Dummy Load - Finally finished!

[jjasilli]

Promised myself (last year) no more amp projects 'till I beef-up my test equipment.  My 'scope skills are now functional (barely).  The dummy load is a switchable Aiken design with test pin jacks for a meter & 'scope, a la Ted Weber.  And it's stereo.  It's built into the chassis of an old broken SS hi-fi amp I've had lying around.  It has large heat sinks to which the resistors are mounted.  (Half the enclosure & faceplate are left over so I can build something else into it in the future.)

Schematic is attached.

Next project:  Power Tube Cathode Resistor Box w/switchable bypass caps -- in stereo.  To be built around Ted Weber's 500R 25W rheostats.  Will have test pin jacks and bias sense resistors.  Have the parts; need the time.  Then back to amps.

[PRR]

> broken SS hi-fi amp I've had lying around.  It has large heat sinks to which the resistors are mounted

Clever idea!

[tubesornothing]

Most excellent.  I am interested to see how the power tube cathode resistor box turns out. 

[jjasilli]

Clever idea!  Thanks!  I originally was saving that chassis (rack mount) for a stereo amp project, a la Hoffman.  But I've got a ton of backed-up amp projects and a dearth of test equipment.  This chassis had to be bastardized anyway, so now it's a dummy load.  

I am interested to see how the power tube cathode resistor box turns out. Maybe I'll put the power tube cathode bias circuitry in the same box as the dummy load.  There's lots of room in that chassis.  I'm too much of a klutz to have alligator clips everywhere, so I want lots of solid connections, like banana jacks.  Otherwise I'm bound to electrocute myself (again) or fry an amp (so far so good, but it’s not like I haven’t tried – they are resilient little tykes).  

The idea is to vary the cathode resistor value with a rheostat.  The rheostat is only 500R so I think I'll put a switchable 470R 5W fixed resistor under the rheostat in series.  On top of the rheostat in series (per side) will be a 1R bias sense resistor.  Test pin jacks for a meter (or two) will verify resistance values; and measure cathode current draw.  In parallel will be switchable bypass caps ranging from 20uF to 100uF.  So, different values can be easily A/B'd before the final circuit is hardwired.  

So much for my '70's era tape labeler.  Looks like I'll need to develop yet another set of skills with google sketchup just to make decals to label the rheostat values.  Any suggestions are appreciated.

[tubesornothing]

The idea is to vary the cathode resistor value with a rheostat.

Good idea - that would be very useful.

So much for my '70's era tape labeler.  

You can always get a modern label maker.  Different fonts, different colour tape - even clear tape.  Or if you have a "straight through" photo printer, you can print on "amula jet" aluminum.  A magic marker works too.  (works for all my home grown test equipment).

[PRR]

> So much for my '70's era tape labeler.

Dymo? If you can live with white tape and 2-3 choices of fonts, I've seen '90's-era labelers as cheap as $19.95 at the Drug-Rite, WalMart, etc. You can pay more for more-fonts, more choices of tape-cart, PC connectivity for arbitrary bit-image..... but you still need a 20-buck labeler for test-gear, workshop boxes, light switches, etc.

[jjasilli]

Dymo it is!  Has a sort of 1950's charm for labels of test equipment.  But a pot or rheostat requires finer gradations.  I need a circular pattern with gradation marks and numerical values, preferably in ohms, for this project.

[PRR]

> pattern with gradation marks and numerical values, preferably in ohms, for this project.

THIS project, I'd think two 6-throw switches. 10, 15, 22, 33, 47, 68 ohms and 100, 150, 220, 330, 470, 680 ohms, or some such spread. You don't need 1% reading. Getting within 10% of target current is plenty good enough. A 10% change in tube current needs about 20% change in cathode resistor.

You would probably be fine with a 12-throw switch and a pack of 100 ohm resistors.

Sharpie hath its charms too.

[craneclimber]

<<I need a circular pattern with gradation marks and numerical values, preferably in ohms, for this project.>>

I used to use dry transfers by Datak back in the day when I was putting together homebrew equipment.
I did not spend a lot of time searching, but this link may prove to be of some use as a starting point if you choose to look for these online.

http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=dry+transfer

Being the packrat that I am, I still have some of these laying around -- see images.
I don't know if they will stick or not, they've been in storage for 30 + years. I'd be happy to send you a sheet as I have no immediate need of these. I have them in black and white.

[dpm309]

I am looking to build a dummy load and stumbled across this old post.  For some reason my computer will not let me open files with the .sch extension.  I have spent the last 2 hours all over the Internet to see how to open this with no success.  Does anyone know an easy way to open the Dummy Load.sch file that jjasilli posted?  Or does anyone have a pdf or jpg of this.

Thanks,

Dan

[sluckey]

Look here...

http://www.el34world.com/Forum/index.php?topic=500.0

[jjasilli]

Due to Isotone's name change to Dummyload, this thread. . . oh never mind   

[DummyLoad]

whaaaaaaadaya mean! i'm done for?  i gotta a few years left in me...  

 

[DummyLoad]

cool idea btw... i have a stack of dead QSC amps in the back garage.... hmmmm

[dpm309]

Sluckey thanks for the link.  I now remember seeing this post previously.  I downloaded it and it works like a charm.  Using jjasilli's schematic and a couple of other schematics I found, I have a attached a crude drawing of a 8 Ohm, 200 watt dummy load that I will build.  Since most of the amps I work on are in the 5 to 100 watt range, I think this should work for me.  I will mount the resistors in a Hammond box with a 1/4" jack.

Dan

{EDIT-- added cropped/sharpened version of drawing -- PRR)

[dpm309]

Sorry for the previous attachment.  I have attached another drawing using ExpressSCH.  Hope this is easier to read.

Dan