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Kiwame carbon resistors

skipper

Member
2007-04-18 3:01 am
OK guys another question. I am wanting these carbon film resistors because I am building a cathode resistor selector. I have a Tubelab Simple SE amp but wondering what wattage value should I use? Should I use the 5W ,3W or is 1/2W OK? For the selector switch I am planning on using a 6-position, 2-Pole, single deck rotary switch. What should the voltage rating be, should if be 250V or higher?
Thanks, sorry about all the question but I need to get these things ordered.
 
That depends on how much current flows through your tube ?

And then the value of the resistor.

Then use ohms law to find out how much you are disspating and multiply with a safety margin to make the resistor run cool.

Bigger isn´t a bad thing in this case as long as you got room.

If you parrallel while lowering the resistance then you distribute the load and can use smaller resistors.
 

chrish

Member
2003-10-20 2:43 pm
Sydney
Is there any particular reason you have chosen a carbon resistor for the cathode resistor? A wire wound high wattage resistor is probably better suited. Inductance from a wire wound is negligible (see Morgan Jones 'Valve Amplifiers). Moreover, carbon produces significant distortion, particularly when placed in a position where it will pass a relatively large current (such as in your intended utilisation).

Something else to consider is how you are going to wire up the cathode resistance with the switch. The worst way would be to have each switch position switch into the circuit the exact value of the resistance required. By this I mean if you require 1200R, 1000R and 800R, you should not have a 1200R resistor, a 1000R resistor and an 800R resistor and simply switch between them. A much better solution is to have the highest resistance required hardwired into the circuit. The switch would then be used to parallel various other resistors with that fixed resistor. This way, a switch failure or an inadvertent switch selection while the amp is powered up will be less disastrous.
 
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Since this is for testing different tubes/settings ?

What i have is a box with simple toggle switches and a bunch of painton 6W resistors in it.

That way i can combine all the resistors individually.

It´s just a piece of perforated metall with switches and some stripboard and 2 leads coming out that attach to the circuit.
 
Moreover, carbon produces significant distortion, particularly when placed in a position where it will pass a relatively large current (such as in your intended utilisation).


It is exactly for this reason carbons are well liked in tube amps. We do need some extra distortion in a tube amp to make it sound good, don't we? :)

And from what i know the Kiwame are rebadged Koa Speer, specially priced for audiophiles. Unlike some carbon comps i find these almost grotesque in their sound signature. In hi wattage applications i seem to like Caddocks.
 
The distortion and noise from resistors is insignificant compared to tubes. Nelson Pass made a very good point to one poster over on his forum a while ago. The poster wanted to know if he should pay extra to get resistors rated at 100ppm or if the 250ppm resistors would be sonically acceptable. I'm paraphrasing here but Nelson replied to the effect of something like "When they start making tubes with less than 10,000ppm, then you might want to consider weighing your resistor choices more carefully. Until then, don't waste your time".
 

skipper

Member
2007-04-18 3:01 am
I was going to use the Carbon reistors only because I had read that they make a good cathode resistor. I would be open to other types, I am still a novice amp builder and pretty much use what is recommended on this forum. That is a good idea to have the highest resistance hardwired in case of a failure.

I am doing this because I what to have a selector switch in place to try different tubes. Instead of changing resistors I can leave them in place and just change the resistance value with the swith.
 

chrish

Member
2003-10-20 2:43 pm
Sydney
See this post for the values I used in my simple se http://www.diyaudio.com/forums/tubelab/130838-simple-se-el34.html#post1706619

As for recommendations for the type of resistors to use in this application, see what Tubelab uses. There used to be a parts list on the Tubelab site for the amp. I am pretty sure Tubelab used standard wire wound resistors here (the ones that look like a white ceramic box). Deciding what is 'the best tool for the job' is not a matt of voodoo, it is physics. There are many different forms of advice on this forum. There is the technical-science-evidenced based you will get from people like Tubelab, Sy and from Morgan Jones books (Valve Amplifiers), and then there is the stuff from people that say that they just use their ears (normally from the less technically inclined/qualified). You will need to decide which camp you belong to and take advice accordingly ;)