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Old 22nd April 2012, 06:13 AM   #11
nazaroo is offline nazaroo  Canada
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Specifically, Can I substitute the following:

BC557C PNP GP 65v 100ma transistor: subwith a 2N2904 GP PNP 60v 600mA?

1N4148 GP fast Diode: sub with a 1N4007 (1000v)?

1N5818 Rect. Diode 30v 1A: sub with a 1N4148 30 volt 1w zener?

QTLP690C superbright 20mA 5v LED: sub with std LEDs?


Because I have these parts in stock.
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Old 22nd April 2012, 08:54 AM   #12
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by nazaroo View Post
(1) The connections for v1 and v2 are those between the plate and cathode? This would need to be modifed to accurately measure current differences. I would not expect the tubes to ever have equivalent resistances except during instantaineous crossovers in transient variations.
I would rather measure the voltage/current through the fixed cathode resistor to ground for accuracy.
The circuit has to connect to the cathode resistor, see sketch below: outside the dotted frame is the existing circuit
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(2) Is the 300v tap necessary? On the one hand, I'll be using a 520 volt supply, and on the other, the HV seems unnecessary for a low voltage circuit connected near ground with an average voltage range of 50-120 volts (bias) and transients at the cathode hopefully not much higher than 200 v (?)
No, you can feed it with a ~20 -25mA current from anywhere.
If the cathode voltage is as high as 120V, you will need other transistors.
Quote:
What is the QTL690C? just a part number for an ordinary red LED?
It is just any LED I used for the sim.

Quote:
(4) Have you eliminated the sampling resistor across the cathodes in my circuit?
See above

Quote:
will any small PNP signal transistors do here?

What is the meaning of the two different types of transistor?
All the transistor are identical, they are preferably small, high gain PNP's, but if you operate at 120V you will need MJE350 or similar.

But I would advise against it: it is preferable to split the cathode resistors to operate at a more reasonable voltage level, like ~12V or 15V.
That way, you will be able to use small transistors.
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Old 22nd April 2012, 09:02 AM   #13
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by nazaroo View Post
Specifically, Can I substitute the following:

BC557C PNP GP 65v 100ma transistor: subwith a 2N2904 GP PNP 60v 600mA?
It would work, but its minimum Hfe is only 40, that is 1/10th of the BC's. It is preferable to use higher gains types

Quote:
1N4148 GP fast Diode: sub with a 1N4007 (1000v)?
The 1N4148 must be the most common and cheapest component in the world, why on earth would you want to sub it?

Quote:
1N5818 Rect. Diode 30v 1A: sub with a 1N4148 30 volt 1w zener?
No, it is a 1A schottky, any similar type will do, but here again the 1N5818 is the most common 1A schottky available.
The area of this diode will influence the center-band width (green LED):with a smaller schottky,,like the BAT54 or BAT85, it will be narrower, with a larger type like the 1N5820, it will be wider.

Quote:
QTLP690C superbright 20mA 5v LED: sub with std LEDs?
Any LED, any color to your taste will do
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Old 22nd April 2012, 10:11 AM   #14
nazaroo is offline nazaroo  Canada
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As for subbing parts, two good reasons:

(1) I have limited parts in my box.

(2) I have no money.

(3) I live in post-industrial Canada, where every decent parts-store has closed down, because people don't build or repair anything anymore. They buy a new cheap replacement from China.

Your parts may be common in large American cities, but not here in my Igloo.

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Old 22nd April 2012, 10:29 AM   #15
nazaroo is offline nazaroo  Canada
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Also, thank you again for all your effort here,
both at providing a workable solution,
and also in explaining the issues and tolerances of substitutions.

This was a very rapid and professional response to the task.
I greatly appreciate your experience and advice here.

Nazaroo
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Old 22nd April 2012, 10:33 AM   #16
nazaroo is offline nazaroo  Canada
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Quote:
But I would advise against it: it is preferable to split the cathode resistors to operate at a more reasonable voltage level, like ~12V or 15V.
That way, you will be able to use small transistors.
This is a great idea. I can put 10% of the cathode resistor at the bottom, and tap from there, sampling about 12 volts out of 120v.

I can use precision resistors down there, and I can then have confidence that the current on each side is very close.

I have to ask just one more question:

1) Which resistor values are not critical and which have more narrow tolerances here for performance/malfunction issues? (I have a limited part-box).

Last edited by nazaroo; 22nd April 2012 at 10:44 AM.
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Old 22nd April 2012, 10:39 AM   #17
Elvee is offline Elvee  Belgium
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The circuit is tolerant and will accept many alternative components. The types I have used as an example are very common and inexpensive, and they would be difficult to beat, but if you have other types of diodes and transistors in stock, the circuit can probably be modified to accept them.

Post your amplifier schematic, your voltage tolerance requirement and your preferred list of semi's, we will see how to make the best out of them.
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Old 22nd April 2012, 10:50 AM   #18
nazaroo is offline nazaroo  Canada
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Thanks!


I was thinking I could take taps as you suggest, splitting the cathodes into series-pairs (10% R precision at the bottom) for about 12 volts on either side.

If I do that, would there be anything wrong with taking the power from the top of the cathodes, by running a diode from each side to the center for a +ve supply?
The opposing diodes would not allow current across the cathodes, but using two would balance the current draw (?).
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Old 22nd April 2012, 11:33 AM   #19
nazaroo is offline nazaroo  Canada
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Okay here's the relevant portion of the schematic (the rest is still being designed).

I am suggesting getting power from the top of the cathode resistors, without interfering with them too much.

And taking readings off the top of a precision resistor bearing 10% of the cathode resistance (for about 12 v).

Now however, the top feed resistor will be unknown.
How should I calculate a safe value?

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Old 22nd April 2012, 11:44 AM   #20
nazaroo is offline nazaroo  Canada
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I currently have 2R2, and 10 ohm 2w,
instead of your original 5.6 ohm.

I had a 120 R 1%, and a 680 R 2w.

I have 1N4007s for 'ordinary' diodes,
and about 10 2N2904s doing nothing.

I don't think I have any Schottky 1A diodes.
Nor do I really know what they are for.

I have powersupply diodes and zeners only.
and a few different color LEDs, normal size.
I also have a dual color LED, but its not a three-color.

I have testbench powersupplies, and I am trying to think of
a test-circuit that would be able to confirm the new circuit works. (I don't need to hook up any tubes for that).
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