Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: Platefire on January 25, 2009, 09:13:35 am

Title: Got my hands on Capacitance and Resistance Decade boxes
Post by: Platefire on January 25, 2009, 09:13:35 am

I had no idea such a thing existed but I was telling my amp tech friend about my 5E3 project with the channel switching betweed differend cathode resistors and bypass caps and he said you need a Decade box---so he let me borrow his for my project. Haven't tried them yet but just looking looks like I might be limited how I can use it with the values available--but all and all cool tool.  :) Platefire

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: tubesornothing on January 25, 2009, 09:19:52 am

For amp designing or tweaking - they are awesome.  I use them **all** the time when starting on a new amp design.  I have two of each resistance and capacitance.

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: jjasilli on January 25, 2009, 05:05:25 pm

Ditto.  Way to go, platefire!

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: Platefire on January 25, 2009, 08:23:31 pm

Well I was commenting on limited values but looking at the boxes a little closer I see that you can turn each one on/off individually so you can combine several values together to arrive at what you might need---so thats a little more flexible than I first understood.

So---in my case on the 5E3 using a SPDT center off--you would have to at least (I think?)go ahead and install the actual components for the center off position to hear the combined effect with the Decade box in the other two positions. I'm just trying to figure how to hook it up with that many switch combinations!! Platefire 

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: jjasilli on January 25, 2009, 10:12:01 pm

There are some boxes that contain sets of both resistors & caps.  Then you can easily put different values of resistors and caps in series or in parallel with one another, using just one box.  I have a number of such boxes.

For noise reduction, I made leads of shielded cable.  Ea lead is terminated as follows:   a) banana plug, no shiedd connection, to plug into the decade box; b) alligator clips on both signal wire and on a long length of braid on the other end.  Clip the signal wire where you want it; clip the braid to the chassis or some convenient grounding point.  This might help reduce noise in the signal under test.  But there will still likely be extra noise caused by running signal on long leads outside of the chassis, through the decade box. 

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: tubesornothing on January 25, 2009, 11:04:03 pm

ooo - I like that shielded idea.  I just put up with all the noise and oscillation.  But I like that better!

ToN

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: Platefire on January 26, 2009, 03:15:47 pm

Ok thanks, it will be a while before I'm ready for testing. Platefire

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: Greasehorse on February 25, 2009, 07:04:57 pm

IS here any concern on the wattages and voltage rating in these boxes? Or they already set up right?

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: jjasilli on February 25, 2009, 08:40:44 pm

Well, there are different types.  There are decade boxes with power resistors.  Otherwise, my vintage boxes have 1 watt carbon comps.  Of course individual resistors may have drifted, perhaps a lot.  So don't blindly trust them without measuring.  You could always replce badly drfited components.  Caps in my vintage boxes range in steps from 100 pF to .22uF.  My RCA box has ceramic & polyester caps.  Eico has those old domino caps.

Title: Re: Got my hands on Capacitance and Resistance Decade boxes
Post by: tubesornothing on February 25, 2009, 09:09:05 pm

Also make sure the switches are clean and work.  One of mine the switch is a little flakey and it really though me for a loop when trying to adjust an amp for tone.