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Old 13th May 2002, 05:50 PM   #1
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Join Date: Jan 2002
Location: Tyrone Ga. U.S.A.
Question Cathode bypass cap ?

Most of the SET amp circutes that I have looked at used a cathode
bypass cap. Would omiting this cap change the effective plate resistance and require a different load resistance?

Thanks, bob12345678
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Old 13th May 2002, 06:48 PM   #2
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Location: Hawaii
Default plate resistance

Yes, the effective tube plate resistance goes way up. Add another (u + 1) * Rk ohms to the "stated" value.

For example, a 12AX7 at 1mA, 150V will have an rp of about 60k. If the common cathode amplifier stage uses a 1k ohm Rk, then add another 101k for a total effective rp of 161k.

The bypass caps preserve gain and keep the output impedance lower. But often at the expense of sound.
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Old 19th May 2002, 05:53 AM   #3
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Join Date: Apr 2002
Location: California
Resources for futher education on this and related subjects:

Bruze Rozenblit's excellent book, Beginner's Guide to Tube Audio Design.

Download the 5 part series from Audioxpress magazines website.

Glass audio has software that an be useful and educational.

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