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Howdy fellas, I've some simple SE questions...

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In the above photo it shows r17 and r27 as the resistors on the board. The rotary switch in this drawing I made has 5 resistors with one terminal on each side open. So when you switch to that position it would be just the value of the on board resistor. At all of the other positions you are paralleling a resistor with the value you have on the board. Hope this photo makes sense.
 
Mine essentially the same as what Russ did in his photo just not as neat ;)

Yeah, his does look pretty well executed, as does the entire build. :)

Nic, as I see this construct it looks like you start with a two pole switch, running jumpers from each to the board. Then the resistors are attached to the different points on the switch. Is that right so far?

Then the other end of the resistor is attached to a common copper wire, is it safe to assume that this bridge wire is isolated from the chassis?

Oh, also, while we're conversing, I'm curious as to what you used for coupling caps. I've been looking at the Auricaps like George used, but wouldn't be opposed to another cap as long as they weren't to pricey.

TIA

Rick
 
Yeah, I did that same thing Paul did. In my case, the permanent resistor is a 680 ohm and six different smaller resistors are switch in parallel. Here is a link to the post with a spreadsheet showing how I arrived at the values:

http://www.diyaudio.com/forums/tubelab/140978-tubelab-simple-se-big-honkin-edcors-9.html#post1835353

Awesome, I just need to read through that thread about 12 more times and maybe it'll finally sink in. :D

Russ, just to be sure on another item, on your bias switch, do you have one wire from each pole running to the board and the other running to your check points (banana plugs)?

Thanks much.

Rick
 
The common for each pole goes to the PCB pad that connects to the tube cathode. The banana jacks also tie in there. That heavy gauge solid wire in the picture is signal ground, which connects to one of the resistors other terminals on the PCB.

The 680 and 620 ohm resistors are mounted to the chassis. That's where the extra wires are going.
 
Just out of interest rknize (beautiful chassis by the way!), why did you go to the bother of wiring all of those resistors? I can understand the beauty of being able to bias for many valves, but was there any particular reason why you didn't use a variable resistor/pot in order to do the same thing without having to be a master solderer?

(Sorry to resurrect an old thread; am trying to go through the old ones in order to not ask the same question over and over)

Regards,

Mark
 
Such a pot would need to dissipate 5W or more. That means it needs to be a big, wire-wound one. Certainly possible, but this method was cheaper. Also, since I know the resistor values ahead of time, I can measure the voltage across them and calculate the plate current.

Would something like this work ok? You would need two, but at $4 each cost is not really an issue. I would be concerned if it degraded sound quality any, which seem to be a reason people avoid adding pots to their power amps.....
 
It would work. It will have some inductance being wirewound. I'm not sure if it all gets swamped by the bypass cap. Also, as was brought up in another thread on this topic, you don't want to entirely replace the resistor with one of these. You want to add some series resistance because you never want to have zero resistance between the cathode and ground.
 
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