Reactive Load Build - Aiken/TGP/Suhr

[johnDH]

I'm realizing now just how much aluminum in proximity to the air core coil affects it. I said something in a previous post about cramming the reactive load build into an aluminum enclosure like a Hammond 1402DV. I wouldn't try to do that.

I don't think we need to worry about an aluminium case. It has magnetic permeability virtually the same as air. Carbon steel is what not to use it has permeability about 100x greater.

https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)

I build my attenuators in an Al case. It's the ideal material for that reason and also for very good thermal conductivity to spread and dissipate heat.

I've checked this too by measuring inductance. No change i can measure when mounted on an Al plate. But put a steel bolt through the coil and it can add 50% to the measurements.

 

[Phrygian77]

I don't think we need to worry about an aluminium case. It has magnetic permeability virtually the same as air. Carbon steel is what not to use it has permeability about 100x greater.

https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)

I build my attenuators in an Al case. It's the ideal material for that reason and also for very good thermal conductivity to spread and dissipate heat.

I've checked this too by measuring inductance. No change i can measure when mounted on an Al plate. But put a steel bolt through the coil and it can add 50% to the measurements.

This is a significant change. It would actually read .452mH if it were sitting parallel. It's at a slight angle because of the way it's sitting on the zip ties.

 

[2L man]

Aiken Reactive Load is great because it is very easy to modify to work Attenuator as well.

There are two teansformer standards. Older is 100V transformer standard. They were used a lot in PA systems like in schools, hospitals, etc... where signal was transfered long distance so it was good to increase voltage. Some tube amplifiers have 100V outputs. Then there was transformer close the loudspeaker which change 100V to loudspeaker impedance.

Other transformer standard is 600 ohm which are used for same purpose and it might be newer used with teansistor amp PA systems and I did use this.

I have both types but my 600 ohm transformer has 12.5W, 25W and 50W inputs and 4, 8 and 16 ohm outputs there are five different winding ratios so I achieve five different attenuations.

I did calculate them but don’t remember them now but I recall highest attenuation is about 22dB. Sound stay clear obviously because of transformer attenuator.

 

[2L man]

This is how my 16 ohm Attenuator looks now. Bottom middle is 10 ohm 200W series resistor. On left is black 1,8mH air core series coil and above it is its 100ohm 50W green parallel resistor. On right corner is resonance circuit: Four parallel connected blue 56uF capacitors, 15mH/600W ferrite toroid coil and 100 ohm resistor. Bottom right is the transformer. Front plate has now loudspeaker screws what I use for transformer setting but I will change them easier to operate turn switch when I build it neater. I will also add three more resistors so that series resistor will be total 10 ohm 800W and it should be good for 200W amplifier.

 

[2L man]

I forgot to write that transformer input is parallel with 16 ohm reactive load so it and loudspeaker which is on its output has effect to total impedance but I have not calculated them.

if anyone needs true ”bedroom tube amp” it is possible to install also another higher winding ratio tube output transformer for higher attenuations.

 

[johnDH]

hi @Phrygian77

Those measurements are interesting, are we seeing the Al plate actually reducing the inductance?

 

[Phrygian77]

Over the last couple of days, while I'm off for winter break, I finally got around to creating some drill templates for the enclosure and a layout/template for the eyelet board. I also got some new 80mm fans in just before Christmas, but I've been too busy with the holiday to test until now.

The enclosure is a Hammond 1458VD4. It's 18 gauge steel, 8"x8"x4", with vented sides. The aluminum resistor plate is mounted on 3/16" standoffs. I may increase that and/or add some vent holes to the top panel if heat is a problem. The 172mm length of the aluminum plate just barely clears the front and rear enclosure panels. It worked out fine, but I should have left more room for error.

Hopefully, I'll be cutting and drilling board tomorrow for the rest of the components.

hi @Phrygian77

Those measurements are interesting, are we seeing the Al plate actually reducing the inductance?

Yes. Steel obviously has the opposite effect because of its permeability. Mine will be mounted 5/8" away from the chassis, so hopefully not close enough to be a problem, but to be sure, I'll check it once it's in place before soldering everything together.

 

[Phrygian77]

 

[Phrygian77]

 

[smuc]

Some really nice work there mate! Well done!

 

[Phrygian77]

Some really nice work there mate! Well done!

Thanks!

I debated about adding a line out and level control. I don't really need it with the Torpedo CAB M.

I've got most of the parts to build another one. I may do a slightly smaller and lower power version without a fan. Those resistors don't even break a sweat at 50 watts RMS.

 

[Len058]

Great build! It looks and sounds the part!