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A few weeks ago I started a build using the guts of an old Silvertone console that I salvaged from the town dump. After some discussion here, I decided to build the RH Designs SEP EL84. I finished it tonight and I'm very happy with it. I had to make a few minor adjustments to the original circuit, namely:-The power transformer I used was capable of 250VDC, not the designed +300VDC. I used UF4007 solid state parts in place of the 5Y3 (or EZ81 in my case). This gets me up to about 270/275VDC.
-Had to use a CRCRC instead of a CLCRC filter in the power supply (no L on hand).
-I used .3uF coupling caps because that was all I had on hand.
-I dropped the screen voltage down a little lower using a ~4kohm resistor.
-As a few on this board hinted, after the first build it played but had 120Hz hum (45mV) on the outputs. Based on feedback from a few friends, I decided to stiffen up the power supply (it was already much looser than originally designed). I did this by bumping the capacitance of the second filter stage up to 430uF (390 + 40). This resulted in ~5mV of 120Hz noise on the outputs, which although greatly improved, was still audible. I was given a nice L (thanks Nate), but the DCR was too high . . . so I bumped the capacitance of the first filter stage up to 430uF as well. I know huge capacitance is unconventional and less than optimal for a power supply, but I was able to make it work with the parts I had on hand. Now my DMM on the mV DC setting reads 0mV at the output and I hear nothing but silence. I also put a bleeder on the first filter stage (50kohm 1W resistor - takes maybe 15 seconds for B+ to reach 0V when turned off).
-The power transformer I used was originally rated for 115VAC whereas my wall voltage is ~123VAC. This resulted in almost 7VAC on the heaters, and a mechanical buzz from the power transformer. To reduce the main voltage I added a 10ohm 10W resistor and CL90 thermistor on the hot side of the AC mains (before the power switch). This drops the voltage down to about 116VAC when warm (which brings all of my other voltages up to snuff, including the heaters) and has the added benefit providing some surge current limiting (given the huge capacitance in the power supply). I calculated that the 10ohm resistor is sinking about 1.6W, but it still gets pretty warm . . . same with the CL90.
Just wanted to say thanks to everyone for the great suggestions, and thanks to andrewbee for pointing out the RH Designs circuit. Now I just need to find a bottom plate for the chassis I used (I'm thinking about giving this amp to a friend but want to make sure it is 100% safe first).
-Matt
Follow Ups:
Do you have any experience with the 2A3 Dht to give us the comparison.I also think the 12at7/ecc81 on the front end is the perfect choice for this application.
Have fun : let us know how it is with the choke, also how about ultrapath upgrade, etc.
Peace, and sunshine in greaterSeattle today
So you like it ;-)I think once you get a choke and sort out the power supply all will be well. Please let us know IF you hear any difference if and when you regulate the screens and if so then if is better to your ears.
I have tried every which way I can think of to run an EL84 SE (I am an EL84 fan) and this circuit has beaten them all.
Thank you for pointing this circuit out again. It really is simple and elegant and sounds very nice.I will let you know when I get the power supply sorted out and if I decide to make up some Maida style regulator PCBs for the screens.
I also love EL84s - they are small and sound excellent in almost any configuration. I actually just came across another very interesting tube that I might experiment with - the ECL86 (it caught my eye as I was rummaging through my box of random tubes). It is a pentode and high gain triode in one envelope. I do not know the details yet, but you could probably build a nice little amp with just a single ECL86 per channel. Simple and small - I like it.
-Matt
Thanks for the heads up with the ECL86, I am familiar with it, I have one but have never done anything with mine either. Its similar to a 12AX7 and EL84 in the same envelope.
A popular dual tube similar to this is the 6BM8 which has quite a following in Japan. The 6BM8 was used extensively in the audio output of TV's.
More like a 12ax7 & a 6v6, notice the power out & beam tetrode construction. I use these in p-p. The Italian TNT www page has a PCL86 circut that is easily changed to 6.3v filament for ECL86 (follow link below). Poke around that article & discover triode-straped or NFB pentode circut arrangement as well as Volume control. The ECL86's *dense & focused* sonic signature has been decribed as light-years beyond the 6BQ5/EL84. Previously mentioned here as exceptionally linear in triode-strapped mode. Widespread economic use (due to the dual sections) in TV & portable phonographs probably gave it an un-deserved low-fi reputation. The ecl-xx family sibling 6BM8/ECL82 has slightly less power out but is/was in current Russian production. Today, 6gw8/ecl86 are becoming scarce.
A 50k bleeder on a 275V B+ is going to shunt over 5mA to ground, and dissipate over 1.5W. A 270k 1W resistor is sufficient, in my experience.Regarding your hum, it's the nature of this topology to be sensitive to hum on the B+ rail. I guess you've nailed it down with all that capacitance, but a filter choke is really called for in this style of amp.
Thank you for pointing this out - I actually used a 91kohm resistor (sorry about that mistake - it was a late night). This should draw about 3mA and dissipate about .82W. I should bump it up a little though - which I'll do tonight when I try to get the filter choke in.
Matt,
Bumping your first cap up to 430uF is a bit extreme. If you are using a tube rectifier you are exceeding its current limit at startup. Even for silicon rectifier at high voltage, this is quite a whack of current. You say you’re using CRCRC which should be enough B+ filtering but choosing the right values is important. Try modeling your power supply in Duncan’s power supply designer to make the best use of your parts on hand. I would try to get that choke in someplace, even if it means dropping a bit of B+ because the DCR is high. Using a choke allows you to drop cap values quite a bit which makes the transformers life easier.
Matt
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
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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