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In Reply to: Re: Thank you! RH Designs SEP EL84 build is done posted by mwiebe on March 27, 2007 at 19:16:46:
Thanks for the feedback. Everything was simulated in PSUD2 before construction. The simulations ran fine with no errors, although I will admit that the surge current is quite high (well within the specs of the UF4007 diodes but certainly difficult on the transformer). The CL90 should be limiting the surge current somewhat. Had I stuck with the tube rectifier, I would have been limited t0 <50uF in the first stage.Anyhoo, I'm new to pentodes - I think the reason I was seeing 120Hz on the output was due to the suboptimal power supply and unregulated screen supply (a few here pointed out this possible problem). The quick fix was to really stiffen up the B+ supply by adding more capacitance. The better way (I admit) is to use the recommended CLC filter and possibly regulate the screen supply using one of the methods mentioned in the earlier post.
I know it's a little extreme to use 430uF in the first stage. Maybe I'll fiddle around with it more and try to shoehorn the choke in and pop out the extra 390uF cap that I added to the first stage.
Again, thanks for the feedback.
-Matt
Follow Ups:
Matt,
Congrads for making music, that's the goal. There is nothing wrong with throwing your 40uf first, add another CR section using the big cap on the end. That should make your transformer happier. But as you will find out, make a better power supply, hear better music.I don’t know what your layout is like, but it is possible that hum is transformer coupling between the PS and output transformers. Try turning your PS trans 90 degrees and see if there is any change, or moving your output trans as far as you can away from the PS. You could also have a ground problem. I really don’t think your B+ supply is your issue, using the values you did pretty much removes any vestige of ripple.
Screen regulation is very important and there is none of that in the schematic you linked. But if you think that is the issue just hook the EL84 up as a triode and see if the hum disappears.
Thank you again for the feedback.Since the hum I was seeing was 120Hz (measured on a scope) it must have been ripple from the full wave B+ supply and not transformer coupling (which would have been 60Hz). Regardless, I followed all of the standard practices when laying this out - outputs are rotated 90 degrees from the orientation of the power transformer and are mounted well to the side and in this case below the chassis.
I checked out my ground connections and didn't see any noise there as well.
I think the original problem was really due to the unregulated screen supply and underbuilt supply. I was thinking of designing some PCBs for the Maida supply using a 317 VR that a few here had mentioned - maybe I'll follow through with that now.
I think I'll try fitting that choke in tonight and going 40uF + 8H + 40uF (B+) + R + 40uF (screen). I can add in the big dogs if need be (390uF caps).
Matt,
Sorry for my comments above, I was spouting off before reading where your tweaking lead you, I see you have no hum issue.I’m not sure what a Maida supply is but I think I turned up something Googling it. Enjoy the music along the way as you play with PS topologies. The choke PS should be audibly different then your big cap setup, and the Maida also a change. Good luck as you tweak.
Matt
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