### Network

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#### gtrdude

Jan 1, 1970
0
I'm building a cabinet switcher for my guitar rig. It will be relay based,
and I need to provide a dummy load for amps that are not in use. The
biggest amp is 120W, but I figure that a 50W resistance ought to be enough
just for safety's sake. There are 4 amps to consider, plus the fact that
all of the circuitry has to fit into a single rack space.

Paul

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#### Ross Herbert

Jan 1, 1970
0
On Sat, 26 Jun 2004 13:27:18 +1000, "gtrdude"

|I'm building a cabinet switcher for my guitar rig. It will be relay based,
|and I need to provide a dummy load for amps that are not in use. The
|biggest amp is 120W, but I figure that a 50W resistance ought to be enough
|just for safety's sake. There are 4 amps to consider, plus the fact that
|all of the circuitry has to fit into a single rack space.
|
|
|Paul
|

I would use Arcol 50W or 75W (they also make 100W thru 600W versions)
resistors available from RS Components for about $8 ea. Depending upon how much heat is going to be generated these may need to be mounted on a heatsink. I would mount the resistors in a rack case positioned at the top of the cabinet (not below other electronic equipment) and fit one or two 40mm fans in it as well. P #### Phil Allison Jan 1, 1970 0 "gtrdude" < I'm building a cabinet switcher for my guitar rig. It will be relay based, and I need to provide a dummy load for amps that are not in use. ** No need for that if the amps use transistor or mosfet output stages. The biggest amp is 120W, but I figure that a 50W resistance ought to be enough just for safety's sake. There are 4 amps to consider, plus the fact that all of the circuitry has to fit into a single rack space. Any comments? ** Relay switching the output of typical valve guitar amps ( eg Marshall, Laney, Orange etc ) while running at full power is a dodgy idea - there will be a brief moment with no load connected and /or a back emf spike from the cabinet as it disconnects. As a result, valve bases may arc across between pins 2 and 3 ( anode to heater ) or the output tranny can suffer insulation failure. ............ Phil T #### Trevor Wilson Jan 1, 1970 0 gtrdude said: I'm building a cabinet switcher for my guitar rig. It will be relay based, and I need to provide a dummy load for amps that are not in use. The biggest amp is 120W, but I figure that a 50W resistance ought to be enough just for safety's sake. There are 4 amps to consider, plus the fact that all of the circuitry has to fit into a single rack space. Any comments? **You don't strictly require a dummy load for MOST SS amps. If you're using a transformer coupled SS amp, then a dummy load might be a good idea, though. You should ALWAYS use dummy loads for valve amps. The cheapest and easiest, is to find an old (or new) jug element. Measure it. It should be pretty close to 20 Ohms. Take a few turns off the element and dunk it in some water. It should cope with lots of power. G #### gtrdude Jan 1, 1970 0 Phil Allison said: "gtrdude" < ** No need for that if the amps use transistor or mosfet output stages. ** Relay switching the output of typical valve guitar amps ( eg Marshall, Laney, Orange etc ) while running at full power is a dodgy idea - there will be a brief moment with no load connected and /or a back emf spike from the cabinet as it disconnects. As a result, valve bases may arc across between pins 2 and 3 ( anode to heater ) or the output tranny can suffer insulation failure. ........... Phil Good point Phil. I already use a commercial cabinet switcher with great success. I'm simply building my own to handle more amps and cabs. Maybe a make before break scheme is needed? What do you think? Paul G #### gtrdude Jan 1, 1970 0 Trevor Wilson said: **You don't strictly require a dummy load for MOST SS amps. If you're using a transformer coupled SS amp, then a dummy load might be a good idea, though. You should ALWAYS use dummy loads for valve amps. The cheapest and easiest, is to find an old (or new) jug element. Measure it. It should be pretty close to 20 Ohms. Take a few turns off the element and dunk it in some water. It should cope with lots of power. Nice suggestion Trevor, but completely unusable inside a rackmount case!! : ) Paul G #### gtrdude Jan 1, 1970 0 Ross Herbert said: On Sat, 26 Jun 2004 13:27:18 +1000, "gtrdude" |I'm building a cabinet switcher for my guitar rig. It will be relay based, |and I need to provide a dummy load for amps that are not in use. The |biggest amp is 120W, but I figure that a 50W resistance ought to be enough |just for safety's sake. There are 4 amps to consider, plus the fact that |all of the circuitry has to fit into a single rack space. | |Any comments? | |Paul | I would use Arcol 50W or 75W (they also make 100W thru 600W versions) resistors available from RS Components for about$8 ea. Depending upon
how much heat is going to be generated these may need to be mounted on
a heatsink. I would mount the resistors in a rack case positioned at
the top of the cabinet (not below other electronic equipment) and fit
one or two 40mm fans in it as well.

Cool. I'll have a look at those on the website. Thanks for the good

Paul

P

#### Phil Allison

Jan 1, 1970
0
"gtrdude"
Good point Phil. I already use a commercial cabinet switcher with great
success. I'm simply building my own to handle more amps and cabs. Maybe a
make before break scheme is needed?

** Make before break relays are scarce items.

.......... Phil

J

#### John Crighton

Jan 1, 1970
0
Nice suggestion Trevor, but completely unusable inside a rackmount case!!
: )

Paul

Hello Paul,
Do you remember the famous Heathkit Cantenna?
A 50 ohm resistor in a one gallon paint tin filled with oil,
Here is a link. Scroll to bottom of the page to see the Cantenna.
http://www.surplussales.com/RF/RFDummy-3.html

You could easily adapt Trevor's suggestion to the oil filled
paint tin and it would be usable in a rack for sure.

The resistors or resistive element as Trevor suggested
is mounted on the lid only, the can is filled with oil.

Solder some metal strips to the can or strap the can to
a bracket with big utilux hose clips. You can buy empty
new paint tins for a few dollars or use a fancier container
and fancier resistors. It is up to you how much you want
to spend. Trevor's suggestion was to get you thinking.

Regards,
John Crighton
Hornsby

A

#### Alan Rutlidge

Jan 1, 1970
0
John Crighton said:
Hello Paul,
Do you remember the famous Heathkit Cantenna?
A 50 ohm resistor in a one gallon paint tin filled with oil,
Here is a link. Scroll to bottom of the page to see the Cantenna.
http://www.surplussales.com/RF/RFDummy-3.html

You could easily adapt Trevor's suggestion to the oil filled
paint tin and it would be usable in a rack for sure.

The resistors or resistive element as Trevor suggested
is mounted on the lid only, the can is filled with oil.

Solder some metal strips to the can or strap the can to
a bracket with big utilux hose clips. You can buy empty
new paint tins for a few dollars or use a fancier container
and fancier resistors. It is up to you how much you want
to spend. Trevor's suggestion was to get you thinking.

Regards,
John Crighton
Hornsby

Somehow the wire jug element in the water trick sounds somewhat safer to me.
A gallon of hot flammable oil seems a tad risky to me. But hey, maybe I'm
just being a bit cautious.

Cheers,
Alan

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