PMillett Tube Preamp Build

[Ricardus]

I saw a post here with someone building this preamp from 2010 or so, and not many comments on it, so I thought I'd start a new one to document my build.

I discovered this guy's site about a year ago when I was sorta randomly googling around for vacuum tube pre kits other than the stuff at Drip Electronics and such. I found this. It looked interesting. I initially ordered one PCB and then ignored it for a while because there were a few things about it I didn't like. He used Neutrik Combo connectors for the output and the input, and the output wasn't balanced. He posted some files to get an enclosure made for it, but I wasn't in love with that option either, because it wasn't a standard rack mount or something more modular like 500 series. A couple months ago I figured the best solution for all of those issues for me was to buy another PCB, and ultimately rack it up as a 2-channel unit in a 2U rack box, and then I could panel mount the toggles and the XLRs and run jumpers from the PCB. I'll also balance the outputs with transformers.

The tube sockets came a week ago so I soldered those, and the Mouser BOM came yesterday, and I'll begin soldering in a bit. I don't have any of the traffos yet, and I didn't order the 2U rack box yet, either. I'll get that from Collective. Their basic 2U units look very nice.

Anyway, here is a pic of one of the PCBs with the tube sockets installed. I'll post more pics as I do more work.

 

[Ricardus]

OK. I soldered for a couple hours and have all of the resistors, all the diodes and various other semis, and all the inductors in both boards. I'll get to the caps tomorrow. Progress.

 

[Ricardus]

Even more progress. Caps are all in, but 4 resistors had to go because a few hours ago I realized they were supposed to be rated at 300 volts or better, but they were only rated at 250. Probably would have been fine, but hey, better safe than sorry.

 

[Ricardus]

Transformers ordered. Lundahl  7903's.

 

[Ricardus]

Lundahl 7903's arrived today and mounted. These things are nice.

Things will be slow going for a bit due to funds and time.

 

[Ricardus]

The 1 watt, 500 volt resistors came today. Soldered those. Now I gotta get the rack box and do some internal layout stuff.

 

[Ricardus]

I'll be ordering the 2U rack box from Collective, tomorrow.

When the guy designed this he spec'd an external switching PSU. It wasn't anything special. Low cost. 48 volts at 1 amp. I have a 2.3 amp 48 volt Meanwell switching supply hanging around that I am going to try out. I just wonder if I'll be able to keep it in the case or not. I'll experiment when I get everything going.

 

[Ricardus]

2U rack case ordered from Collective.

 

[Ricardus]

I designed a front panel for this. It's about $150 so it's not in the build budget right away, but it will be.

 

[letterbeacon]

Looks great so far!

 

[Ricardus]

Looks great so far!

Thanks!

Right now the only thing slowing me down is cash. I have both PCBs stuffed and ready to go. I ordered the 2U case from Dan over at Collective Cases so I'm waiting on that, and once some people pay for some work I did for them, Thursday, I will be able to order the Sowter 3603's and the 6922 tubes, and some various odds and ends to finish this. Things like the IEC inlet that Dan's cases have a hole for. I also need to restock on hookup wire and some balanced install wire.

 

[shabtek]

I did not search for schematic and you are not the first to build from this pcb rev. but--couple things that came to mind;
600:600 output transformer?
is that switching power supply with all those inductors on-board?
consider external/remote power supply?

 

[Ricardus]

I did not search for schematic and you are not the first to build from this pcb rev. but--couple things that came to mind;
600:600 output transformer?
is that switching power supply with all those inductors on-board?
consider external/remote power supply?

He built a switching power supply for the B+ voltage onto the PCB. But the main PSU that supplies the 48 volts can be put anywhere.

To be honest I have no idea what output transformer would be best here. I emailed Sowter and they recommended the 3603. The output of this preamp is unbalanced. The 3603 is being used to balance the outputs. Do you have any other suggestions?

His site with this mic pre can be found here:

http://www.pmillett.com/micpre.htm

 

[Ricardus]

Tubes are ordered. JJ, matched just for the hell of it. Collective has shipped the 2U case, and the Sowter balancing transformers are ordered also.

 

[Ricardus]

The 2U Rack Box arrived today from Collective Cases. I had to do a little fab to the front panel to get it to screw to the back box, but otherwise it's perfect (just had to ream the holes a tad bigger). They do nice work there.

Here it is mocked up with the IEC, Star Ground, and XLRs.

 

[Ricardus]

Tubeziez arrived today. Almost ready to drill some holes and mount some PCBs.

 

[analag]

To be honest I have no idea what output transformer would be best here. I emailed Sowter and they recommended the 3603. The output of this preamp is unbalanced. The 3603 is being used to balance the outputs. Do you have any other suggestions?

His site with this mic pre can be found here:

http://www.pmillett.com/micpre.htm

You would be better off going with a 10K:600 or 4:1 output transformer, for better impedance matching between the tubes and whatsoever you plan to have them drive. So omit R6 and C4 at minimum should be 10uF to avoid a resonant peak in the lower audio spectrum.

 

[Ricardus]

You would be better off going with a 10K:600 or 4:1 output transformer, for better impedance matching between the tubes and whatsoever you plan to have them drive. So omit R6 and C4 at minimum should be 10uF to avoid a resonant peak in the lower audio spectrum.

Hmmm, OK. I'm out of my league here with design. I ordered the Sowters some time ago so they're on their way no matter what.

 

[Ricardus]

You would be better off going with a 10K:600 or 4:1 output transformer, for better impedance matching between the tubes and whatsoever you plan to have them drive. So omit R6 and C4 at minimum should be 10uF to avoid a resonant peak in the lower audio spectrum.

Ya know, I was just searching through some emails a moment ago, and I found an email from the preamp designer, PMillett, who suggested adding a 600:600 traffo at the output to balance it.

 

[analag]

Here's a little reading on the SRPP output stage. 
https://www.tubecad.com/may2000/page16.html
Even without an output transformer the frequency response is not flat from what I see in the graphs especially in the low end. Perhaps if global feedback was used instead of that pseudo plate follower SRPP configuration?