Returning the plate snubbing capacitor to the cathode
Returning the plate snubbing capacitor to the cathode
Fellow radiophiles,
Nearly all tube radios have a capacitor wired to the output pentode plate. This capacitor is usually in the range of 0.01uF to 0.05uF and is rated for several hundred volts. The purpose of this capacitor is to absorb the energy that is stored in the core of the output transformer, in the event that the plate current driving this transformer is suddenly cut-off. The purpose of the capacitor is to "snub" excessive voltages.
This plate current cut-off could happen due to static noises, or any other negative impulse at the pentode control grid. If this capacitor were absent, a sudden cutoff of current to the output plate, could easily develop over a thousand volts and this voltage could damage transformer primary insulation, or cause internal arcing in the output tube.
The following schematic excerpt from a classic American AC/DC 5 tube radio, a Teletone 148, shows this capacitor marked as (8), and with a value of 0.02uF. I will call this capacitor C8.
However, C8 does not return to ground, or even to the power supply side of the output transformer (32). C8 returns instead, to the cathode of the output pentode and to the 150Ω cathode bias resistor.
This connection has little effect in the ability of C8 to absorb the energy stored in the core of the output transformer, especially when considering that the AC impedance of the transformer is on the order of a few kΩ.
The effect of returning C8 to the cathode and the bias resistor, is a doubling of the high frequency audio bandwidth due to a 50% positive feedback. The cathode impedance is 1/gm=150Ω. Half the output high frequency current gets recirculated into the cathode.
The following LTspiceIV simulation compares the frequency response with the 0.02uF cap returned to ground, to the frequency response with the return to the cathode. The high frequency cutoff point is 3.3kHz with the 0.02uf returning to ground, and 6.6kHz with the 0.02uF cap returning to the cathode and 150Ω bias resistor.
Summarizing, the cathode return of the plate subbing cap allows the full AM radio 5kHz audio bandwidth to be amplified, while protecting the output tube and transformer from common static spikes.
This snubbing capacitor sometimes appears wired from the plate to ground or from the plate to power supply side of the transformer. From the design engineer's point of view, these are electrically equivalent, but the ground return is easier to install in a crowded chassis. Having a constant high voltage across the cap is a greater liability in the case of leaky paper caps. If there is leaky paper cap used as snubber, it may have to be replaced if the leakage is excessive, while the same cap could be left in place if it were wired directly in parallel with the transformer primary between the plate end and the power supply end.
Regards,
-Joe
p.s.:I have attached two pdf files with the contents of the .ASC schematic and the waveform setup file .PLT for simulation with LTspiceIV copy the text contents of each of these files with a text editor into files with the appropriate extensions of .asc and .plt, and you are ready to simulate.
When making the .ASC file, make sure that 0.02μF is represented properly. My text editor represented the greek character "μ" incorrectly. I changed this to "u" after opening in the LTspiceIV. Also be sure that the last "TEXT" command in the .ASC file fits in one line.
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