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EL803S-6CW5 / EL86 amp design check (schematic attached)

Hi tube gurus,

This is my 1st time designing tube amps using the EL803S and EL86 pentodes. I'd love to hear your opinions on the design, as well as any recommendations on the operating point.


The EL803S is running rather too hot and I'm planning to reduce the plate voltage and current. Since the EL803S triode connected curve are not available (I can't find it anywhere. Does anyone know where I can get hold of the EL803S triode curve?), I've very little idea what operating point I should set for this tube.

Thanks in advance!



diyAudio Moderator Emeritus
2011-01-05 8:34 am
In Transient
Can you re-post the schematic, it is too small to read... You can get the triode curves by measuring it yourself on the bench either manually or with a curve tracer; or you can kind of faking it by plotting the triode-connected plate curve using the pentode SPICE model in a simulator, although the accuracy is questionable, but it might be good enough for a rough estimate.

Thanks Jaz. Will do so. Just trying to see if there are any measured or tried operating points available for EL803s. :)

Andreas - for the cathode bias, actually it is taking both the current from 2 channels and therefore R3 / R6 are halved. :) This is something done in Decware amps and I'm thinking of copying to see the effects of shared and separate cathode bias.
DF96 - I read somewhere that connect it as such act as kick-back protection. My thinking is that if the power stage draws too much current, the diode will prevent the droop impacting the following stages by the reverse charge from C11/C10 to C6. What do you think? Hmm.. this reminds me that I need to add bleeder for C11/C10 with this kickback diode in place.

Thanks Andreas! :)
I can't think of any good reason to share cathode bias between channels, and I can think of a few good reasons why not to do it.

Me too. For the sake of experimentation, I was thinking of just giving it a try. Worse is I just separate them if it turns out bad. Some says that it will create more even order harmonics but I do not see any reason that would happen.

Perhaps anyone in this forum can attest to it? ;)
D1 might have the action you describe for D2. D2 will not, as it will always be forward biased when in use. C10 will need a bleeder resistor. D2 can be omitted.

Shared bias just ensures that the valve can't adjust themselves as their emission changes and will introduce inter-channel crosstalk. I can't think of any reason even to try it, apart from saving a few cheap components. If you were worried about saving money you would not use this design anyway.

Why are you running the driver stage as a power driver when the EL86 grid just needs some voltage?
DF96 - got it. Updated the schematic, incorporating all the suggestions. :) I was using the datasheet operating point for the driver stage. I've changed it after getting Andreas's help on the triode curve for EL803 and reduced the operating point lower.

Attached the updated schematic.
Try this:
If does not work, try this:

Thanks guys! Appreciate it very much!
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