Building a dummy load

[oguinn]

I’m beginning work to recap a Pioneer SA-7500 II. Their service manual states there needs to be an 8 ohm dummy load for testing. Does anyone have a preferred dummy load build they could share? Or is it more cost effective to buy a pre-built one?

[Deke609]

Hey Jameson - It might be cheaper just to buy one (or two): http://www.parts-express.com/8-ohm-200w-non-inductive-dummy-load-resistor--019-030

At 45 wpc, I think you'd want a 200W dummy resistor to keep it from getting super hot.  So that's 4 x 8R/50W resistors, or 2 x 8R/100W resistors -- plus something to screw them into to act as a heatsink.  Glancing at Mouser, that looks pricey.

cheers, Derek

[oguinn]

Cool, thank you!

While I have you, what are your thoughts on a Variac? Is it essential to spend a ton of money on those, or would something like either of these be alright?

https://www.ebay.com/itm/283924405477
Beleeb 20Amp Variable Transformer Auto AC Voltage Regulator Power Supply, 2000VA Max, 0~130 Volt Output https://www.amazon.com/dp/B07JV8ZCG8/ref=cm_sw_r_cp_api_i_NWclFb94KSZBQ

Any recommendations?

[Paul Birkeland]

I've worked on a 9500 or two, and you don't need to worry a whole ton about the wattage rating of the dummy load, as there doesn't generally need to be signal going through the amp to set the bias.  (If that 9500 has original power supply caps, it's absolutely mandatory to replace them, even if the amp passes signal)

The variac that I have that I like is an autoformer variac that can take in 120V and make up to 250V, but I don't see any available in the usual places.  A 20A unit would be pretty overkill, but it's not insanely expensive. 

[oguinn]

Is there much benefit for a hobbyist like me to have something that passes 250V? It seems like I’d be fine with something that passes voltages below and up to line voltage.

I’m also wondering whether this is a purchase where the name brand or cost actually matters or if I can get something economical for my purposes.

Also thanks for the tip on the 9500. If you have any other advice I certainly wouldn’t turn it down.

[Paul Birkeland]

Name brand isn't likely super critical.  The step-up is just nice for flexibility if you need to work on 240V gear.

The last 9500 I worked on passed signal just fine but sounded terrible.  I had to look at the frequency response into a load and then I saw the issues pretty clearly!  A good solder sucker makes the recapping job a lot easier.

[oguinn]

Awesome, thanks Paul. I’ve got the capacitors already, just trying to find some time with the new baby to get started. As it turns out babies need a lot of attention!

[Deke609]

  A 20A unit would be pretty overkill, but it's not insanely expensive.

I have the same 20A Amazon model (different name, same item).I got the 20A and am happy that I did b/c I've been using it to bump up my mains supply voltage to my BH amps to 120+/- 1V and mostly just leave it on.  It's only stated to be good for max. 10hr use durations ... but I figure windings rated for 20A can dissipate the heat generated by 3A more or less indefinitely. [I still need to build that voltage buck/boost box that I have the parts for, but lost enthusiasm for building]

So I'd go with the 20A for it's versatility.

cheers, Derek

[Deke609]

The step-up is just nice for flexibility if you need to work on 240V gear.

That makes sense. In which case, you could get a 1:2 step up transformer (high current rated), and pair it with the 20A, which you;d need to derate to 10A.