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LED bias indicator

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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
 

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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
 
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
 
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
 
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
 
One thing that is handy for a Dyna ST70 style amp is to use separate resistors for each cathode (4 resistors on a ST70). 3.18 Ohm cathode resistors will allow you to check that the "matched" pair EL34s are really matched, both when you install them new, and as time goes on. 50 mA = 1.59V

Just use 2 1640 Ohm resistors in place of R3 to use your LED comparator circuit. One from one 3.18 Ohm, and the other 1640 Ohm to the other 3.18 Ohm resistor.

The old Black coupling caps on the ST70 get really leaky as they age. I had a good and well matched pair of EL34s that were passing 60mA on one, and 40mA on the other EL34.
That will show 100mA on the original 1.59 Ohm resistor, but does not show that there is a major problem.
 
I’ve also been looking at Curcio’s bias control circuit and mating it with
The window comparator LED indicator once the bias control has adjusted to the
Proper bias the green LED would come on, would be a cool addition to any amp.

BTW if You use just one of the six gate inputs of the 4049, the data sheet
Indicates grounding the other 5 inputs or tie them to Vcc.
 

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I found this circuit online that might work it’s a window comparator, I’m not sure it would work within a window of less than .2V or so but might be worth looking at.

There are a couple of things a bit crappy about the circuit of post # 8:
1. The op-amps run on a single-sided supply of 5 V, but they are not guaranteed to work below 20 V difference between the positive and the negative supplies.

2. There is nothing except leakage current pulling down the cathodes of the 1N4006's.

It should be more reliable when you change the op-amps into TLC271 or so and add a 100 kohm pull-down resistor after the diodes.

Actually that won't do either, because the output of the TLC271 doesn't go high enough.
 
You need an op-amp that works on the supply that you use, and that can lift its output high enough for the 4049 to reliably see it as a high level. With an LM741 it may or may not work, depending on luck.

By the way, you could make an even simpler circuit with an LM393. Connect both outputs, the anode of the LED and a resistor to the supply together, connect the cathode of the LED to ground and connect the inputs as they are connected in the circuit of post # 8, but with the + and - inputs reversed.
 
I don't get it. You run your op-amps on a single-sided +5 V supply, and still they manage to produce -5 V at their outputs?

If you want to see whether the circuit is sensitive to board leakage, put a 1 Gohm resistor between the input of the 4049 and the +5 V. Is the reverse current of the diodes then still large enough to pull down the inverter's input?
 
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Including the 1 Gohm resistor? Then apparently the diode's reverse leakage is high enough. Was that the reason for choosing 1N4006 diodes rather than small diodes such as 1N4148?

Regarding the op-amp, I think you are right. The LM741 is only specified to work from 20 V to 36 V or 44 V (or +/- 10 V to +/- 18 V or +/- 22 V with split supplies), but when you look at the internal schematic, you see that they should do something on any supply exceeding about five junction forward voltages. The only effect of a low supply voltage will be much reduced performance, because the biasing current through R5 will get much smaller, but that won't matter for a LED bias indicator.

If you model the op-amp as a high-gain controlled source with limits at 0 V and +5 V, that's a bit optimistic, because the LM741's emitter follower output stage will stop working somewhere between 0.6 V and 1 V below the supply. Nonetheless, 5 V - 1 V - 0.5 V or so (for the 1N4006) is still high enough to be seen as a high level by the 4049.
 

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I’m going to play with it somemore, and I believe I’m going to model 12V for the opamp using a dropping resistor and Zener for the inverter, as well as doing some more work with it in B2 Spice, the iOS app I used on my iPad I’m not sure
How good it is. I’ll eventually do some breadboarding with this as well.
 
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