Building a dummy load - suggestions please

[cjlectronics]

I am an effects/keyboard/solid state music equipment repair shop and have been getting more and more requests to repair amplifiers. I've turned away many amp repairs but they continue to keep coming in. So, I'm looking into a good dummy load for amplifier repair.
I've searched this forum and found a few suggestions such as power resistors from Mouser (which others say are expensive) and a 1000W heater coil (which looks promising). What about old speakers where you remove the cone and magnet and only use the voice coil? If I'm way off base here understand that I am still in the research stage of taking on this venture.

Another thing, with these high voltages and currents where is a safe place to attach your oscilloscope to look at the output? Should I float my scope to avoid damage to my scope?

Thanks
CJ

[stokes]

Get a couple of 100watt 8ohm resistors,you can use one for 8ohm loads and use two of them in series for 16ohms and parallel them for 4ohm loads.To look at the output just connect the scope to the dummy load.There will be times you need to use the scope to look at the signal earlier in the circuit,just connect the scope to the coupling caps on the "grid" side of the cap.

[Ptron]

Another thing, with these high voltages and currents where is a safe place to attach your oscilloscope to look at the output? Should I float my scope to avoid damage to my scope?

The ground lead of an Oscope is tied directly to the mains ground so you can't really "float" it. Maybe that's not what you meant, though.

[cjlectronics]

Thanks for the input. The 8 ohm resistors is a good idea.

Yes, you are correct, the ground lead of the scope is tied to the mains ground. However, to measure high currents and voltages you can lift the mains ground of the scope and this will float your scope off of ground. I know this because years ago I blew up a scope probe working on a high voltage circuit that was not referenced to mains ground. In order to look at the output signal I had to use an isolated Variac and then I could probe directly across the output (probe on one side of the load and the ground lead on the other side of the load ) without blowing up the probe or scope. For obvious reasons this is not safe and I'd prefer not to do this.

Now here is the disclaimer, this happened a long time ago and I didn't understand what I was doing then and even now I'm a little unsure and this is the reason for my question.

So, I guess I should restate my question. Is the output of a tube amplifier referenced to mains ground? Is it safe to keep my scope ground attached to the chassis of the amp while I probe the speaker wires?

[Mark Black]

The output of a guitar amp is not always directly tied to mains ground.

I prefer to have my test equipment plugged into the same strip (common grounds) and let the signal generator accomplish the grounding for the scope. This usually works pretty well unless you need to measure very low-level signals.

I hate having the grounding leads on my scope probes because they seem to have a high level of attraction for energized points in the circuit .

[bob p]

The ground lead of an Oscope is tied directly to the mains ground so you can't really "float" it.

isolation transformer.

[bob p]

stokes' idea about using an array of 8R resistors and switching them for 4/8/16 R loads is a good one. i've done that with a gang of 4R resistors for 2/4/8 R loads.

one thing that you might notice, though, is that 8 R resistors aren't always available in the wattage rating you're looking for, and when you can find them they can be rather costly.

another option is to use another value that is close. bearing in mind that 8R is really a nominal resistance, i kept an eye on the eBay auctions for a while and found somebody liquidating a box of Ohmite 100W 10R resistors. I ended up getting 10 of them for $10. then I took an old Compaq 486 PC mini-chassis and gutted it of everything except the PSU. I bolted the resistors down to the chassis on their Z mounts, and ran wires to a rotary switch on the front panel so that I could select 5/10/20 R loads. when I run the load box I switch on the computer PSU and the fan takes care of forced air cooling.

with this type of setup I was able to confine my dummy load into a stackable box that's only about 3 inches tall and 14 inches wide. it sits on a shelf on the bench and doesn't take up too much space.

for those really insane high powered amps like the Carvers and the QSX that put out a kilowatts of power, i use a completely different dummy load -- a toaster. the hot and cold Z of the nichrome wire element is 16 R. yes, you can use a carver amp to make toast. i've also used a 120 VAC 1200/1500W selectable range Arvin space heater. At the 1200W setting the Z is about 8 R.

Other people I know have used heating elements from electric washing machines. Those babies can handle a HUGE amount of power.

[Regis]

Parts Express has dummy load resistors for reasonable prices.


I wonder if these would be good for building an attenuator?

[Enzo]

Looking in Mouser, I see:
Arcol brand aluminum housing power resistors,
8,4 ohm 100 watt - $12.22
8,4 ohm 150 watt - $15.79
8,4 ohm 200 watt - $46.51
(but why pay that much when two $12 100 watters works as well?)

I don't think $12-15 is all that expensive for a load resistor. Series/parallel them for higher wattages.

For that matter, the 50 watt ones are only $3.19 each, which means a 200 watt load can be made for your $12.

The Vishay/Dale listing only goes up to 50 watts, but those are $4.26, and down to $3.67 in 10 lot.

I like the aluminum ones. I have a set of Dale 250 watters here. But the plain old ceramic tubular ones will work. Vitreous enamel I guess it is.

The Ohmites come in 4 and 7.5 ohm - close enough - and the 100 watt ones are $9.45. The Vishay/Dales are $6.64 each.

Lots of reasonable options.

I would never float my scope, I would float the unit under test if anything. The only time I don't have my scope referenced to the chassis or other circuit ground is when working in switching power supplies and some low voltage supply is perched on the -170v rail or some such. In a tube amp I'd have to think hard to come up with a need for floating something.

In any case, if you float the scope, and reference it to a hot portion of a circuit, then the scope frame, case, probe connector, and everything else is now sitting right on your bench at whatever elevated voltage you are working with. SOunds dangerous to me. Run the UUT off an iso so it floats, now your bench remains at a nice polite ground level, and the only place where anything sits funny is in the amp chassis.

I'd say an awful lot of amps reference the speaker to ground, but there are amps like the Mode 4 Marshall - MF350 - which is actually a bridged output, and neither side is grounded. It boils down to knowing what you are working on.

[bob p]

I would never float my scope, I would float the unit under test if anything. The only time I don't have my scope referenced to the chassis or other circuit ground is when working in switching power supplies and some low voltage supply is perched on the -170v rail or some such. In a tube amp I'd have to think hard to come up with a need for floating something.

Good point about the floating potentials. Its important to know what you are doing!!!

Hijacking for a moment... I was looking at the Service Manual for the Carver M-1.5t the other day, as I have a dead one of these sitting on the back burner waiting for me. The manual lists the following test equipment being necessary on page 11:

Scope (isolated)
DA (with signal source)
22A Variac
Line current monitor
Line voltage monitor
8R load
DVM
ACVMs

I guess that this amp could fall under the category of a switching power supply, no? Without an isolation transformer, I think it would be pretty hard to do diagnostics on the PSU. What do you think?

[bob p]

Regarding finding cheap ways to make power load boxes, I've found that a good way to exploit the cheap-factor is to keep an eye on eBay. I ended up finding one fellow who sold LOTS of resistors, and I ended up buying 4 of his auctions and having him gang ship things to me. I practically stole the hardware.

As I mentioned earlier, I got a gang of 10, 10R, 100W resistors for all of $10.

I also got 50 ohmite, 15R, 30W "Stackohm" resistors for about $20. They normally cost $12 each. If I ever get around to it, I'll be using these to design a series of stacked T-type attenuator circuits, or perhaps a ladder-type attenuator instead of the old "trainwreck" L-type design which doesn't maintain constant Z.

The bottom line is that if you're creative with ohm's law, you can find all sorts of good deals on surplus gear for next to nothing. Of course, if you need something right away, there are plenty of off the shelf solutions available.

Have fun.

[Enzo]

The Carvers are commutating amplifiers. The power supplies are more or less linears, with the transformer controlled instead of wide open. The power supplies may be switched in and out - commutating - at who knows what speed as they follow the waveform, but the AC being rectified is 60Hz. I would scope it as I would any other linear supply, though I would be aware that the primary mains circuit won't be referenced to chassis.

A switching power supply starts with DC, usually the mains is directly rectified into about 170vDC, or doubled to 340vDC. (But you can also start with +12vDC from a battery or 48vDC from a phone system battery.) If you want to work on the primary side there, none of that is referenced to ground. In the 340v example, the control circuit for the switching xstr is probably referenced to the -170v. Your scope would have to be grounded to that to view say the waveform at the base of the xstr.

SInce the switcher is running at that high rate, the filter caps on the secondary side only have to hold up the output voltage for 1/100,000th of a second instead of 1/120th. The transformer is small because it only has to replenish the charge spent in that brief interval.

In any case, that DC is switched on and off through the primary of a small transformer at a high rate - like 100kHz.

At the output of the supply, you wind up with whatever voltage you wanted, and the amp circuits don't know any different - linear or switching, either way the voltage is sitting there waiting.

The Carver wants the isolated scope, but what matters is that the scope and amp are not grounded together. I'd iso the amp as mentioned before. If you iso the scope, and it is on a riser like mine, then not only is the scope hot as I mentioned, but also mine sits right next to the signal gen and the riser is metal, so I have to make sure the scope doesn't touch anything around it. Too much hassle for me.

The only place the scope is an issue on the Carver is the primary triac circuit.

[bob p]

The only place the scope is an issue on the Carver is the primary triac circuit.

thanks. that is what i had gathered, that you needed the isolated scope for observing those funny looking triac waveforms.

perhaps one reason that Carver recommends the use of an isolated scope is that its technically easier to isolate the scope than the amp? there are lots of us that have a 3A isolation transformer that we could use to float the scope, but not many of us have a 22A isolation transformer that would be needed to float the UUT.

[Mars Amp Repair]

yes,
I bought 4 used irons at Savers thrift store for about $1.50 ea. The nominal dc resistance is about 12.6ohms, but I'm only using them to cook the higher powered amps. you can also calculate power out using that value since the transistor amps aren't as particular about impedance as the tube amps are...also you rarely run into a 1000watt tube amp anyway!!

Otherwise I used the 8ohm 50watt aluminum cased resistors from Mouser in parallel & series/parallel to make my 8 & 4 ohm loads.

[Steve Conner]

Hi guys,

I don't much like floating scopes for safety reasons, and I try to avoid it wherever possible. I've never come across a situation when I needed to do it, in the context of hi-fi gear or musical instrument amps. I just clip scope ground clips to the chassis and it all works out.

For tube amp work, I like to use 100:1 high voltage rated scope probes instead of the regular 10:1 type. You can happily prod around power tube plates and rectifiers, and they also load preamp circuits 10 times less than the 10:1 type do.

For working on stuff like SMPSs that is tied to the AC line, I keep an isolating transformer around. When working on a SMPS you just fire it up on the isolating transformer and clip your scope ground to the DC bus negative, et voila. The 100:1 probes are great here too.

The wiring on my isolating tranny preserves the ground connection, so any metalwork on the DUT that was grounded before will still be grounded: only the live and neutral wires will be floating around inside. I like it that way for safety.

I actually have two isolating trannies, a 500VA unit that I keep permanently hooked up to a variac, and a big mutha one that must be at least 1kVA continuous. Like all transformers, you can load them at least a factor of two over their ratings for short periods, so you could possibly make do with a smaller one than you think. They were invaluable when I was working on solid-state Tesla coils a couple of years ago.