CHAMP ELECTRONICS -" THE VINTAGE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

CHAMP DUMMY LOAD - STEREO DIGITAL TRUE RMS POWER METER

(1000 Watts per Channel)

 

 

"From about nine years old I have messed around with valve (tube) amplifiers. I can remember in those early days, and through into the 1960’s, using banks of wire-wound resistors and (or) electric fire bars with jubilee clips wrapped around them to achieve the desired impedances for testing valve amps. Back then, all impedances were usually 3.75 / 7.5 / 15 ohms whereas nowadays it is simply 4 / 8 / 16 ohms. As we moved into the transistor age, impedances requirements went lower, so 1 / 2 ohms is now not uncommon too.

In 1967, at 17 years old, I’d had enough of this fire bar/wire wound resistor scenario, so I ordered from a company  who I think were called Cressall (?) and who advertising on the back page of a Practical Wireless magazine, 4 x 3.75 ohms, 1% 500 watt vitreous enamel resistors complete with mounting brackets! In their advert they quoted “we can make any resistor to your requirements”. They cost me £23 as I seem to remember, quite a lot of money back then! When I received them I was impressed. At about 12” long and about 2” diameter, they were great! 

I found a suitable case down at the local army surplus stores, made a front panel, and cut all the relevant holes for cooling fans (one sucking, one blowing) and the single large analogue meter, switches, sockets etc. A quick coat of silver paint, wire up and there she was - up to 2000 watts of direct readout at the various impedances as required.

I used the unit in this mono state for many years; in fact until about the mid-eighties at which time I was getting increasingly more transistor amplifiers in to work on and almost always stereo. This being the case, it was time to give the dummy load a “face lift” and a "re-think" to get it into stereo!

Apart she came and the making of a new, more serious, front panel began. This time I had included 2 digital readouts which are driven by 2 true RMS A/D converters. More types of sockets were added along with two huge 100 amp switches (compliments of our good old Army Stores again), and this time 2 x 10/1 step down, isolated scope outputs were installed.

A good new coat of grey Hammerite, a full re-build, and there she was - a 1000 watts per channel stereo version with 2 / 4 / 8 ohms switch able per channel. (I have added to each individual 3.75 ohm resistor a .25 ohm, 100 watt to achieve the 4 ohms, readily available nowadays!) Also there is now a mono switch on the front panel which parallels the two loads on to the left hand meter. This gives a further variety of impedances between 1 & 4 ohms in various steps via combinations of both switches. Useful frequency response is 1 – 200 kHz - more than enough for audio. I have also added a 3-position switch to the fans to provide "off", "slow" and "fast" speeds. Most of the time the fans don’t get used, but for the 400 watt stuff they go on at half speed. On the big 1000 watt per channel transistor jobs they run flat out. You could dry your hair on what they blow out!!

To buy even a 100 watt version of something like this would be quite difficult, and that is without considering the matter of price!? To buy a 1000 watts per channel version like this would be almost impossible, and would most probably cost in pounds something similar to the unit's power rating!?

Since I first built this dummy load back in the sixties followed by the re-build in the eighties, it has been in constant use, virtually on a daily basis, and is one hell of a God-Send! I couldn’t be without her now! "

John.

 

 

 

 

 

This photo shows the unit with the back hinged down. The size of the resistors, switches and cooling fans can be clearly seen.

 

 

This photo shows the unit with the front hinged down. Once again everything can be clearly seen including the mammoth switches.

 

 

 

On this photo the circuit board containing one of the true RMS A/D converters is shown (about in the middle), to the right of this is also one of the 10/1 step down isolating transformers for the scope outputs. At the bottom of the picture is the rear of the right hand digital meter assembly.

 

 

 

This shows one of the two 5 volt regulated power supplies for the A/D converter and meter readout. Separate ones were used to stop any cross-talk readings between channels.

 

 

 


 

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