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Cathode bypass cap ?
Cathode bypass cap ?
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Old 13th May 2002, 06:50 PM   #1
woody is offline woody
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Cathode bypass cap ?
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, 07:48 PM   #2
hagtech is offline hagtech
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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, 06:53 AM   #3
Aud_Mot is offline Aud_Mot
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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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