Decade Resistance Box

[Fresh_Start]

A search here didn't turn up as much as I hoped for.  The link to plans in this thread is dead:
http://www.el34world.com/Forum/index.php?topic=9350.msg84893;topicseen#msg84893
Then Buttery whipped up a drawing with the resistors in parallel instead of in series...

Bottom line, I'm starting a build which is going to involve a LOT of experimentation and a decade box seems essential.  Is it reasonable to try to buy one off eBay?  There seem to be Eico and an occasional Heathkit boxes, but I'm not sure what to look for.  Was Eico a kit brand also?  I'm thinking that would be better because I can always replace resistors provided that the rotary switches work OK.

Or is it more economical to just build one?  I tried to find a double-pole/10-throw (or 12-throw) rotary switch on Mouser and didn't find the right thing.  There were 6-throw switches, but even those aren't cheap. 

Any help and advice is greatly appreciated!

Chip

[tubesornothing]

I bought 2 heathkit resistor decade boxes and 2 heathkit capacitance decade boxes from ebay.  About $30 each.  My time is limited so, spending the bux worked for me.  The values have drifted somewhat, but it gets me very much in the ballpark.  One of these days I will replace all the components with high precision components.

[HotBluePlates]

Well, I found a few resistor and capacitor substitution boxes from Heath on Ebay, and as there were duplicates I gave a couple to Tubenit.

If I were in your position now, I'd look at the price of a couple rotary switches and a box or 2, and compare that to the prices the boxes have sold for on ebay. If you add the as-sold price to the likely amount for shipping, is it much more or less than buying the completed box?

If the price is comparable at all, the buying is as good as building. Just remember that these things are on there all the time, so set a very firm limit for your bid, and don't place your bid until the last few seconds of the auction. If you can't get it for your price, know that you'll see more over the coming weeks.

[Boots Deville]

These might be the images that were linked to in that thread.  I saved 'em off from somewhere recently:

http://lh4.ggpht.com/_rYf9n9CQ3bQ/S_wEElv1jjI/AAAAAAAABac/7o8cgRF5gM8/RkCk_sub_box_schem-1.jpg
http://lh4.ggpht.com/_rYf9n9CQ3bQ/S_wEEof15-I/AAAAAAAABac/WVNI_9LGdPM/RK-CK_subbox_6x3.jpg
http://lh4.ggpht.com/_rYf9n9CQ3bQ/S_wEEof15-I/AAAAAAAABac/WVNI_9LGdPM/RK-CK_subbox_6x3.jpg
http://lh4.ggpht.com/_rYf9n9CQ3bQ/S_wEEWOaISI/AAAAAAAABac/W_obrw_VRN0/s800/RkCk_sub_box_panel-1.jpg

Also go to parts-express.com and search on "substitution"

They have a couple reasonably priced boxes that might be worth it just for the box and some other components for the basis of a DIY version that might better suit your needs.  I don't own one, so I can't speak to their quality.

-John

[Fresh_Start]

Thanks guys.  From Mouser, it looks like NKK rotary switches are about $15 each.  Even at $40 including shipping, buying one with the front panel labelled, etc. looks like a good bet even if I end up replacing the resistors.

John - that cathode R/C network box is just plain cool!  Put a honking big plate resistor in there and parallel it with a pot or decade box and you can dial in anything.

Cheers,

Chip

[eleventeen]

I'd look to buy something used if I could find it on ebay. Of course you can save a particular search. By the time you buy a switch...dual banana posts...a box...label the thing so that it doesn't look goofy...

$10.      http://cgi.ebay.com/Heathkit-CS-1-Condenser-Substitution-Box-8856-/220638026404?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item335f0d1aa4

It may not be a "decade" box....

There are a half a dozen others for $20.

[Fresh_Start]

Thanks eleventeen - I was searching this afternoon and stumbled across a "condenser" substitution box.  Didn't know that was the word for "capacitor" way back when.  Don't need decades for caps IMHO.  One for coupling caps and a second for cathode bypass caps would be great.

Chip

[eleventeen]

How about this for a silly idea:

You look around for a dual-potentiometer and put it in a box. If you look carefully, I bet you can find really nice Allen-Bradley surplus ones which are rated at 2 watts and I would bet the two sections track each other pretty closely. You can buy a Chinese DVM for under $10. (Yeah, I know, it's just an idea)

example: http://cgi.ebay.com/100K-OHM-DUAL-ALLEN-BRADLEY-TUBE-PRE-AMP-LIN-TAPER-POT-/400132619033?cmd=ViewItem&pt=Vintage_Electronics_R2&hash=item5d29c33b19

But you find something for $2, not $20. Maybe you get 2 qty 1 Megs and 1 qty 10K and 1 qty 2K. It'll take a bit of work to locate them and they'll of course NEVER have the same shaft configuration. (short, long, screwdriver)

Now you repeat that times 3 or 4 or 5. 3 qty dual pots; 3 qty dual banana posts. 1 box. In the box, you also have a switch that can switch among and thus read the second sections of each of the pots, and cause the DVM to switch to and thus read any of the three or four potentiometers. 1 more set of dual banana posts for your "dedicated" DVM. Make sure you use a 2 pole switch and keep every "in-circuit" pot electrically separate!

Now you have the ability to diddle/substitute 3 or 4 resistors at the same time, and, you don't have noise or transient parallelling issues with make-before-break. The problem with sub boxes is that you're typically forced to only change 1 resistor at a time.  While that is a "nice" "discipline" it's also a PITA!

For a little more than what it would cost you to build a switched thing, you could effectively dedicate the cheapo DVM and have 3-4 boxes in one. WIth a digital readout. You wit me? Get color coded clip leads for the different pots and you'd be sportin'!

Oh, and by the way: If you're searching on ebay, don't just search for "resistor substitution box"... or "resistor decade box". Also search for "resist"ANCE"..Ideally, you want to buy srom someone who doesn't know what they have and is just reading off the front panel.

[HotBluePlates]

The cap substitution box that Eleventeen linked is a more modern version of what I bought. They work like a charm, and you really don't need an actual "decade box" most of the time in a guitar amp application.

[eleventeen]

Oooooooh decade sub box pr0n! Seller has 2 qty.

http://cgi.ebay.com/General-Radio-Decade-Resistor-1432F-/280535088735?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item415131be5f

16 hours from 16:30 PDT Snooze not!

If you want 'em. You need 1 ohm steps, 1 - 11110 ohms? I don't that range and that fineness of variation does one much good on tube design.

Still, GR is always topnotch gear.

Another one.

http://cgi.ebay.com/General-Radio-1434-G-7-Decade-Resistor-0-1-1-111-111-/300446690933?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item45f404ae75

This one is cool!

[Fresh_Start]

eleventeen and I have been talking via PMs.  He had an interesting idea about using a pot instead of the rotary switch.  Here's one approach which may have merit.  The idea is to have a pot, then place progressively smaller resistors across the pot to reduce the effective resistance.  This would give you multiple ranges of adjustment.  I do know that this approach produces unexpected effects on a pot's taper, so that might become a problem.  However, this looks like relatively simple and cheap way to get a very wide range of potential values.

Cheers,

Chip

[sluckey]

You may consider a 10 turn pot.

[eleventeen]

[PRR]

> a pot, then place progressively smaller resistors across the pot to reduce the effective resistance

OK. Put it in the 1K position then turn the 1Meg pot to each mark on a 0-10 dial.

1K || 1Meg = 0.999K
1K || 0.9Meg = 0.998,9K
1K || 0.8Meg = 0.998,7K
 . . . .
1K || 0.2Meg = 0.995K
1K || 0.1Meg = 0.990K (1% change)
1K || 0.0Meg = 0.0K (? end-resistance on 1Meg pot may be 1K)

So your pot gives 1% useful variation on the 1K range.

K.I.S.S.

WHY do you want "decade"? No good reason. Actually you want equal-interval, like the frets on a guitar, or like standard resistor value-series. 100K, 150K, 220K, 330K, 470K, 680K.... if the 100K-1Meg sound you want isn't near those values, it probably isn't in between.

And that's a 6-way switch you can buy anywhere for $2.

12-way rotary switches are out there for $2.



Call it 10-way and dink around with 1ooK, 200K, 300K, 400K... 900K, 1Meg resistors. (Note the 2:1 gap 100K:200K and the 1.1:1 gap 900K-1Meg.)

Either way, one switch covers a decade. Five switches in decade spacing will give 100 to 1meg. That's $10 of switches and you should already have a many-pack of resistors.

[Fresh_Start]

OK, the taper effect is worse than I guessed.

Bought an Eico resistor substitution box for $17.50 shipped.  Needs all new resistors (1956 carbon comps WAY off spec) but switches and box look great and work fine after a bit of contact cleaner.

Thanks guys!

Chip

[Fresh_Start]

OK, my buying binge is over... temporarily at least.  I managed to buy a Heathkit decade resistance box, an Eico resistance substitution box, and a Heathkit capacitor substitution box, all for $56.50 total including shipping.  Patience is a virtue when trying to buy on eBay.  I also found that some sellers are open to offers if their item doesn't sell at auction.  Actually got 2 of the 3 boxes that way.

I'm going to replace all of the resistors and capacitors in the substitution boxes.  The old cap values are all over the place, but it's only about $5.00 to replace them.  Replacing the resistors will be even less.  I don't care about the calibration of the decade box since I can just measure whatever it's doing.

This was definitely less expensive than trying to make the boxes from scratch.

Sluckey was nice enough to give me some 2-conductor shielded cable a while back, so I'm going to make a couple of test leads with it.  Spade terminals on the box end, and 3 alligator clips on the other end with one for the shield.  Probably overkill, but what the heck.

Cheers,

Chip