For maximum heat transfer it is recommended that a heat sink compound be applied between the resistor base and heat sink chassis mounting surface. It is essential that the maximum hot spot temperature of 200°C is not exceeded, therefore, the resistor must be mounted on a heat sink of correct thermal resistance for the power being dissipated."
https://www.ohmite.com/assets/docs/acl_hs10.pdf?r=false

 

[gadget73]

hm, revisiting something I was babbling about 3 years ago. What did I have for breakfast again?

My dummy load is built on a big finned hunk of aluminum heat sink, well secured with thermal compound. Basically all the usual stuff you'd do. I was just thinking that if I needed more heat dissipation capacity than my chunk of aluminum can provide for a short period I could dunk it in water to increase heat dissipation. Not a permanent solution and probably not really an ideal one but good enough in a pinch. I've never needed to do this so it remains a hypothetical that I will hopefully not need to test.

 

[conductivity]

hm, revisiting something I was babbling about 3 years ago. What did I have for breakfast again?

My dummy load is built on a big finned hunk of aluminum heat sink, well secured with thermal compound. Basically all the usual stuff you'd do. I was just thinking that if I needed more heat dissipation capacity than my chunk of aluminum can provide for a short period I could dunk it in water to increase heat dissipation. Not a permanent solution and probably not really an ideal one but good enough in a pinch. I've never needed to do this so it remains a hypothetical that I will hopefully not need to test.

Beyond interface material, good airflow on the dummy loads makes around the same difference oil would. Water and heat corrode metal. Oil and water have higher electrical conductivity and dielectric constant than air.

So I wouldn't use oil or water, since they are like a little short circuit between the dummy load terminals. Just wanted to mention these properties, since there are some who have used oil or water as the interface material and maybe didn't need to.

 

[gadget73]

I have access to de-ionized water at work. Non-conductive. If I really needed to force my dummy load to handle serious power I could just bring home a 5 gallon bucket of the stuff. Wouldn't affect the load resistance in the slightest. Doesn't do good things to metal long term but if I actually needed a seriously high power dummy load long term I'd just build one.

 

[conductivity]

Doesn't do good things to metal long term but if I actually needed a seriously high power dummy load long term I'd just build one.

Another thing is how wise it is to test a vintage high power amplifier at full wattage. Electronic components deteriorate from high temperature. Anyone who thinks that is a concern or should I test my Yamaha M-2 at 200 W per channel?

 

[gadget73]

The original test conditions were probably vastly more abusive than anything you are likely to subject it to. It often involved a long period of partial power as a "warm up" phase, then full power for some length of time. I've never felt the need to run sustained long duration full power tests on anything, but I'm not afraid to run it up to where it starts to clip just to confirm its doing what it ought to do. If it blows up in the several seconds that requires, it was going to break anyway.

I really don't enjoy equipment that I have to be concerned about all the time. If I can't just use it without fear of it self-destructing from old age, it needs a rebuild or it needs to be owned by someone else.

 

[Bill Ferris]

So I wouldn't use oil or water, since they are like a little short circuit between the dummy load terminals. Just wanted to mention these properties, since there are some who have used oil or water as the interface material and maybe didn't need to.

Oil has been successfully used in all power company`s distribution AC power transformers for many, many decades without any significant AC current leakage issues to help heat sink (effectively radiate the heat to the surrounding air) the windings, and often thermostatically controlled electric fan assisted at the substations big distribution transformers..

And very large TV/radio transmitter final output tubes use pumped "pure water" (de-ionized) to cool them with tubes HV anode exposed to the water in a closed loop configuration..
The water`s purity is monitored via a sensitive ( low milliamp) current meter connected between the water and the transmitter`s ground circuit, as after a period of time the once non conductive cooling water becomes slowly conductively polluted from the tubes metal components immersed in the water releasing it`s conductive metal particulates, and the water cooling system must be drained/flushed and replaced with fresh pure water.

As I remember studying my Howard W. Sam`s 1st/2nd class Radio License FCC training books in the mid seventies, plus the on duty transmitter operator at a ch. 6 TV (WSIX, I think)) transmitter site, who generously offered to give me a several hr. and complete tour of the TV transmitter`s building`s inner workings down in South Miami (really Homestead FL.) in 1979, after I curiously putted up to the transmitter building on my motorcycle one early spring afternoon shortly after the station had greatly increased it tower height and power output and replaced the multi ton antenna structure.

The transmitters onsite work shop also had a IIRC correctly, a 75kw rated water cooled RF dummy load for checking out/testing the final 50kw TV transmitter`s huge visual output tube.
Very interesting and up close informative personal tour that individual provided me, and helped that he was a fellow motorcycle rider in befriending me, and found out I was also an electronics technician interested in high powered RF power generation.

 

[conductivity]

Oil has been successfully used in all power company`s distribution AC power transformers for many, many decades without any significant AC current leakage issues to help heat sink (effectively radiate the heat to the surrounding air) the windings, and often thermostatically controlled electric fan assisted at the substations big distribution transformers..

And very large TV/radio transmitter final output tubes use pumped "pure water" (de-ionized) to cool them with tubes HV anode exposed to the water in a closed loop configuration..
The water`s purity is monitored via a sensitive ( low milliamp) current meter connected between the water and the transmitter`s ground circuit, as after a period of time the once non conductive cooling water becomes slowly conductively polluted from the tubes metal components immersed in the water releasing it`s conductive metal particulates, and the water cooling system must be drained/flushed and replaced with fresh pure water.

As I remember studying my Howard W. Sam`s 1st/2nd class Radio License FCC training books in the mid seventies, plus the on duty transmitter operator at a ch. 6 TV (WSIX, I think)) transmitter site, who generously offered to give me a several hr. and complete tour of the TV transmitter`s building`s inner workings down in South Miami (really Homestead FL.) in 1979, after I curiously putted up to the transmitter building on my motorcycle one early spring afternoon shortly after the station had greatly increased it tower height and power output and replaced the multi ton antenna structure.

The transmitters onsite work shop also had a IIRC correctly, a 75kw rated water cooled RF dummy load for checking out/testing the final 50kw TV transmitter`s huge visual output tube.
Very interesting and up close informative personal tour that individual provided me, and helped that he was a fellow motorcycle rider in befriending me, and found out I was also an electronics technician interested in high powered RF power generation.

Thermal conductivity is still paramount for preventing hot spots on dummy loads used for power amplifiers, and a 1% tolerance can be used for repeatable measurements of distortion at 8 or 4 ohms. Mineral oil and water have much worse conductivity, but better heat capacity than aluminum.

I oversize my aluminum heatsink as well as chassis mount resistors and use airflow. I use 4 resistors per channel, so that their aluminum chassis provide heat dissipation. Each resistor has 2 ohms, so the load is configurable at 2, 4, 6 or 8 ohms that are nominal impedances for various loudspeakers, albeit only 8 ohms have the full wattage.

 

[Bill Ferris]

Thermal conductivity is still paramount for preventing hot spots on dummy loads used for power amplifiers, and a 1% tolerance can be used for repeatable measurements of distortion at 8 or 4 ohms. Mineral oil and water have much worse conductivity, but better heat capacity than aluminum.

I oversize my aluminum heatsink as well as chassis mount resistors and use airflow. I use 4 resistors per channel, so that their aluminum chassis provide heat dissipation. Each resistor has 2 ohms, so the load is configurable at 2, 4, 6 or 8 ohms that are nominal impedances for various loudspeakers, albeit only 8 ohms have the full wattage.

When decades ago I decided I needed to ramp up my seventies purchased Heath Kit that used (4) 8 ohm resistor rated at 50 watt per, I installed (shoe horned) (4) 320 watt at 8 ohms each non inductive 5 % into the housing with a 120v box fan offset from the vented bottom by 1" standoffs, so to be able to inexpensively, & quickly deal with the ever increasing wattage of high powered commercial sound type SS amps being brought to me in the early nineties.

The design of the front panel of the Heath Kit dummy load`s 5 way binding post`s spacing made it easy and quick to strap the resistors using shorted dual banana plugs that I created to configure/bridge for any variation of load resistance/power handling that I would need for most of my servicing requirements.
From 1280 watts @ 2 ohms, 640 watts @ 4 ohms X 2 channels, or 2 x 640 watts @ 16 ohms, or 1280 watts @ 32 ohms with all the load resistors connected in series.
Though I don`t use it any more, it`s nice to be able to easily take to any of my multiple room`s stereo system`s amp for on the spot power testing, if needed.