dirt cheap load box

ecc83

Anything known? Yesterday on the HR forum I stalled a guy who was probably about to wreck his H&K by driving it JUST into a Red Box!

Nobody it seems makes a simple resistive load box for around a nifty?

Dave.

timmysoft

Rat amps do one 

ICBM

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

ecc83

ICBM said:

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

Yeah, I can give the guy chapter and verse on how to build one but he seems very reluctant to do so IC.

Will have a gander at Rats Timmy.

Cheers  each.

Dave.

guitarmangler

ICBM said:

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

'The Harness 'made by Allan Holdsworth : 

http://ofeuillerat.free.fr/otherdocs.html

basically uses the coiled spring heater element to an old electric fire as the load for running a valve head down to line level . 

It was Allan that designed the rocktron juice extractor, too ( except he didn't like it).

vale

ICBM said:

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

does this set up need to be earthed? & screened? & is there any chance of shocks.

a kettle is a bulky thing so would want to take element out & rehouse (small wooden box). so am wondering what other things i need to pay attention to. screen with foil? earth wire?

ICBM

vale said:

does this set up need to be earthed? & screened? & is there any chance of shocks.

No. Bear in mind that a kettle or heater is designed to be safe when plugged straight into the mains . There's no more need to earth or screen it than there is with a speaker cabinet.

vale said:

a kettle is a bulky thing so would want to take element out & rehouse (small wooden box). so am wondering what other things i need to pay attention to. screen with foil? earth wire?

That would be a fire risk with anything more than a very low-power amp. The point of using a kettle or a heater is that it's purposely designed to get hot, safely.

This is not theoretical, by the way - when I've worked on high-power amps like Marshall Majors and Hiwatt 400s I rigged up loads like this, since no commercial attenuator/load will handle the full power... especially not the Hiwatt, both the ones I've worked on produced around 450W clean and over 700W fully overdriven.

vale

^ thanks for the safety info @ICBM
just seems a funny thing to have a kettle wired up the the amp. looks a bit odd. but if you think best to leave as is, i will.

ICBM

Yes, it’s simplest and best. If you’re going to use it as a normal part of your set-up rather than a workshop test load, I would also hardwire across any switches (including thermostats) and not just tape them - the last thing you want is it accidentally going open circuit, either by being switched off or a duff contact.

mart

vale said:

ICBM said:

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

does this set up need to be earthed? & screened? & is there any chance of shocks.

a kettle is a bulky thing so would want to take element out & rehouse (small wooden box). so am wondering what other things i need to pay attention to. screen with foil? earth wire?

More importantly, how long will it take to boil enough water for me to have a cuppa?

Sporky

Do different kettles affect the sound? 

maltingsaudio

mart said:

vale said:

ICBM said:

Find an old kettle or an electric heater of around 2kW, preferably a really ancient one with no power switch if you remember those... if not, gaffer tape the switch to 'on'.

Cut off the mains plug and fit a 1/4". Instant (roughly) 16-ohm load .

does this set up need to be earthed? & screened? & is there any chance of shocks.

a kettle is a bulky thing so would want to take element out & rehouse (small wooden box). so am wondering what other things i need to pay attention to. screen with foil? earth wire?

More importantly, how long will it take to boil enough water for me to have a cuppa?

Hill audio use to demo their PA amplifiers like this at exhibitions and make tea from the boiled water!

ecc83

https://www.vishay.com/docs/50020/rssd.pdf I made up a good 1/2 doz  welded ally boxes of 2mm metal using the 450W RSSD 4ox 370mm adjustable resistors. Two 4r7 in series then the band was tweaked to get a very precise 8 Ohm load as read on a special 4 wire "low Ohms" meter.

The high power handling and precision is needed for test purposes but is a very expensive overkill for general workshop testing or practice.  They do run out very expensive mind, even sans my time!

The kettle scenario is an interesting one! Checking my Russell Hobs I see it is rated 2400 at 240 V so a "burning" resistance of 10 Ohms, near enough for jazz for both 8 and 16 Ohm amp taps. However, when I measure is stone cold it reads 25 Ohms and of course that will only get higher as it heats up.

25R is of course fine as a "safety" load for biasing etc but I would not like to drive a valve amp to clipping on such a high resistance, not even on a 16R tap.

Ah! Jusfort! I have a mains consumption tester somewhere, will check the kettle kW.

"O'il be beck"!

Dave.

maltingsaudio

Thing about busses! Spotted in amp for sale section http://www.thefretboard.co.uk/discussion/133581/rat-dummy-load-box-palmer-bonamassa-di-audix-f5-mic#latest

ICBM

ecc83 said:

The kettle scenario is an interesting one! Checking my Russell Hobs I see it is rated 2400 at 240 V so a "burning" resistance of 10 Ohms, near enough for jazz for both 8 and 16 Ohm amp taps. However, when I measure is stone cold it reads 25 Ohms and of course that will only get higher as it heats up.

25R is of course fine as a "safety" load for biasing etc but I would not like to drive a valve amp to clipping on such a high resistance, not even on a 16R tap.

Not enough coffee yet Dave - 2400W @ 240V is 24 ohms not 10 .

It’s fine anyway, it’s well within the natural impedance range of a 16-ohm speaker cab at a bit higher than 1KHz.

Bridgehouse

Sporky said:

Do different kettles affect the sound? 

You’ll get into hot water asking questions like that..

ecc83

ICBM said:

ecc83 said:

The kettle scenario is an interesting one! Checking my Russell Hobs I see it is rated 2400 at 240 V so a "burning" resistance of 10 Ohms, near enough for jazz for both 8 and 16 Ohm amp taps. However, when I measure is stone cold it reads 25 Ohms and of course that will only get higher as it heats up.

25R is of course fine as a "safety" load for biasing etc but I would not like to drive a valve amp to clipping on such a high resistance, not even on a 16R tap.

Not enough coffee yet Dave - 2400W @ 240V is 24 ohms not 10 .

It’s fine anyway, it’s well within the natural impedance range of a 16-ohm speaker cab at a bit higher than 1

Ooops!

ICBM

ecc83 said:

Ooops!

Ha, I did something similar yesterday in the battery-powered amp thread . Forgot to convert 240V to 12V for working out current draw...

olafgarten

ICBM said:

ecc83 said:

The kettle scenario is an interesting one! Checking my Russell Hobs I see it is rated 2400 at 240 V so a "burning" resistance of 10 Ohms, near enough for jazz for both 8 and 16 Ohm amp taps. However, when I measure is stone cold it reads 25 Ohms and of course that will only get higher as it heats up.

25R is of course fine as a "safety" load for biasing etc but I would not like to drive a valve amp to clipping on such a high resistance, not even on a 16R tap.

Not enough coffee yet Dave - 2400W @ 240V is 24 ohms not 10 .

It’s fine anyway, it’s well within the natural impedance range of a 16-ohm speaker cab at a bit higher than 1KHz.

I thought that looked off at first glance, but didn't want to say anything because I wasn't sure. 

ICBM

olafgarten said:

I thought that looked off at first glance, but didn't want to say anything because I wasn't sure. 

A speaker's impedance curve goes from about 3/4 of the nominal impedance at DC (0Hz), to a sharp peak at the resonant frequency (usually around 50-80Hz), back down to the nominal impedance at around (usually) 1KHz, then rises to several times the nominal impedance at the top end of the audio range.

This means that the 'average' impedance - especially for distorted sounds which include more high-frequency content - is higher than the nominal impedance, so if you're using a resistive load (which does not change with frequency) it's usually better to pick a value slightly higher than the nominal if you're trying to duplicate the load on the amp under normal 'speaker' conditions. Somewhere between the nominal and double it will be about right, so a resistance of 24 ohms for a 16-ohm amp is pretty much perfect.

I have certainly never had any trouble testing amps into these sorts of loads, even running at the maximum continuous distorted power for long periods.

DJH83004

Agreed IC they are ideal whenever the amp output is not sufficient to have any significant heating effect on the element (hence changing the resistance), and even then I am only talking about if you are trying to take accurate otput measurements at a given frequency. Some of the 2kW PA amps make them cough a bit though