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Old 1st June 2009, 01:34 AM   #1
pforeman is offline pforeman  United States
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Default simple se cathode resistor question

I'm a little confused.
Can you run the simple se on different tubes (EL33/KT88 etc.) by just switching tubes (assuming that the power transformer is up to the task providing the correct B+ voltage) or do you have to change the cathode resistor for each tube type?
I've seen the chart on George's web site, I assume that you don't want to be in the red, and to choose a cathode resistor that is in the black.
I was also hoping to assume that I could just use the resistor thats in the parts list.
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Old 1st June 2009, 08:19 AM   #2
Tweeker is offline Tweeker  United States
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If your going to switch from EL34 to 6L6GC or KT88 yes you will need a different resistor if you want to run the higher dissipation (KT88, 6L6GC) tubes at full dissipation. You would also need a change to run tubes of significantly different mu.
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Old 1st June 2009, 12:10 PM   #3
oldmanStrat is offline oldmanStrat  United States
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Or you can be lazy like me and just use one resistor value for all of the tubes. It's not perfect for any one tube, but the 560ohm specified in the parts list works for all of the tubes for me.

You just need to make sure your B+ is close to the values in the table or you will have to take out the meters...
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Old 1st June 2009, 05:28 PM   #4
rknize is offline rknize  United States
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At 450V B+, 560 ohm is a good compromise. It will run EL34s fairly hard, though.
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Old 1st June 2009, 06:08 PM   #5
porkchop61 is offline porkchop61  United States
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I have a rotary switch with a different set of resistors depending on which tube I'm running.
I've seen some people add several resistors this way to fine tune the bias.

My SE amp schematic

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