If you wan't to make an RF filter, add an external capacitor and keep the resistor at around 1k or so. f=1/(2piRC).
The AM Broadcast band starts below 600kHz. If you want a reasonable amount of attenuation here from a single pole / -6db R-C input filter, you need to have a -3dB frequency much lower - say 100kHz.
Suppose your CF tube buffer has a Cin of 15pF (including strays). For a 100kHz f3 that would require a resistor of around 100k.
Such a large resistor in series with the signal source would be a pointless generator of excess noise.
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Thanks Mr Horneman... I was'nt really know that!
In fact, I already have a LPF just after the tube. And in my listenning test I did not hear any HF interference. Actually, in the simulation I've seen this danger and increased the grid resistor to 20K. But may be I can test with 100K or more...
Thanks.
In fact, I already have a LPF just after the tube. And in my listenning test I did not hear any HF interference. Actually, in the simulation I've seen this danger and increased the grid resistor to 20K. But may be I can test with 100K or more...
Thanks.
Short tube grid to the ground (with no other input, of course). Is hum still there ?
If not, it is definitely not a heater supply related problem.
If there is still audible hum, try battery heater supply. Is hum still there ?
FWIW, my multimeter has a frequency counter function in addition to all the normal stuff. It never ceases to amaze me that holding the probes in my hands will almost always yield a correct measurement (of 50 Hz, rather than 0 Hz) - enough voltage is induced by the mains wiring radiation in my body to have measurable effect even with such a crude tool. This makes it easy to understand how hum can creep into circuits made of considerably better materials (as far as electrical properties go) than human flesh.
If not, it is definitely not a heater supply related problem.
If there is still audible hum, try battery heater supply. Is hum still there ?
FWIW, my multimeter has a frequency counter function in addition to all the normal stuff. It never ceases to amaze me that holding the probes in my hands will almost always yield a correct measurement (of 50 Hz, rather than 0 Hz) - enough voltage is induced by the mains wiring radiation in my body to have measurable effect even with such a crude tool. This makes it easy to understand how hum can creep into circuits made of considerably better materials (as far as electrical properties go) than human flesh.
That's interesting as I've got a hum with the transformers I'm using at the moment. Tried out my aikido and another project and hum present. Star grounding etc.
Not sure if it's the power transformer falting or the heater AC.
I want to eliminate a heater hum possibility as I've never had problems using AC before.
So if I disconnect any source input, place my headphones on, and connect a shorting wire from the grid to the ground, if the hum is still present it is a heater supply issue, if no hum then another problem.
Please correct me if I'm wrong.
Not quite; this would isolate the hum to that (or any succeeding) stage. In your case it could also be coming via power supply, not only by the way of heater-cathode interaction.
Once you isolate the stage where the hum appears, you should check each potential cause (i.e. use battery for heater supply and eliminate the heater as this is usually far more convenient than obtaining high voltage battery for anode supply).
Once you isolate the stage where the hum appears, you should check each potential cause (i.e. use battery for heater supply and eliminate the heater as this is usually far more convenient than obtaining high voltage battery for anode supply).
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