• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Curcio Daniel Preamp Mods?

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Hello,
I was wondering if anybody who has one of these preamps has modified it to get better sound? Please share any experience...
I was thinking about replacing the 20K plate resistor with Pimm's Self Bias CCS. Has anybody done anything similar?
As a general question, if I replace the plate resistor with a CCS, should I bypass the cathode resistor with a cap? Is this needed?
Thanks in advance...
Daniel
 

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OK, so generally speaking, when and where would a CCS make a positive difference? And in what kind of tube circuits?
Looking through past posts here, it seems like people tend to use them a lot in the plate of voltage amplifier stages, in the cathodes of differential phase splitters, basically everywhere but in the power output stage.
What am I missing?
Daniel
 
People use them in power output stages, too. Like anything else, it's a tool, no more, no less. But a very useful tool.

A CCS will cause a triode's load line to be nearly horizontal. That reduces distortion due to curve bunching at low currents. It also runs the tube at its maximum gain (mu, to a good approximation). Because of its large dynamic impedance, it will change the output resistance of a stage and thus the associated rolloffs. Followed by a buffer or with a sufficiently low rp tube, that's not a big deal, but in some situations, it is. Remember, though, that the tube's plate load is not just the plate resistor or CCS but also the load that it's driving, even on the other side of a coupling cap.

Once a tube's plate load is 5-10x bigger than rp, the CCS will have considerably less beneficial effect. And for circuits where the plate resistance is high (like degenerated common cathode or a cascode), the CCS can be detrimental.

In my preamp, I use a CCS to set the DC operating point of a cathode follower while allowing the driven load to dominate the AC loadline for the tube.
 
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