Could you check the values for this current source?

[Bas Horneman]

There have been a couple of changes in this part of the Baby Huey schematic.

It should sink around 2 to 2,5mA of current. Are these resistor values ballpark?

I'm asking because there have been a couple of changes and I have zero knowledge of transistors.

The values in the schematic and what is on the pcb differ.

 

[planet IX]

Hi Bas,
if we assume Uled=1,7V and UbeQ2=0,6V then
Isink= (Uled-Ube)/R18
= 1,1V/560R
= 2mA

Regards, Boris

 

[Bas Horneman]

Thanks very much Boris,

So the R18 sets current. Very helpful.

is R17 like a gate stopper or something? And R16 some kind of reference to ground?

 

[pieter t]

Take care of "if we assume"....
Best is to measure the voltage drop across R18; then apply Ohm's law to calculate the current.

 

[Ketje]

Yes, current about 2mA, that is 1mA pro triode.
That's to much with anode resistors of 220k and 300V power.
Drops 220V, leaving 80V for the ECC83, no good.
Keep R18 at 1k as original, gives aprox 0,6mA triode current.
R16 provides the base current and with R17 divides the -12V over the two transistors.
No need to use 2N5551, simple BC547 or BC548 is ok.
Mona

 

[planet IX]

I suppose the kathode of the LED is on a negative potential,
so R16+R17 determine the LED current from GND to V- and
R17 generates also the voltage for Q12 (cascode with Q2)

Daanve is right: to be safe better measure the voltage across R18

 

[Bas Horneman]

Thanks all. I did a search in the original thread...1mA should be the target.

No need to use 2N5551, simple BC547 or BC548 is ok.

Sure. But I just went and bought what the schematic said.

 

[Bas Horneman]

So 1k looks like just the ticket. This is the actual value printed on the PCB.

 

[Bas Horneman]

Measured current is 1.056mA. So that is ballpark if not perfect.