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Shared CCS on output tubes in PP?

Posted by jyourison (A) on December 3, 2010 at 06:30:49

All the potential output tubes I have on hand for P-P use are unmatched, but I'd like to use them if possible by applying a CCS and/or maybe gartering if needed.

In looking at a schematic for a 6AS7 push-pull amp, I see that the builder uses a full, cascoded CCS for each cathode using two DN2540s per tube section. Recently I saw a schematic for a 6CW5/EL86 push-pull that uses only a common CCS in the cathodes.

I think the DN2540n5 in the TO-220 package can handle up to 150 mA of current and dissipate up to 15W with a good heat sink. What are the considerations for using separate or common CCSs in the output stage?

To be honest, I'm just trying to keep things as simple as possible and keep component count down as well.

Thanks for any help in understanding this.

I found this post from diyAudio member "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."

I'm having some trouble picturing his suggestions.

Here are some links to examples:

Common CCS in a KT88 PP:
http://jastrid.xs4all.nl/Dalojan/Schema_KT88PP.html

Common CCS in an EL86 PP:
http://lagarto.web.infoseek.co.jp/el86pp.htm

Individual CCS in a 6AS7 PP:
http://www.triodeguy.com/Triodeguy%20PDF%20files/6AS7%20CURRENT%20BALANCED%20PP%20AMP%20SCH.pdf

Thanks for any of your recommendations,

--Jeff

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