Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: lego4040 on February 26, 2015, 12:44:16 pm
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If you dont have a speaker when your about to test a amp and get its voltage readings, can you use this? i have it set up for 4/8/16 ohm settings and its a 150 amp power resistor
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Yes. You can use a 'load' resistor of proper ohms and wattage in place of the amps speaker.
They are also nice to use when poking around and testing an amp with a signal generator for power output and you don't want any sound coming out.
Brad :icon_biggrin:
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I test my power amps for maximum power always with this kind of resistors and when they get too hot I put that resistor into a plastic bucket with water.
And if the water gets too hot then I let the cold water flow into the bucket.
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I hope you mean putting the resistor in a plastic bag in a bucket of water. My father did similar thing when he was testing his high power Ham Radio's
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Not in a plastic bag. I isolated the connections and put the whole thing into streaming cold water.
Don't touch it during testing, just measure the voltage.
I do this up to 5KW without any problems.
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The adjustable resistor seems very sturdy, only remember that using a 20R resistor at 16R - 8R - 4R, the dissipation ability is only a fraction respect the whole R Wattage
K
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This is my DL, when I did CB repair we used a "paint-can" load, a paint-can full off resistors soaking in transformer oil with an N type connector on the lid, was good to 1 Kw.
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If you already have the resistor you may as well use it. Some comments.
* It's good to use a speaker when testing an amp, because you can hear important things that would be masked by a dummy load: Bad noises, hum, squealing, etc. "Good" noises like slight pops when a voltage probe is touched to a plate pin lug tell you the amp is working.
* The wattage rating should be 2X or more of the expected wattage in a circuit. A 50W amp under distortion puts out 100W. So a 200W dummy load is preferable in that scenario.
Duncan & Aiken have great ideas and schematics. Below is schematic I adopted from there for my dummy load box. The resistors are mounted with bolts tapped into to large heat sinks I salvaged from an SS amp.
You can use a dummy load, and a speaker at reduced volume, if you tap a Line Out from the Dummy Load.
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I had a 7.5 Ohm 44 Watt resistor, and I ran it in water even for overnight stress-tests. Lasted 30 years with minimal corrosion at the terminals.
At higher voltages than 20V-30V, or at higher impedances, the electrolysis or leakage might be significant.
I would think if it is hot enuff to need added cooling, it is hot enough to melt a plastic bag?
The class coolant is oil. Dry oil has super-low conductivity, and even the (toxic!!) damp oil you drain out of your engine won't leak much by speaker-impedance standards. However oil leaks and never evaporates. HeathKit used to sell a "Cantenna" which was a gallon paint-can with the RF-standard 50 Ohm resistor mounted to the lid. I think there was a vent which didn't leak "much". You filled it with mineral oil (clean and inexpensive) to load-up your transmitter when you didn't want your problems to go on-the-air. However RF has the special problem of short wave-lengths. If the load resistor is not "small" compared to a wavelength, it acts funny (the wave does not reach all corners at the same time). Audio electric wavelengths are OTOO a mile or more. 2-Meter radio is 7 feet, and hams explore even shorter waves. A compact resistor is better.
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I have one of those radiator type oil filled heaters,1500w rated, and at high setting, it runs about 9.3 ohms. No water needed for most amps :laugh:
It makes a great dummy load, and has a 900w, 600w setting for other loading values. I assume they vary a bit from heater to heater, but no big deal (close enough for tube work)
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I gotta contact my pops this weekend and ask if he still has his setup from his Ham Radio days. He rebuilt a WWII era Ham Radio with giant tubes. When he would mic the damn thing all the electronic items in house would go bonkers, we had to isolate everything. The coolest thing with that Ham Radio is that it was from WWII and Europe. My father and our family were forced out of Belgium during the Nazi Blitz and my Dads never been back since. He used to be able to talk to people all over the world and they would send each other flags of their areas. My dad starts talking to this guy in Belguim and he winds up being from my dads area Brussels. Long story short, he winds up being my dads neighbor and my grandfathers best friend, gets better, next time they talk over the Ham he suprises my Dad and brings my fathers uncle over, they haven't seen each other or spoke since the war. Talk about a small world reunion
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Very nice story
Franco
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I have my old Heathkit Cantenna it was a dummy load for Ham radios
I wonder if that would work it is oil filled the size of a Gallon of paint
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I have my old Heathkit Cantenna it was a dummy load for Ham radios
I wonder if that would work it is oil filled the size of a Gallon of paint
It'll work as a 50Ω load. You'll need a different resistive element for a speaker d/l.
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I do have one of them big Green Resistors