Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: BrianS on August 18, 2014, 11:40:00 am
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...and could use some suggestions as to the connections and terminals.
I'd like to be able to use the (4) 8ohm resistors (all are 100watt) to make 4, 8 and 16 ohm loads. I know how to hook them up to get the loads I need, but I'm wondering what the best connectors and terminals would be.
The heat sink I've had in a box for 20 years, and it is a substantial piece of aluminum. I think I'm going to have to sand the back of the resistors flat before I connect them to the heatsink, as I don't think they will contact good enough if I don't. I also have heatsink compound.
So, banana jacks? Alligator clips? 18 gauge wire? Switch types?
Thanks!
(https://lh4.googleusercontent.com/-FlWixjy6phY/U_Ip6tGBDvI/AAAAAAAABmc/WBYFyGadXXk/w829-h622-no/001.JPG)
(https://lh4.googleusercontent.com/-HJP4Xyj53AQ/U_Ipx0ToeLI/AAAAAAAABmU/_dlFeyiAGHU/w829-h622-no/002.JPG)
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Here's my simple D/L. Might give you some ideas. I just used a terminal block for changing resistance. I have 8Ω/50W between black and green (or white and green). I have 16Ω/50W between black and white (Have to move a wire on the terminal block). And if I jumper black to white, I have 4Ω/100W between black and green. My wire is 18 or 16 gauge.
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Cool. Thanks! That looks like a good system.
I wish my resistors had the bolt holes on both sides like yours.
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You look like you are good to go. I like Sluckey's deal, I would avoid switches; most often rotary switches are maybe 2 amp rated and that could be light. Alligator clips can come off at inconvenient moments. Bananas or terminal strips is how I would go.
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I wish my resistors had the bolt holes on both sides like yours.
That's no big deal. just put some heatsink grease on the bottom of the resistors. I'd use those resistors in a minute! :icon_biggrin:
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Pretty much done:
(https://lh6.googleusercontent.com/UyGnV4GHb5tifaswFErMspM2_UaGfvkHqpNIXeXNxGc=w829-h622-no)
Terminal strip idea was golden...wish I would have thought of it myself! :icon_biggrin:
I figure I could run the other 8 ohm resistor in series with the 8 ohm configuration of the other 4 and get 16 ohms.
Thanks, Sluckey!!
*EDIT:
I suppose I should mention that I purchased these resistors off of ebay, directly from China. They were $15.95 total cost and were delivered within about week. They measure right on. Hopefully they are not overrated as far as wattage and last a long time.
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> wish my resistors had the bolt holes on both sides
Lay a hunk of angle-iron across the resistors. Drill 1/4" through iron and sink between resistors 1-2 and 4-5. Run 1/4" bolts through and clamp those suckers down.
If you were doing transistor-only work, what you would want is four 16 Ohm parts. Transistor (no OT taps) amps usually make much more power in 4 Ohms than in 16 Ohms. With a few shorting straps you could have 16 Ohms 100W, 8 Ohms 200W, 4 Ohms 400W, which echos the output of fairly high-power amps. (Crown DC300: 104W 16r, 190W 8r, 340W 4r.... and we used to call the '300 a VERY High-Power amp.....)
Did these claim "non-inductive"? It hardly matters for stage-amp work; the worst wire-wound doesn't have enough inductance to matter (or as much inductance as a loudspeaker). It can be important for precision specs-manship: you may read a high voltage at 50KHz and claim high power beyond the audio band, when in fact the amp is not really working hard (high impedance) and some critic will call you out for this.
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http://el34world.com/Forum/index.php?topic=9925.msg104987#msg104987 (http://el34world.com/Forum/index.php?topic=9925.msg104987#msg104987)
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^ :icon_biggrin:
PRR,
Thanks for the tips. I am doing a fair amount of solid state work, and anticipate more coming in. Thing is, how do I deal with those big 800 to 1000 plus watt power amps? I recently worked on a Crest VS900 where I replaced a bad part, but felt I could not do a thorough test of the amp because at the time I did not have even this dummy load, just a 75watt guitar speaker. The customer says the amp works, but the LEDs are not lighting correctly, or maybe they are, but something is still wrong with the amp.
When do I need a load on a solid state amp?
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For those HUGE 1000W amps
http://www.chambonino.com/construct/const3.html (http://www.chambonino.com/construct/const3.html)
K
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^That is AWESOME.
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> how do I deal with those big 800 to 1000 plus watt power amps?
Bigger resistors, obviously.
1,000W resistors are available; IIRC DigiKey lists to 3,500W (not cheap!!).
Seems to me that iron fence-wire would work. Cheap enough.
TractorSupply sells 17 gauge galvanized "electric fence wire", 1/4 mile (1380') for $13.
I do not know why it is 17 gauge.
#17 Copper is 5.16 Ohms per 1000 feet.
Steel is 6 to 10 times the resistance of copper.
So #17 Steel is 30 to 50 Ohms per 1,000 feet.
Say we want 16 Ohm units (for a 4-8-16 array). Each would be 533' to 320'.
The whole array (one channel) would two rolls tops.
Fusing current for #17 Steel in open air should be >10 Amps, or 160V 10A or 1,600 Watts per 16r unit, 6,000+ Watts for a 4r array.
For $26 and a pair of wood fence-posts to wind a few thousand feet of wire around. (Lace some bamboo stakes vertically, this could be a dandy garden-fence.)
A bit bulky. So you would need a long run of heavy speaker-cable to run from the bench to the garden. (OTOH you could zig-zag it across the rafters in the garage or shed you use for a shop.)
It looks like the fusing current for #17 Copper is 98 Amps. Steel will throw 6 to 10 times the heat per ampere, but has a higher melting point.....
But wait. Metal resistance changes with temperature. You do not want to run it too hot or your "8 Ohms" becomes 10, 12, 20 Ohms. (It is not unusual to run PA speakers hot enough for the VC resistance to double.)
Possibly a smaller gauge leads to an acceptable mix of size and power. I have a spool of nasty iron wire that I probably picked-up in the cement aisle, possibly sold for tying rebar steel rods. If it is half the diameter we would need a quarter of the length, maybe 100 feet for 16r, 10 runs across a 10 foot span, which would fit on a shop wall.
A sturdy plywood box and a 24" box-fan would allow a compact design with higher dissipation.
Doing a load-soak test on a 2*1000W amp may be nice in the winter but very uncomfortable in the summer.
> When do I need a load
You can always run them without a load. Proves it swings. Does not prove it can carry the weight. If an amp has 20 output devices, and all but 2 are blown, it will do fine at infinity or 40 Ohms but fail to swing 4 Ohms to full power. If the amp depends on fan-cooling and the fan is gummy, it may "seem OK" (if you don't know how the fan sounded new) but over-heat or shut-down after many seconds of 500W-1000W into a load.
The last 150W amps I beat-on, I held them in slight clipping for hours at a time. I used a 44W resistor in a can of water.... test was interrupted when too much water steamed off.
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This one is more expensive ($73.50)
http://www.aliexpress.com/store/product/1000W-Power-Wirewound-Aluminum-Resistor-5-8-ohm-1pcs/705952_544342382.html (http://www.aliexpress.com/store/product/1000W-Power-Wirewound-Aluminum-Resistor-5-8-ohm-1pcs/705952_544342382.html)
This is less expensive ($34.56)
http://www.ebay.it/itm/1000W-8ohm-aluminium-shell-braking-resistor-resistance-Dummy-Load-for-Audio-/121155803337 (http://www.ebay.it/itm/1000W-8ohm-aluminium-shell-braking-resistor-resistance-Dummy-Load-for-Audio-/121155803337)
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I used a 44W resistor in a can of water
This remember me something :icon_biggrin:
(http://upload.wikimedia.org/wikipedia/commons/thumb/8/89/Cantenna.JPG/450px-Cantenna.JPG)
(http://www.railroad-signaling.com/heathkit/hn31.jpg)
K
p.s.: Cantenna uses standard resistor
I don't know if the following may be an idea for a DIY Dummy Load for our purpose
http://www.qsl.net/k5lxp/projects/SaltLoad/SaltLoad.html (http://www.qsl.net/k5lxp/projects/SaltLoad/SaltLoad.html)
http://home.earthlink.net/~jimlux/hv/rwater.htm (http://home.earthlink.net/~jimlux/hv/rwater.htm)
http://www.nessengr.com/techdata/liqresistor/liquidresistor.html (http://www.nessengr.com/techdata/liqresistor/liquidresistor.html)