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Old 1st April 2013, 12:54 PM   #1
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Default R17 & R27 selection.

I've had my SSE up and running for a few days now and all is going very well.

I'm about to order some CL90 inrush resistors from the US as no one in the UK has them and I want to get a few other bits along with those.

I've been reading Georges simulations on the Tubes and Transformers page but I'm not sure if I've understood it correctly and I don't want to order the wrong parts.

I'm running Tung Sol 6L6GC STR and some Svetlana KT88 tubes and I'd like to get corresponding resistors for each.

My Power transformer is the Edcor XPWR059 and opt's are CXSE 25-8-5k (5000k load?).

For the 6L6's, with a b+ of 450v I'm looking at a resistor value range of 430 to 680, is that correct?

And for the KT88's, same b+ I believe I'm looking at a range of 330 to 560?

If I'm right so far (and its from here I'm a bit lost) how do I select resistors within that range?

Also, if its advisable to gather a range of resistors to future proof my tweaking is there any suggested recommendations? I don't have the space to rig up a rknize style resistor selector switch but I have mounted everything in such a way as to make the resistors easily accessible and only 5 minutes to change.

Last edited by mudchute; 1st April 2013 at 01:18 PM.
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Old 9th April 2013, 02:15 PM   #2
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Quote:
Originally Posted by mudchute View Post

For the 6L6's, with a b+ of 450v I'm looking at a resistor value range of 430 to 680, is that correct?

And for the KT88's, same b+ I believe I'm looking at a range of 330 to 560?

If I'm right so far (and its from here I'm a bit lost) how do I select resistors within that range?
Yes, you are correct.

A couple of things to consider:

The power dissipation numbers in red are busting out of the typically published numbers for each tube type. The power numbers in black are within the tube specs. You can usually get away with this without issues although the harder you drive the tubes the shorter their life will be.

The cathode R's set the bias current through the tubes; lower value R equals more current. You can get a good approximation of the power dissipated by multiplying the B+ voltage by the current, or the anode voltage (Vp in the tables) times the current (a little more accurate).

So, for example for a KT88 running at 450 B+ and 109 ma (Rk=330R), we get 450*.109=49W or Vp*Ik 429*.109=46.7W. It looks like George has the max power spec for the KT88 set at around 35W, although some specs rate them as high as 44W or so.

In the tables, load Z=output transformer primary imp (5K in your case), Rk is cathode R value, Ik is bias current, Diss (idle)= power dissipated as heat out of the tubes, Vk is cathode voltage, Pwr out = output watts and DF is damping factor.

As far as picking Rk values, just pick one or two within the range specified for each tube type. It all depends on how much idle (bias) current you want to run.
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