[CButterworth]

I know what the LEDs indicate:  Red indicates that power is on to the tube filaments, and green comes on when the B+ reaches voltage.

My question is

How does this simple circuit work?  I'll either be replacing my Aikido preamp PSU with a new higher quality transformer, or changing the ss bridge for tube rectification.  Either way, I'd really like to be able to incorporate the Red/Green LED.  The tubes run on 6.3 regulated DC. The B+ will be between 290VDC and 340VDC.

Thanks,
Charlie

[hagtech]

It is a comparator circuit.  The two PNPs form a differential pair.  The resistor dividers on either side decide which voltage is lower, and sets the base voltage for the transistor to hog all of the current.  The over-simplicity makes it kind of precarious, and is designed to work specifically in this one situation.  It doesn't take much to pull it out of range.

jh

[Bill Epstein]

I haven't got 'round to wiring the LED in the new Cornet box. Any issues?

[CButterworth]

Jim,

So you're saying that it is sensitive to the voltages that it sees?  Surely the resistor values could be changed for other voltages.

I wish I knew about these kind of things.

Maybe the would be for me to simply wire a single LED to B+ using a sufficient voltage drop resistor.

Thanks,

Charlie

[BobM]

Maybe it might be easier to just put an LED on the + side of the incoming AC voltage so you know when it's on or off, and not install this indicator at all. (that's what I did)

Enjoy,
Bob

[CButterworth]

Bob,

Yes I could do that, but it would be good to understand the details.  As far as I can tell, R104 sets the current for the LED.  R105/106 and R107/108 form voltage dividers that are seen by the transistors.  I'll need to measure the voltages at R105 and R107 to be able to figure out what the voltage drops actually are, and the voltage at R104 to figure out the current for the LED.

It seems that Ohm's Law will play a big factor in these calculations, although there's always the transistor voltage drops/currents and Hfe to take into account.

I suppose that as long as I can set R104, R105 and R107 in my version to values that give the same voltages as seen in the Cornet2 application, then the LED thing should work.

Parts for this are cheap, so it'll give me opportunity to dabble and learn.

Charlie

[hagtech]

Mostly it is a simple voltage comparator.  The current either goes through one collector or the other.  The problem is keeping the transistors from saturating, which turns off the LED.  Minimizing the range of the voltages on the bases will make circuit work better - and more adaptable to other machines.

jh