CCS placement

dgta · 9th July 2011, 11:00 PM

Suppose we have a simple voltage amp, like the first stage of a Williamson.

We want to improve operation by introducing a CCS. We use a simple BJT or JFET CCS that doesn't take much voltage to operate. So we're able to place the CCS in either the plate or cathode (Assume Vgk=8 to 9V).

Assume the next stage is a direct-coupled concertina, no grid leak. What, if any, advantage(s) would be realized from placing the CCS in the A or K?

How does the answer change if there is more output loading by the following stage?

Alastair E · 9th July 2011, 11:12 PM

Putting in the Anode-circuit, you can tailor the reference-voltage to get the phase-splitter grid-voltage correct, for optimum operation....

SpreadSpectrum · 9th July 2011, 11:58 PM

I'm having trouble understanding what you are getting at. If you put a ccs in the cathode of the first stage, you will get no gain as the ccs will try to cancel out the input signal.

If you don't have equal loads on a concertina, you will have an unbalanced splitter.

dgta · 10th July 2011, 01:02 AM

Spread, I assume you can avoid cathode degeneration by bypassing the cathode, no?

As to the concertina, you misread. I'm not proposing a CCS in the concertina, only in the voltage amp. I will edit for clarity.

Oops, too late to edit my original post. Sorry.

SpreadSpectrum · 10th July 2011, 01:33 AM

Quote:

Originally Posted by dgta

Spread, I assume you can avoid cathode degeneration by bypassing the cathode, no?

As to the concertina, you misread. I'm not proposing a CCS in the concertina, only in the voltage amp. I will edit for clarity.

Oops, too late to edit my original post. Sorry.

Ahh, yes. That makes more sense.

CCS in the cathode (assuming it is bypassed) is going to give a more stable current than a resistor, otherwise it is about the same as a resistor.

I would put the CCS in the anode where it will improve linearity and offer more gain. I would also bias with LED in cathode circuit.

dgta · 10th July 2011, 04:42 AM

Do you know any LEDs that work well at 2mA?

DF96 · 10th July 2011, 02:35 PM

Remember that with a triode there is a relationship between Vg, Va and Ia (and this relationship changes with ageiing). You can fix any two of them and the triode will attempt to set the third. Fix Ia (with a CCS) and Vg (with an LED) and Va will then set itself where it chooses. The result is that you will need to keep adjusting the CCS as the valve ages, otherwise the phase splitter could get upset.

If using a CCS in the anode, I would use a resistor in the cathode so that the necessary adjustment is shared between grid and anode voltage. This gives better long-term stability. This issue is less of a problem if the next stage does not use DC coupling.

SY · 10th July 2011, 02:51 PM

Quote:

Originally Posted by dgta

Do you know any LEDs that work well at 2mA?

With a CCS in the plate, nearly all of them.

dgta · 05:32 PM

Quote:

Originally Posted by DF96

Fix Ia (with a CCS) and Vg (with an LED) and Va will then set itself where it chooses.

Yes indeed. I haven't done much reading on the LED biasing, but my understanding is that LEDs are essentially used as a fixed voltage reference. With a CCS, seems to me you have the same thing already. A constant current across the fixed (cathode) resistor.

SpreadSpectrum · 11th July 2011, 02:35 AM

I'm having trouble understanding how a resistor would give more DC stability than the LED. Like dgta said, fixed current through a resistor gives a fixed voltage, independent of the tube in the circuit.

I would think that the LED would be slightly better. With real world CCSs, AC impedance will not be infinite so there will be some small AC voltage at the cathode in the case of the resistor. In the case of the LED, this will be less due to the (presumably) smaller AC impedance of the LED. True, this may be an insignificant improvement but I like to take better when it comes at no additional cost. If I can't find an LED with the right forward voltage I would have no problem using a resistor, though. I have to admit that another draw to the LED solution is that it glows.

9th July 2011, 11:00 PM	#1
dgta diyAudio Member Join Date: May 2011	CCS placement Suppose we have a simple voltage amp, like the first stage of a Williamson. We want to improve operation by introducing a CCS. We use a simple BJT or JFET CCS that doesn't take much voltage to operate. So we're able to place the CCS in either the plate or cathode (Assume Vgk=8 to 9V). Assume the next stage is a direct-coupled concertina, no grid leak. What, if any, advantage(s) would be realized from placing the CCS in the A or K? How does the answer change if there is more output loading by the following stage?

9th July 2011, 11:12 PM	#2
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	Putting in the Anode-circuit, you can tailor the reference-voltage to get the phase-splitter grid-voltage correct, for optimum operation.... __________________ Das Beste Oder Nichts

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9th July 2011, 11:58 PM	#3
SpreadSpectrum diyAudio Member Join Date: Nov 2007	I'm having trouble understanding what you are getting at. If you put a ccs in the cathode of the first stage, you will get no gain as the ccs will try to cancel out the input signal. If you don't have equal loads on a concertina, you will have an unbalanced splitter.

10th July 2011, 01:02 AM	#4
dgta diyAudio Member Join Date: May 2011	Spread, I assume you can avoid cathode degeneration by bypassing the cathode, no? As to the concertina, you misread. I'm not proposing a CCS in the concertina, only in the voltage amp. I will edit for clarity. Oops, too late to edit my original post. Sorry.

10th July 2011, 04:42 AM	#6
dgta diyAudio Member Join Date: May 2011	Do you know any LEDs that work well at 2mA?

10th July 2011, 02:35 PM	#7
DF96 diyAudio Member Join Date: May 2007	Remember that with a triode there is a relationship between Vg, Va and Ia (and this relationship changes with ageiing). You can fix any two of them and the triode will attempt to set the third. Fix Ia (with a CCS) and Vg (with an LED) and Va will then set itself where it chooses. The result is that you will need to keep adjusting the CCS as the valve ages, otherwise the phase splitter could get upset. If using a CCS in the anode, I would use a resistor in the cathode so that the necessary adjustment is shared between grid and anode voltage. This gives better long-term stability. This issue is less of a problem if the next stage does not use DC coupling.

11th July 2011, 02:35 AM	#10
SpreadSpectrum diyAudio Member Join Date: Nov 2007	I'm having trouble understanding how a resistor would give more DC stability than the LED. Like dgta said, fixed current through a resistor gives a fixed voltage, independent of the tube in the circuit. I would think that the LED would be slightly better. With real world CCSs, AC impedance will not be infinite so there will be some small AC voltage at the cathode in the case of the resistor. In the case of the LED, this will be less due to the (presumably) smaller AC impedance of the LED. True, this may be an insignificant improvement but I like to take better when it comes at no additional cost. If I can't find an LED with the right forward voltage I would have no problem using a resistor, though. I have to admit that another draw to the LED solution is that it glows.

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