CCS in Otput Tube Cathode - Does It Make Sense?

[oshifis]

Currently I am using capacitor shunted cathode resistors for setting the DC operating point (807 triode connected, 425V B+, 560 ohms parallel 2 x 10uF / 160V) that sets about 50...60mA.

What happens if I connect a constant current source, like an LM317 with a 25 ohms current sensing resistor in place of the 560 ohms cathode resistor? How do I calculate the necessary shunt capacitor? Does it make sense at all?

[Shoog]

It makes most sense in a PP amp where it fixes the current through the output transformer, eliminating saturation currents from the core.
In SE amps, as you are describing, with fully bypassed cathodes - I would say that it probably makes absolutely no difference since all of the AC signal is going through the cap in either case.
Still my RH807 SE amp is setup to use LM317 CCS in the cathode and it certainly does no harm.

Shoog

[oshifis]

I intend to use it in a push-pull amplifier, but I've never seen any circuit diagram using a CCS in the output tubes' cathode. My problem is with the shunting capacitor. The CSS is virtually an infinite resistor, so the determining element is the output impedance of the cathode, right? How do I get it from the curves?

[d2134]

Use the same shunting capacitor as for the 560 ohm resistor.

[cerrem]

It does not make much sense to me, if your using a common tied cathode from both output tubes...
Saturating currents in the transformer occur from IMBALANCE in the output tube pair in P-P amps.... Since the DC cancels when it is balanced...
Unless you are using a "seperate" CCS for each output tube independently.... Then you would bypass each with a cap based on the resistance looking into each single cathode, which is basically 1/gm ..... Once you know the R, then you set the time constant for a -3dB POLE at no higher than 6 Hz....
In common cathode P-P outputs, you would not have an AC signal at the cathode due to cancelation, but in real life the tubes do not track with perfect balance, which makes for small degenerative feedback, which may be good...or use a bypass cap to bypass what it not canceling...

Chris

[Johan Potgieter]

If output operation is studied it will show that there is mostly an increase in current near max. output and for lowest distortion. A CCS will thus limit max. output power to a little lower than obtainable with cathode bias.

[dragos_g]

See Dalojan's schematics for an example. It keeps the output stage, KT88, in class A:
http://jastrid.xs4all.nl/Dalojan/Schema_KT88PP.html

Regards,
Dragos

[Shoog]

If you use a single CCS for an output pair you need to include a current balancing pot to null current imbalances and minimise static core saturation.
The better way is to have independent CCS for each cathode and bypass them seperately. This can be turned into a differential pair by connecting the two caps cathodes together and referencing the node to a 1meg resistor, which then goes to earth. This unfortunately puts your caps in series and so effectively halves their value - so a cap of twice the size is needed. Bypass the caps with film caps and bypass each valve cathode to the other with a film cap. This works very well and needs no rebalancing as the valves age.

Shoog

[oshifis]

Thanks for the ideas to all. I tried independent CCS in each cathode (an LM317 with 25 ohms current setting resistor, 50 mA idle current). The bypass capacitor is 2 x 10 uF polypropylene. It works, but the available output is lower than with autobias. I think this is because there should be a minimum 3 V voltage drop on the CCS, which reduces the g1 voltage swing. The voltage drop is about 36 V on the CCS. So the maximum voltage swing is 33 V peak or 23 V RMS. I could drive the output tubes in autobias with 26 V RMS indeed.

[7N7]

A constant current sink in the output stage of a p-p amplifier, apart from any direct advantages is a nice idea for DC-coupled amplifiers.

I once built a DC-coupled amplifier using a 6528A as the output pair I sat this on top of a 12E1 beam tetrode as a sink. I recall that the amplifier sounded very nice.

7N7