Power-supply-design for Tube-preamp

dryman · Jun 24, 2012

Hello,

i am just rebuilding an bass-tube-preamp (S*T) from my own bass amp for redording. And i don't know how to design the power-supply. The preamp uses 3x 12AX7
In germany we have 230V.

I need following voltages:
* 275V DC Platevoltage
* +/- 15V DC Voltage for Opamps
* Heater-voltage (6.3 / 12.6 V DC)

For heater-voltage i prefer 12.6V regulated by an LM317. The original amp uses 40V in series.

But i don't know, what is the best way to get the plate-voltage. Gyraf G9 uses an TL783, and some z-diodes (ZD39v) to get 245V DC. How can i modify it to get 275V.
http://gyraf.dk/gy_pd/g9/g9_sch_small.gif

If i see correctly, the original preamp uses resistors to get the voltage. What is the best way?

I think i use 2 transformers, similar to the G9 Powersupply. But i think i can use 2 2x15V, because rectifying 230V AC will give me about 325V DC effective. Or do i need an 2x12V as secondary transformer?

thanks

Stefan

ruffrecords · Jun 24, 2012

The zener diodes in the Gryaf HT supply are simply to protect the TL783. It can take a maximum of 125V between its input and output so if you use it to make a 24%V HT supply as they do in the Gryaf then if you accidentally short the output the TL783 will die instantly without the zeners there to protect it. I know this is true because I recently modified my own 48V phantom supply PCB, which uses a tL783, to make a 275V HT supply. I had no protection zeners and I accidentally shorted the output and the TL783 died.

The best way to set the voltage with a TL783 is with resistors as they do in the Gryraf. For my 275V supply I used 150 ohms from the output terminal of the TL783 to the adjust terminal and 33K from the adjust to ground. Note the 33K dissipates over 2 watts so I used 4 x 2W 33K resistors in series/parallel.

Cheers

Ian

CJ · Jun 24, 2012

205 to 210 sec volts will rectify and filter up to about 275 volts dc,

this is if you use a solid state bridge, a tube rect will chew up more B+ so you need to add some volts for that,

so get yourself a 230 volt ac (50 or 60 hz?) to 210 volt sec with a 12.6/6.3 winding.

the 12.6 will be perfect for making 15 volts dc for the opamps,

what are the opamps for? got a schematic?

if for low level audio then you need good filtering on the 15 vdc supply.

we have places to buy cheap pwr transformers over here, but the shipping would be too high,

so you will have to dig around for a european source,

no wait, you need plus/minus for the opamaps,

scrap the 12.6 idea, looks like you need an aux winding of 30 vdc/1.3 = 24 volts center tapped (rounded up to the nearest popular voltage)

so you need a 230>210/24/12.6CT

Power-supply-design for Tube-preamp

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