Tube preamp PSU

laksaja

Member
2010-09-17 11:06 am
Hello!

One day i decided to build tube phono preamp for my brothers Technics turntable, using two ECC83 tubes. Total current draw of the preamp is 4,7mA(@260v). Since i wanted to incorporate old tubes and modern technical possibilities into small box, i decided to use switchmode HV converter. I know that many tube lovers against it in principle, but i havent solid transformer at hand and i was reluctant to wind thousand turns hair thin wire :). So i googled and found someones Nixie HV supply (i apologize in front of the author, but i really dont remember his name), based on MC34063 step-up ic. This design was useful for my intentions and i build it. The first prototype was really meant for Nixie tubes, the smps worked satisfactorily in the 180v area, not more, but i wanted 260v. Im shame to admit, but i tried at least 10 different coils, starting from 100uH to 800uH, using 0.2mm wire to 1mm......and the performance was bad. Coil get really hot to touch, so the mosfet. Then i looked at old transistor radio and the bulb in my head got to glow instantly :). The solution was ferrite rod from transistor radio´s receiving antenna, cutted by diamond knife to 2 cm in length. I used LC meter to wind 0.2mm wire (only!) around this ferrite rod to achieve 470uH (i dunno the exact turn count). With 1nF timing capacitor and 470uH inductor, the smps frequency should be around 40-50kHz. I dont have oscilloscope and neither low frequency meter, but this design runned cool even 5 hours continous work. More important, the preamp is dead silent!. Only if im using very sensitive in ear monitors Shure E5 with power amp headphone out, cranking the volume all the way up (100w), i hear small motorboat other side of the river :). And all of this using only 22uFx450V filtering cap! Total current draw from 12v wall adaptor is 390mA with 12.6v heating for EEC83 tubes
Unfortuantely i cannot attach schematics, because it will log me off when i try to attach it, weird.
Sorry for the long thread, but im sure that smps can be used to build small tube preamp. BTW, im looking forward to build offline smps which gives me 12.6v for heating and as well as 260v HV.
 

laksaja

Member
2010-09-17 11:06 am
I managed to upload the schematics and pics as well :)
 

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laksaja

Member
2010-09-17 11:06 am
Hello!

How is this doing for you almost 2 months after posting schematic?

12V supply to filaments?

Honestly saying, this construction have been worked since i posted the schematic. I also built el84 stereo tube amp for that preamp since it gives 1v audio out easily which i need to drive the power amp.
However, im using quite strange setup. Since i tried to supply the preamp tube filaments through cheap LM7812 stabilizer and 18v wall adpter the lm7812 got so hot even with big heatsink, so i scratched my head couple of days. Its more convenient and easy to supply the preamp with stabilized 12v, so the switchmode wall adapter game into play :). Its not normal to supply the switchmode HT supply with switchmode wall adapter :), but it works even after 2 months perfect and silent. Right now im working on with the switchmode offline supply to get 2 amps 12v DC for filaments and 260v HT. The prototype works but it still too big for fitting it into that small box, heatsinks and etc.
Little note also, i have tried to supply the el84 power amp with modified ATX supply (400V), it works and runs silent, but the sound is "thin". Its ok to use switchmode supply for preamp, but the power amp should stay on the regular transformer (toroidal in my case) and the difference is remarkable :)

Regards
 

laksaja

Member
2010-09-17 11:06 am