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Cathode bypass capacitor related question
Cathode bypass capacitor related question
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Old 3rd August 2018, 07:31 AM   #1
zlab is offline zlab  Hong Kong
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Default Cathode bypass capacitor related question

For a first stage of power amp (voltage stage), if it is plate CCS loaded, is there any reason to use cathode bypass capacitor?Also, for the last stage of power amp, if it is cathode CCS bias, is there any reason to use cathode bypass capacitor?

Thanks.
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Old 3rd August 2018, 08:30 AM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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One example to get you thinking: Take a single ended output tube, and put a perfect current source in its cathode. The current source will keep the cathode current constant, no matter what the grid voltage is. That would include the DC grid bias setting if using fixed bias, and also the grid signal from the driver tube. There will be no change in the cathode current, and so no change in in the plate current, and so no output from the output transformer when the grid is driven by signal (music). That would be a good reason to bypass the cathode current source.

For an input stage, if the plate current source is perfect, the tube plate still has to drive the coupling capacitor that drives the next stage's grid resistor to ground. Therefore, there will be a change of plate current. the changing plate current means there will also be a change in cathode current. A cathode resistor that is unbypassed will have a changing current in it, and so have a changing voltage across that cathode resistor.

With a perfect plate current source, the voltage across the unbypassed cathode resistor will be according to the ratio of the cathode resistor versus the next stages grid resistor.
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Old 3rd August 2018, 09:16 AM   #3
DF96 is online now DF96  England
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For an input stage, the anode still has to drive the next stage grid resistor too.

For an output stage, CCS cathode bias is a good way of ensuring that the bias is mostly wrong. This is because it fixes the wrong current: average current instead of quiescent current. A resistor would be better. Fixed voltage bias better still, although it will not self-adjust to different valve samples or valve ageing.
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Old 3rd August 2018, 09:19 AM   #4
Merlinb is offline Merlinb  United Kingdom
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When using a CCS load on a regular gain stage (assuming not much additional external loading), if you include the cathode bypass cap you get:
1. Pretty much the same gain;
2. More distortion;
3. Smaller output impedance.
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Last edited by Merlinb; 3rd August 2018 at 09:22 AM.
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Old 3rd August 2018, 04:41 PM   #5
zlab is offline zlab  Hong Kong
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Hum...Get info now, thanks. Output need cathode bypass capacitor for AC current path for proper functioning. Voltage stage does not need it.
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Old 4th August 2018, 10:18 PM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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Some tube types have more heater to cathode leakage than others. Some tubes heater to cathode leakage degrades over time. 1mV of injected hum from the heater to the cathode, will produce the same amount of hum as if you injected 1mV of hum into the grid. In either of these cases, you may experience hum because you did not bypass the cathode resistor.
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Old 6th August 2018, 08:08 PM   #7
tapehead ted is offline tapehead ted  United States
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As for calculating an appropriate bypass cap- I'm reading that the cathode resistance, just for the tube itself, is (Ra+ra)/(mu+1). That's in parallel with the cathode resistor for calculating the bypass capacitor. Since the CCS has a huge output resistance, am I correct that it could be ignored and the bypass capacitor calculated by (Ra+ra)/(mu+1)?
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Old 6th August 2018, 08:32 PM   #8
6A3sUMMER is offline 6A3sUMMER  United States
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tapehead ted,

I had not even considered eliminating the bypass cap when I modified a circuit recently. It now has a current source in the plate circuit, and the plate drives the RC coupled grid of the next stage.

Just for fun, I will measure what the change is with, and then without, the cathode bypass cap. The grid resistor of the next stage is 270k Ohms, and that in parallel with an equal or even higher impedance of the current source, and when that is divided by u +1, is going to be many many times the resistance of the self bias resistor. I am predicting a change far less than 1dB.

I probably do not have enough spur free dynamic range in my FFT to see any difference in the harmonic levels between bypassed and unbypassed self bias resistor in the triode that has a current source in the plate circuit. I also do not expect the gain to change significantly. I shall see.

Where I would expect to see a difference between bypassed and un-bypassed is if I overdrive that stage and draw grid current. The self bias should shift when the bypass cap is present. And that bias shift, no matter the size will have recovery time due to the capacitance and the unbypassed impedance of that node (the total RC time constant).

If there is noise on the filament secondary, and if it has lots of high frequency energy, I do not expect it to introduce significant noise onto the cathode, because the self bias resistor is much lower than the Xc of the small pF filament to cathode and the large resistance of the filament to cathode leakage.

Last edited by 6A3sUMMER; 6th August 2018 at 08:35 PM.
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Old 7th August 2018, 05:50 PM   #9
tapehead ted is offline tapehead ted  United States
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Quote:
Originally Posted by tapehead ted View Post
As for calculating an appropriate bypass cap- I'm reading that the cathode resistance, just for the tube itself, is (Ra+ra)/(mu+1). That's in parallel with the cathode resistor for calculating the bypass capacitor. Since the CCS has a huge output resistance, am I correct that it could be ignored and the bypass capacitor calculated by (Ra+ra)/(mu+1)?
Sorry, I was thinking constant current source in the cathode circuit here, not at all sure I made that clear.

Very interested to hear the experimental results.
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Old 7th August 2018, 07:32 PM   #10
tapehead ted is offline tapehead ted  United States
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So I'm designing an input stage that is direct coupled to the next stage. So there is no coupling capacitor and no grid leak resistor to drive, but there will be a grid stopper. So will a bypass capacitor be required for a constant current sink in the cathode circuit of the input stage?

Right now I'm thinking of just using an anode resistor so it's easier to set a fairly precise operating point for the sake of the DC coupling.

Also, I'm concerned about keeping noise and especially signal from ground from getting on the cathode of the input stage. My ground looks to be messier than my HT in this regard, so I'm wondering if the constant current sink in the cathode circuit will better reduce noise and unwanted signal from the ground. It looks to me like I could have a very large resistance from ground to cathode, and then a much smaller cathode resistance (Ra+ra)/mu+1) to form an effective potential divider.

It looks like a bypass capacitor could only interfere with this perceived advantage in reducing noise and unwanted signal from ground.

Thanks for any light anyone can shed here.
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