Drip Redd 47 Ultra tube vs SS rectification

Hi there!  I'm hopefully nearing the end of my Drip Redd 47 Ultra build and due to a mistake along the way, and a burned out 6x4 rectifier tube, I contemplated for a moment using a SS rectifier.  I've seen people build their units both ways and I'm curious which design I will actually prefer--each design seems to have its sonic merits.

Once my unit is up an running, I'm planning on testing it out both ways and sharing my thoughts.  I figured there might be a few others out there who have tried the same, or are curious like I am--so please share your thoughts. 

One technical questions that springs to mind: should I be concerned about compensating the higher DCV that SS rectification will provide?  The Drip build uses a bank of three 47uf foil caps (rated at 350VDC)**** for filtering, and also a pair of OA2 regulator "glow tubes," which bring the B+ down to where it needs to be for the amp.  Will the voltage rise with a SS rectifier be enough to cause worry about those parts?  My rectifier plates will be seeing 309ACV, FWIW.

Anyway, I'd love to hear from anyone who has a strong opinion or who has tried it both ways in this build.  Cheers!

****WRONG!  Drip revised this info.  Three 22uf 450V caps were spec'd in place of the 47uf 350V caps that are written on the PCB.

Shattersignal said:

The Drip build uses a bank of three 47uf foil caps (rated at 350VDC) for filtering.....

.... My rectifier plates will be seeing 309ACV, FWIW.

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You should be using 450V caps for this. Initial DC should be close to 400V with the 6X4 and a little higher with SS.

should I be concerned about compensating the higher DCV that SS rectification will provide?

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Maybe, if the OA2 has a max input voltage rating? (I don't know if it does).

On the original EMI schematic there are four 1K resistors (R18-R21) between the B+ input and the OA2, if you need to drop more voltage, just increase one or more of resistors.  If the channel uses about 20mA, every 1K increase in those resistors should get you another 20V drop.

Hi Mitsos.  Yes, the Drip board specs a chain of power resisters that total 1.72k, when built for use with with the tube rectifier.  It also calls for 350v caps, although it warns that they are close to their tolerance***.  I'm just going off the tested and proven build guide that Drip provides for their PCB as far as the voltage rating is concerned--I hope I won't end up needing to replace those spendy Mundorf Evo foil caps.  Thanks for the input--I'll make sure to measure the B+ after the power resistors when I decide to install the SS recto, and up the resistance if needed.

I can't tell what the voltage tolerance of the OA2 is, even after looking at some data sheets.  I'm also not entirely sure why there are two of them in the circuit.  From what I gather on the Wikipedia page, although I'm guessing a bit here, the OA2 is designed to regulate to 150V, which would explain why two of them are needed to regulate the B+ to the 290-300V that the amp needs. 

https://en.wikipedia.org/wiki/Voltage-regulator_tube

***correction!  The latest BOM states that 22uf 450V cap should be used here instead.  I bought the caps a couple years ago, and I got the ones that are incorrectly listed on the PCB, which are only rated for 350v...  Looks like I need to buy some new caps.

This is all you need:

http://www.sowter.co.uk/schematics/EMI-REDD47.pdf

The schematic is at the end.  Notice there are FOUR 1K resistors  (R18-R21, in the B+ filters at top left).  R21 is shown jumpered, so maybe they put it there in case some PSU had too high voltage.  Anyway, there is 3K series R in the B+ line.  That doesn't mean you have to use the same number or even same resistance value, use whatever you need to get you to the level you need to be at.

Thanks, Mitsos.  I've been fairly trusting of Drip in the past--the Pultec I built had no errors in the build guide.  Why would Drip tell me that the power resistor chain should total 1.72kohm?  Very strange and disconcerting, if in fact this figure is wrong...

I made 2 P2P unit and made a SS power supply, RC filter and ended up with a really clean, no noise pres. Here is my schematic.

Thanks, Deepdark.  That's rad.  This is only the second tube amp I've built, so I opted for the comfort of the Drip PCB.  Given how much I've needed to learn (due to a lack of a build guide), I could have easily done a point-to-point build by now.  I suppose I'll do that next time 

I still have the Drip 670 PCB waiting in storage as a future project--that one would be a little too much for me to tackle as a P2P.  Knowing how simple the Redd 47 is, now, I can see that it would be a pretty ideal first P2P project.  Very cool--I'm sure yours sounds awesome, too.

Shattersignal said:

Thanks, Mitsos.  I've been fairly trusting of Drip in the past--the Pultec I built had no errors in the build guide.  Why would Drip tell me that the power resistor chain should total 1.72kohm?  Very strange and disconcerting, if in fact this figure is wrong...

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I don't know  where his info comes from. It's possible that Drip has calculated that resistance for someone using his exact list of parts.  The reality though is that, because of differences in your mains, or mfg differences in trafos, or whatever, you may need to tweak your resistors for the proper voltage.  Then again, it's regulated so, maybe it's just academic. 

Deepdark said:

I made 2 P2P unit and made a SS power supply, RC filter and ended up with a really clean, no noise pres. Here is my schematic.

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Me too, a couple P2P and a couple on PCBs (mine). All of them used SS regulated power supplies.  I posted a schematic in a thread called AK47.  I made a few PCBs back then, but the circuit is so simple, it can easily be built on perfboard. 

http://groupdiy.com/index.php?topic=60343.msg765335#msg765335
*Credit for the B+ circuit goes to Kingston who posted it in his M670 build. 


The nice thing is, you don't need all those CRC filters any more,  Just the decoupling cap for the EF86 (C1 in the Redd47 schem) and one for the output tube if you want.  I've driven two channels with it, no problem.  Best of all, you have a trimpot to adjust your voltage.

Hey Mitsos, when is it okay to forego an output coupling cap?  I've spent some decent money upgrading coupling caps in most of my gear--and heard considerable improvements.  I'd imagine that the best sounding cap is no cap at all.

No, the cap is there to block DC from the tube anode. You need a cap there, wima polypropylene is great. 

Mitsos--you indicated that the coupling cap on the output tube was optional.  That's what threw me off. 

Hey, if you think a Wima is nice, you'll be stunned by the upgrade to a Mundorf SGO Supreme.  You'll add a couple hundred dollars to the cost of a build, but if you use your equipment for professional purposes the quality improvement is definitely worth it.  I upgrade all the audio path coupling caps in my gear now that I've done some extensive comparisons. 

Shattersignal said:

Mitsos--you indicated that the coupling cap on the output tube was optional.  That's what threw me off.

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Sorry if I wasn't clear, I meant an additional PSU decoupling cap on the output tube, not the audio output cap. 

Also, adding some bypass cap, 0,1uf ceramic on B+ , near the tubes is great for cleaning highs freq. Noise.

A little update:

I just completed the build last week and now I'm trying out some tubes.  Everything works perfectly--including with the resistor values that Drip specified for the build.  Man, this preamp sounds way way nicer that I had even hoped for.  I wasn't expecting to be this impressed.  I suspect my BAE 1073s will be seeing far less use on single source recordings. 

Thanks for all the help, folks!

I may still audition a SS rectifier at some time, but I'm so in love with the sound right now that I doubt I'd ever feel compelled to make a big change, for the benefit of some sonic improvement.

Shattersignal said:

I may still audition a SS rectifier at some time, but I'm so in love with the sound right now that I doubt I'd ever feel compelled to make a big change, for the benefit of some sonic improvement.

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Did you at the end use 6X4 or solid state rectifier ?

Peace.

lampas said:

Shattersignal said:

I may still audition a SS rectifier at some time, but I'm so in love with the sound right now that I doubt I'd ever feel compelled to make a big change, for the benefit of some sonic improvement.

Click to expand...

Did you at the end use 6X4 or solid state rectifier ?

Peace.

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FWIW, I got the impression he went tube, "I may still audition a SS rectifier at some time,"

Yes, I used a 6x4 in the build.  It sounds fantastic that way. 

I also just increased the cathode bybass caps from 100uf to 470uf and also upgraded to Nichicon KZ Muse.  The increased bypass reservoir really helps!  The sound is even more open, natural, and realistic.  There's a bit of a bass boost with this amp, and the new cap value seem to allow those bass frequencies come through with less sense of congestion.  It seems to be a very nice mod--highly recommended!