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Sharing cathode resistor and capacitor

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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.
 
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.
 
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.
 
DF96: in particular, the remaining good tube will have its current defined by:

i = [(gm Vcutoff) / (1 + gm Rcathode)]

so, with algebra,

i2/i1 = [big equation, reduces to] = [(1 + gmR1)/(1 + gmR2)]

This will be the INCREASE in current, because R1 always looks "larger" than R2. When the two tubes are sharing the current (exactly equally, the limit case), R1 looks 2x R2, the native nominal value. So, this reduction can go to:

R1 = 2R2
i2/i1 = ((1 + 2gmR) / (1 + gmR))

if gm = 0.005 S = 5000 uS
and R = 300 ohms
then i2/i1 = 1.60 times; 60% more current flow.

GoatGuy
 
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