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In Reply to: RE: CCS for PP tube amp - self biasing? posted by raq on November 25, 2007 at 22:16:47
Done properly, the conversion of a PP amp from "fixed" bias to common constant current source bias results in a "freer", more dynamic sound, with more clarity and detail.
There is no getting around providing enough heat sink for the current sources needed. The only safe way to proceed is one current source per output tube and a large heatsink. Two of my Cascode current source kits with the large heatsinks will work, if the heat sinks are up above the chassis where they will receive good air circulation. If you can't spare the space, then you shouldn't do the mod.
You will no longer need the negative voltage supply, if you do this. You will need to do the following:
Remove the "lower" end of each grid leak resistor for the 6550s from the negative supply and connect it to ground.
Leave the 10 ohm resistor attached to the joined cathodes of the 6550s, but remove the other end from ground.
Attach the free end of the 10 ohm resistor to the + pad on each current source board. Attach the - pad on each current source to ground.
Once installed and amp is turned on, adjust each current source trimpot about the same amount (so that the two current sources share current more-or-less equally) until the DC voltage crop across the 10 ohm resistor is about 1.4v (assuming you seek a bias current of 70mA per output tube).
Kevin Carter
K&K Audio
www.kandkaudio.com
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