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Old 31st May 2012, 03:53 AM   #1
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Default Sharing cathode resistor and capacitor

In PP amp, each PP power tube has its cathode resistor and capacitor usually, but some are not. The pair of PP power tube shares a single resisotr and capacitor.

Could someone advise me of difference? Thank you very much!
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Old 31st May 2012, 04:47 AM   #2
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A shared RC bias network for each PP pair can be advantageous. Aside from the cost savings, tube matching requirements are slightly relaxed. Tight matching of gm is essential, but a small difference in cathode current is acceptable. If either tube starts to conduct too much, the bias voltage goes up and corrects the imbalance.
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Old 31st May 2012, 05:53 AM   #3
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Do you mean separate resistors correct imbalance, while common one increases it?
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Old 31st May 2012, 07:29 AM   #4
Yvesm is offline Yvesm  France
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Sharing cathode resistor voids balancing current in tubes and the OPT does not like that.

For this specific point of view a bypassed CCS in each cathode is the best but . . . on peaks and specially if the tubes are used in AB class, the voltage accross the CCS increases very quickly and that is bad !

A quick solution is to add a beeffy zener accross each CCS at says 1.3 to 1.5 the nominal expected bias voltage.

This look a bit complicated but the advantage is that you no longer have to bother about bias setting nor tube wearing.

Incidently note that, being bypassed, the CCSs AC behaviour is "don't care".

Yves.
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Old 31st May 2012, 09:55 AM   #5
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You will find a very interesting analysis here

Amplifier auto bias circuits: Alan Dower Blumlein's garter circuit
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Old 31st May 2012, 03:27 PM   #6
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Thank you very much for your information!
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Old 31st May 2012, 03:32 PM   #7
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Thank you very much
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Old 24th January 2013, 04:35 PM   #8
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what happens when one of the tubes fail? would one tube be able to handle the current through the shared cathode resistor?
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Old 24th January 2013, 04:42 PM   #9
DF96 is offline DF96  England
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The shared cathode resistor does not have a current. It and the single remaining valve agree on a current, based on the voltage developed across the resistor. This current is likely to be smaller than the original current through the resistor, but larger than the original current through one tube when both tubes were present. Whether the remaining tube can handle this current depends on how close the original circuit ran the tubes to their limiting current or dissipation.
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Old 24th January 2013, 04:59 PM   #10
Merlinb is offline Merlinb  United Kingdom
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An article on the effects of a shared cathode bypass capacitor:
http://www.aikenamps.com/cathcap.pdf
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