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Old 15th January 2020, 05:45 PM   #1
cogitech is offline cogitech  Canada
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SSE Cathode-Bias Resistor Value Selection
Default SSE Cathode-Bias Resistor Value Selection

Hello all,

I've been busy learning and planning in preparation to build my first SSE. Most of the board components are on the way. In haste, I went ahead and ordered 2 560 ohm cathode-bias resistors, however this doesn't appear to be the right choice for the voltages, tubes and OPTs I plan to run (EL34 at ~450v into 5k).

George's simulations seem to indicate that a 560 ohm cathode-bias resistor (red) will result in 28 watts of idle dissipation, which is well beyond the class A spec for an EL34 (25 watts).

It looks to me like a 750 ohm resistor (green) is the best bet. Idle dissipation of 22 watts, slightly higher 2nd order harmonics - however considerably lower 3rd order.

Is there anything I am misunderstanding here? I know these are only simulations, but if they were totally useless and misleading I don't think George would provide them on his site.
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Old 15th January 2020, 10:04 PM   #2
Tubelab_com is online now Tubelab_com  United States
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Quote:
I know these are only simulations
They were simulations done nearly 15 years ago using data from the original tube manuals. Many of the modern new production tubes don't always match the old data.....unfortunately most of the modern tube makers either provide NO curves for their tubes, or provide copies of that old data, which does not match what they sell.

560 ohm is the best compromise for an amp that may use all three of the common audio tubes, KT88, EL34 and 6L6GC. A well constructed EL34 will work fine with a 560 ohm resistor, BUT not all EL34's are well constructed. I usually used a 680 ohm resistor in an amp built strictly for EL34's.
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Old 15th January 2020, 11:05 PM   #3
cogitech is offline cogitech  Canada
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SSE Cathode-Bias Resistor Value Selection
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Originally Posted by Tubelab_com View Post
They were simulations done nearly 15 years ago using data from the original tube manuals. Many of the modern new production tubes don't always match the old data.....unfortunately most of the modern tube makers either provide NO curves for their tubes, or provide copies of that old data, which does not match what they sell.

560 ohm is the best compromise for an amp that may use all three of the common audio tubes, KT88, EL34 and 6L6GC. A well constructed EL34 will work fine with a 560 ohm resistor, BUT not all EL34's are well constructed. I usually used a 680 ohm resistor in an amp built strictly for EL34's.
Thank you, George!

Exactly what I needed to know. I will be sticking with EL34s in this amp.

I did manage to find a current spec sheet for the EL34s that I'll be starting off with https://solen.ca/wp-content/uploads/el34eh.pdf

Only using those because I have 3 left over from a quad that I ran in my EL34 P-P amp for a while (one tube died prematurely due to another issue in the amp).

No point buying new tubes till I have to. I also have some NOS GE 12AT7s here so I am all set, except for the rectifier. I have a Shuguang 5AR4 on the way...

Last edited by cogitech; 15th January 2020 at 11:12 PM.
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