Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Madison on April 07, 2012, 09:05:59 pm
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Going to build one, I think.
I found a simple plan for a box with two 12 position pots.
I think I will use 2 watt resistors in an aluminum box.
Any ideas of what would be good resistor values to install?
This would be used for tube amps only.
Or, if anyone has a link t some decent info on a simple box, turn me on to it.
Thanks.
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This seems like less trouble.
http://www.ebay.com/itm/GENERAL-RADIO-DECADE-RESISTOR-BOX-VINTAGE-FIND-/280858306093?pt=LH_DefaultDomain_0&hash=item416475a62d (http://www.ebay.com/itm/GENERAL-RADIO-DECADE-RESISTOR-BOX-VINTAGE-FIND-/280858306093?pt=LH_DefaultDomain_0&hash=item416475a62d)
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You may want to save the General Radio decade boxes for precision measurement, rather than breadboard-type use. The resistors inside are made to be precision, and not necessarily handle power or large currents.
As for what values... If you're planning to use a pair of 12-position switches, then what 24 values seem the most useful to you? Is the plan to have a box for quick resistor substitution to verify a good-sounding value in a circuit? What things might you see yourself tweaking?
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>>Is the plan to have a box for quick resistor substitution to verify a good-sounding value in a circuit?
Yes, that's right.
>>What things might you see yourself tweaking?
Preamp 1/2 watt stuff typically.
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Here's a box which shows typical sub-box values: http://www.ebay.com/itm/HEATHKIT-RS-1-RESISTANCE-SUBSTITUTION-BOX-/280833791241?pt=Vintage_Electronics_R2&hash=item4162ff9509#ht_500wt_969 (http://www.ebay.com/itm/HEATHKIT-RS-1-RESISTANCE-SUBSTITUTION-BOX-/280833791241?pt=Vintage_Electronics_R2&hash=item4162ff9509#ht_500wt_969)
N.B.: The terms decade and substitution box are often used interchangeably. Technically, a decade box is for precision lab use and offers a range of precise values in small increments like 1 or 0.1Ω. Sub boxes offer a selection of useful values in large increments.
In my experience sub boxes on eBay often actually sell for about $30 - $55 or more, plus shipping, with occasional grand bargains. Parts Express has a kit for about $15. Actually that's not bad for the finished box alone, in which you could install higher Watt resistors if you want to.
There are vintage Eico (#1140) & RCA (# WC-412A) sub boxes which contain both resistors and caps. The Eico box allows switched choices: open circuit; short (box bypassed); R only; C only; R/C in parallel; R/C in series. The RCA box allows those choices depending on how you insert the banana plug, except open & short.
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Buy that thing. You'd like to have 2-3 of them, for ready tweaking of the plate and cathode resistors of a stage at a time.
The innards are probably 1-2w carbon comps. I got a few of those a while back, and gave Tubenit one.
You don't really need precision, but if you insist, you can swap out the internal resistors. Everything else is ready for you, and these units don't use the wild precision resistors made of resistance wire wound on mica sheets, a la General Radio.
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I went ahead and bought one of these because my head got tired of thinking and trying to price it out.
I think it will cover a lot of what I will do.
May still make/mod a specialized one in the future.
Thanks.