I broached this topic earlier in the year, but until today, it just remained as mental vaporware.
Since I had pondered the availability of a "ready to play" HV DC SMPS for tube amp B+, and could not find anything suitable, I decided to try what is basically a "crazy mad scientist" eperiment.
Some time last year, I "rescued" a number of NIB 48VDC, 380 mA SMPS
from the dumpster at work. Along with numerous others including one for 12V, 5A DC. These 48VDC, 380 mA PSUs reportedly have an output impedance of approximately 10 ohms. Rock steady stuff!
I did some resistance checks and found that the secondary side of these SMPS were totally isolated from the AC primary side, with only a 0.1 uF, 1KV disc cap connected from the third wire safety ground to the DC negative side. So, stacking these things in series, for more voltage, theoretically, should work.
Since today was a gloomy coolish rainy day, and I had nothing much else better to do, I ended up hanging five of these supplies in series, for 240VDC, and connected a 6.8K, 2W resistor across each supply, just to act as "sharing" and bleeder resistors.
Upon the first power-up into no external load, I got a very steady 241VDC.
OK, lets hang a load on this "trainwreck"... I dug up a 40W, 230V Philips bulb, and connected it across the 241VDC. Powered it up, and it lit up as it should. Measured the DC voltage, still 241VDC, nice and steady, with approximately 167 mA current loading..
Left it to run for about an hour, the five SMPSs barely got warm.
OK, I really needed about 285VDC for a project, and 288VDC is more than close enough, so I added a sixth unit to the string, and got a nice steady (unloaded) 289VDC. The bulb drew about 200 mA, and was quite bright, but it took the "abuse" in stride. 58 watts from a 40 watt Philips bulb.
While I was at it , I added the 12V, 5A PSU, and a 6A "FCC" type power line filter, and neatened up the wiring with some terminal strips.
Even out in the open, this thing is pretty quiet, RFI wise, and I was still able to hear an AM radio station on 550 KHz from about 60 miles away, with no interference.
Just in case there are any PSU failures, I still have another three "spares" should I need them.
Next step is to put this mess into a metal chassis, and filter the outgoing DC lines, so that all the switching EMI noise is contained.
I figure, take the 288VDC, then run it through a 1N5408 rectifier, into a hefty (220 uF or higher) 450VDC electrolytic, and then through a coaxial DC filter which I already have. The purpose of the diode is to prevent the HV electrolytic from back-loading into the series connected SMPS when it is powered down. Just an extra bit of added electrical safety...
I'll filter the 12VDC separately. I have a perfect size "dumpster" chassis to contain everything, and it will even come out looking fairly neat and clean, too.
Update 3/16/11:
I finally got this all assembled into the afore-mentioned "dumpster" chassis, it produces virtually no detectable RFI on a nearby AM radio, and outputs 288VDC @ 380 mA max, and 12VDC @ 5A max.
It all fits nicely inside of a 19X 1.5X 11 inch aluminum box, black powder--coated, the DC is brought out on a six screw terminal block on the back. I still have to add the DC coaxial filters, if I decide that they are really needed, and so far it appears that they probably won't be necessary.
I used a 6 amp AC input RFI filter which really does a good job of keeping the switcher noise from conducting back out on to the power line.
The fuse and AC power switch are on the back, and the front panel is "clean", though I may add a couple of pilot lights for the 12V and 288V DC.
I still have to add the 1N5408 "safety" rectifier and the hefty electrolytic cap, though they may just go into the amp or device under test, instead.
I could build up a suitable 12V6GT / 12SL7 PPP amp into another of these dumpster chassis, and mount it on top of the PSU, and have a nice clean looking, if not somewhat over-sized 14-28WPC PP amp. Or go crazy with a boatload of 13EM7s
If not that, then it will serve as a convenient bench- test power supply for future tubular projects.
/ed B