Curcio Daniel Preamp Mods?

[danFrank]

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

[SY]

Why do you want to do that? Do you need the extra gain?

With a plate CCS load, cathode degeneration is a Bad Thing. You could bypass with a cap (it will have to be a darn big cap) or go to an LED or battery bias scheme.

[danFrank]

I thought that replacing the plate resistor with a CCS was a good thing for fidelity. I must be way off base, or they are way more complicated to implement than I thought.
Daniel

[SY]

Not universally. A CCS is a useful tool, but like any useful tool, it has a specific function. Ask yourself this: if a CCS were a Universal Good as a plate load, wouldn't everyone use them everywhere?

[danFrank]

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

[SY]

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.

[eduard]

hello,
try replacing the 6 20kresistors by 20k 5 watts wirewound mills yoou will be surprised At least i was