build a high watt dummy load

[Randall]

I have a vintage Ampeg SVT I need to set the bias on. One of the 6550s white capped and flamed out one of the 3.6 ohm plate fuse resistors. It's back up and running, but I absent mindedly turned the bias adjustments when I was spraying them out (the amp was horridly grimy). It seems one needs a 350 watt 4 ohm dummy load to bias according to Ampeg instructions. The dummy loads I have seen are pretty spendy for how often I will need one.

SO, I'm thinking of buying 8 of these:
http://www.mouser.com/Search/ProductDet ... -THS502R0J
2 ohm 50 watt resistors and wiring them up on a heat sink. One string of 4 in series = 8 ohms 200 watts. Wire another identical string in parallel with the first and I should have 4 ohms, 400 watts, correct?

Now my concern is, is this any more inherently risky then using one major wattage resistor? If any one of those 50 watters opens up, I'm down to 8 ohms 200 watts. Will I notice this if it happens? Because if I don't it won't take long to burn out the other side. My fear is suddenly going to no load under power. Bad news. My thinking is the 50 watters should not get overloaded because even if the amp is capable actually putting out 350 watts with used Russian 6550s, that is about 44 watts per resistor, or 88% of maximum rating.

Does this make sense?

[Leo_Gnardo]

More or less what I did with a collection of 40 x 75 ohm resistors I bought from a surplus shop @ 30 years ago. Bolt 'em to a rack panel or chassis or a castoff heat sink and wire up - teflon insulated wouldn't hurt, hard to melt (& please don't, teflon fumes are toxic). A fan on the resistor array would be good. Also, I schmeared heat sink grease intended for transistors between the resistors & the plates they're mounted to.

Practically, I find most SVT - the originals - top out 240-260W at clip. One of the adjustments - balance - they want you to clip the amp so you can definitely see it on a scope. 400W should do with a little room to spare. Just don't leave it there for long.

Bias adjustment is done with no signal going through the amp. You could plug it into a speaker cab or load resistor while you dial up 72 millivolts on each side. It IS important to set balance, with the amp in clip & watching the scope, so your resistor array will come in handy for this.

Randall when are you gonna put your thumbs-up dog avatar on here?

[Structo]

I have a 8 ohm 100w dummy load like these.

http://www.parts-express.com/cat/dummy- ... stors/1535

[Milkmansound]

you can find high wattage low ohm resistors at surplus sales of Nebraska for much less money than that

http://www.surplussales.com/Resistors/N ... d_list.htm

[Randall]

I have looked at both those suppliers and I don't see how they are cheaper? My mouser idea is $20.

But really, is my math correct? Does four 50 watt resistors is series handle 200 watts?

[JazzGuitarGimp]

8R, 300W single resistor. Digikey $15.29:
[http://www.digikey.com/product-detail/e ... -ND/269990]