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Bruce's cathode bypass chart

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8/8/2000 4:52 AM

Bruce's cathode bypass chart

Bruce,

I don't get it. I can't find it at Randy Aiken's site either. But I think I believe it. I can hear it and I think the frequencies in your chart are right.

Dan Torres has a chart ? Kevin O'connor says its just the R C pass formula. If i'ts not too difficult some of us would like to know how to figure these frequencies for different anode load values.

Thanks Tracy

8/8/2000 7:14 AM

Try clicking on this to go directly there:

http://www.aikenamps.com/RC-FHz.htm"

/>
Randall has it listed under

TECH INFO
look for...
OTHER AUTHORS
then you should see...

quote:
""A table of lower -3 dB frequency points using standard Rk/Ck combos, applied to 12AX7 w/100K plate resistor
by Bruce Collins, Mission Amps
Lower -3 dB frequency points using standard Rk/Ck combos applied to 12AX7 w/100K plate resistor""

This was pretty simple math and I got all the info I needed to draw that graph up right from Randall's site.

Bruce

8/8/2000 8:55 AM
Dave H.	I don’t think it’s just the RC pass formula. The "R" you need is the cathode resistor in parallel with the impedance looking into the cathode of the tube. The impedance looking into the cathode will be around 2k. If you have a 1k5 Rk the parallel combination will be 850 ohm. A 0.68u cap and 850 ohm have a -3dB point of 1/(2pi8500.68u)=275Hz Dave

8/8/2000 11:09 AM
SteveG	Dave, I entered your formula into an excel spreadsheet and checked the results with the table. They're close, but not quite the same. Are the results in the table empirical, or calculated? Steve

8/8/2000 11:50 AM
Dave H.	I think my formula is a bit off because I just guessed at 2k for the impedance looking into the cathode. The actual impedance will depend on the tube and load resistance. For a 12AX7 with a 100k plate load the impedance looking into the cathode works out closer to 1.6k. 1.6k in parallel with an Rk of 1.5k is 775 ohms. 775 ohms and 0.68u gives a -3dB point of 302Hz Dave

8/8/2000 1:56 PM
SteveG	That's it. It works fine now. BTW, If anyone would like a copy, e-mail me. You'll need Office 97. Steve

8/8/2000 3:26 PM

incidentally, the impedance seen looking "into" the cathode itself is very close to 1/gm.

so:

12ax7
gm=1.5mA/V
Zk=666R

12at7
gm=6mA/V
Zk=166R

12au7
gm=2.5mA/V
Zk=400R

6bq5
gm=11mA/V
Zk=909R

el34
gm=11mA/V
Zk=909R

6l6
gm=5mA/V
Zk=200R

6v6
gm=4mA/V
Zk=250R

6n1p
gm=7.5mA/V
Zk=133R

natch, the gm varies with the Ip, so these numbers are approximations... but they're closer to the real numbers than before.

ken

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