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High voltage power supply - tube amp - low power

post #1 of 5
Thread Starter 

Hi all,


i made a tube amp (push pull EL34, with 12AUX preamp ) and i wanna present one of the electronics i concieved for :


I made many cards, one of them is a high voltage power supply, low power, for the preamp part of the amp (not the high voltage for EL34), here are the characteristics :


  • name : AL_BP_01
  • in : 250V AC (power),  15VAC (control)
  • out : +300V 15mA, -150V 15mA, current limit on each output
  • Microcontroller to control voltages, ripple, rise time, shortcut, power on and off
  • Insulated serial communication link to communicate through the system with other cards (can communicate with others to be switched on and off or report error)



This is part of the amp, here is the video of the first test (nearly a year ago), now the amp is in my living room for testing !



i can't attach files or images (....????? ....) so here is a link to the schematic :




description :


1 - first page :

connectors for high voltage in,

connector for high voltage out

connector for low voltage in

relay to switch on or off high voltage



2 - second page:

+300V regulation

T2 is the ballast transistor, feedback for output voltage through R10 and T3

Current limit with R9 and T5


Working :

- The current through T2 is constant thanks to T4, D7, R8 .... C6 to filter noise, R5 and R6 to draw base current, two resistor because this is 1/4W resistor not supposed to whistand more than 200V !!

- with R3,R4, D8, D9 we have a reference for 300V, filtered by Cè and C8

- T2 gives current to the outcapacitor C9, limited by the current source above described.

- When out voltage is above 300v, T3 start conducting, taking current from T4, current that is no longer going through T2, thus limitating output voltage.

- there is a current limit with T5, R9:  when current drawn by T2 is too much, T5 starts conducting, thus stealing current from T4, T2 stop conducting -> current source.

With my scope, i can see a strict line at starting up, thus this is a current limited source.


Benefits ? capacitors are not stressed by high current starting, capacitors are small (look here only 10µF, thats enough)

With the microcontroller who can measure output voltage, in case of overcurrent, voltage drops, power supply is switched off with error reporting, -> no damage in case of shortcut etc !!


Third page :

Same regulation as above, but for negative voltage, -150V (i used a schmitt phase inverter so i needed negative voltage to get minimum distortion)

note D13 !!  needed if external capacitor are present !

you must have a heatsink for this part because input is -350V and output is 150v -> many heat !


Fourth page :

insulated serial link with microcontroller, note you can connect it on your computer to comunicate and test the card !

communication through one wire, like LIN bus protocol.

you need to supply the PC side (insulated) with +5v (at LIN_VCC), and have a pull down for the LIN_DATA line (i didnt put it because i have many cards, only the "master" have the pull down)


5e page :

Microcontroller, measures  +300v, -150V (note ref for this voltage is at +5V and not AGND like for +300V !!! )

4 leds to indicate status (very usefull for debug)

measure for +5V supply (in case of supply failure)

the lines with "provisions" means this is for additionnal capabilities, i let out not used pins just in case i wanna add or test something ...




in my amp, this card communicates with the "master" card, when iflaments are hot, the master tells this card to start. when this card is ok, then high power high voltage is started, output tubes calibrated, and then let's ear !!


please fell free to ask me more details...


sorry i'm not gonna comment everything, it's just too big for this week end :p



additionnal photos for the AL_BP_01


starting of the +300V (the plateau is due to the high current drawn at starting and the fall of ac input voltage, at steady state there is no such things)


starting of the -150V out, same comment, current controlled because we have a perfect line.


comment : i got 10µF on the board, 15µF on the preamp board : total load = 25µF

get get 150v in 0,234 s  -->  i = C* dV / dT  --->  i = 25µF * 150/0,234 =  17,2mA

looking at schematic i have a 22ohms resistor, T9 start conducting at 0.4V ( starts, conducts well at 0.6 but starts at 0.4) so T9 start current limiting at 0.4/22 = 18mA

my current limiting is correct !!!

same remark for +300V


photo of my board :





have a nice days guys !

Edited by guanglier - 2/24/13 at 10:12am
post #2 of 5
Thread Starter 


Edited by guanglier - 2/24/13 at 10:13am
post #3 of 5

Wow! This is so overengineered... And you designed and built this as a DIY project?


The whole amplifier is so complex it would probably qualify as a project at the end of an Electrical Engineering degree. Are you an EE student or something?

post #4 of 5
Thread Starter 

Hi !


it's just for DIY , i like to be complicated ... :(

the whole amplifyer is with solid state power supply (linear, low caps, no inductor, overcurrent protected)

automatic synchro of start and stop

auto tuning of the EL34 tube bias ...


if this can be usefull for someone ...


(i'm embedded software engineer, electrical engineer )

post #5 of 5

Ah! So you ARE an engineer... I figured you were. wink.gif


Seul les ingénieurs créent des projets aussi complexe! biggrin.gif

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