LED bias indicator

[scottw]

Hey,

Below is a circuit used in later "ST70 type" amps that indicates the bias level by having two led's match their brightness when the correct bias level is reached. It seems to be accurate and sensitive. This circuit is set to measure 100mA for the pair of EL34's (or 50mA per).

How could the circuit be changed if one wanted it to correctly measure a lower bias current? Say I wanted 40 or 45mA per tube?

If it is just sensing the 1.56V on top of the 15.6R, then it may be as simple as treating the 15.6R and 820R as a voltage divider and swap the 820R for a different value??

Thanks,

Scott

[Wavebourn]

Increase R5 proportionally

[gingertube]

Hope I'm not "teaching Grandma to suck eggs" but here is how it works.

Think of Q1 and Q2 forming a comparator.

When Q1's base voltage is higher than Q2's base voltage then Q1 hogs all the current and holds Q2 off, LED1 (LO) will be on. When the reverse is true Q2 hogs all the current holding Q1 off , LED 2 (HI) will be on.

R5 and R6 set the reference voltage to Q1 base

Vref = 12 ( R6 / R5+R6) = 1.59 Volts

R3 is the current sense resistor.
At 100mA through R3 you get 1.56 Volts which is less than 1.59 and LED 1 (LO) will be ON.
At 104mA through R3 you get 1.62 Volts which is more than 1.59 and LED2 (HI) will be ON.

To work at a different current you can either:
Change the reference voltage at Q1 base by changing R5 (or R6)
OR
Simply change R3 so that the current you want will give 1.59 Volts.

That is new R3 = 1.59/ Current required.

Cheers,
Ian

[scottw]

Cool, a consensus.

And Ian, thanks for the tutorial, it was just right for my limited "egg sucking" ability. Very helpful.

I'll adjust the reference voltage to Q1. Any reason I couldn't put a cermet trimmer in place of R5 (~10k) and adjust it with a DMM appropriate to the reference voltage I want?

If you'll indulge me one more question, could this circuit be adding "noise" to the cathode of the el34's? What might the nature/frequency of the noise be and how could it be reduced? Ferrites? Did I say one question?


Thanks,

Scott

[gingertube]

Scott,
You could certainly use a cermet trimmer to set the reference voltage to Q1 base.
I would actually put the trimmer in place of R5 with the noise supression cap you see across R5 between the 0V end of the trimmer and the trimmer wiper. Use a 1K trimmer and say 4K7 for R6. That would allow stting of Q1 base between 0V and 2.1 Volts.

The current sense resistor in the cathodes of the EL34 will not be introducing noise.

There is actually anecdotal evidence that a resistor in the cathodes like this does not ONLY give a convenient place to measure the current BUT also acts to reduce distortion in the output stage.

Cheers,
Ian

[scottw]

So, I modified the circuit with a 1k trimmer and a 4k7 resistor and it works slick.

But, at closer inspection I noticed when the LED's were illuminated equally the voltage across the cathode resistor was a little off. And it seems off proportional to the variation of V forward between the pairs of LED's. So, I may replace the LED's with matched V forward.

Diffuse green LED's with Vf of about 1.7 volts are what is currently in the circuit and most of the green LED's I see available for replacement are about 2.1 to 2.2 Vf. Some of the Series II had blue LED's (all the other component values were the same) and these tend to have substantially higher Vf (around 4 or 5 V).

So, in this circuit, would a LED with higher V forward be more or less sensitive/accurate than an LED with lower V forward?( or, Would the blue LED's with the higher Vf be less/more accurate?)

Thanks,

Scott

[Serge66]

Hi,
I saw an interesting application of the TL431 in the data sheet.
You can download it - for free and NO registration - at www.alldatasheet.com.
The circuit uses two LEDs switched on when a lower then a higher threshold has been reached.

Serge