Tube DIY Asylum: Confessions of a plate choke convert & a few CCS questions (long) by Doug Flynn

Hi DIYers

My DIY 300B SET amp with 6J5 driver was beginning to sound a bit boring, so I decided to experiment with adding a better driver. I wired up a 6V6GT (triode-strapped) as the driver, with the 6J5 now doing duties as the input tube. I used a 5K resistor as the 6V6 plate load, cap coupled to the grid of the 300B. There was much more gain (obviously - it's now a 3 stage amp) and drive - the 6V6 really is nice triode-strapped, although I am going to try out KT66s and even 10Ys. However, there was a graininess to the sound, and when I cranked the volume I got smearing of instruments and the whole thing basically "lost the plot". I had the 6V6 biased at -9V, so it wasn't overload causing the problems. I also tried a 10K resistor, but to no avail.

Quite frankly I was about to rip out the 6V6 and go back to the 2 stage 6J5/300B configuration because I liked the sound of the 6J5 more, despite its problems. Then on a whim I decided to replace the 6V6 load resistors with some 100H, 100ma C-core plate chokes I bought with a set of parafeed output trannies recently (for a project I haven't started yet...). These plate chokes are big buggers and are actually a bit bigger than my SE output trannies. The resultant circuit is virtually the same as the Komuro DC 300B SET amp (except it's cap coupled, not direct coupled).

I'm glad I made that change because it made a big difference. I hate hopping onto the net and declaring something to be a "night and day difference" (a la Thorsten!), but this really did blow me away. Suddenly all the grain disappeared and was replaced with a lush, vibrant, sweet, smooth and highly detailed soundstage. All the compression and muddiness of the plate loaded 6V6 vanished. Wow! I'm now going to get my tranny winder to wind me some more plate chokes for every tube in my amp, preamp and phono stage. I can't conceive of ever using a resistor again to load a tube. This is gonna cost me some dough.......

I've been reading a lot about pentode CCS (thanks to Gary P) loading of tubes. CCS looks a bit fiddly, and I can't say I'm thrilled at putting all that silicon in my amp. However, if it sounds better I'll do it. My question is whether anyone has done any serious comparisons between pentode CCSs and (good quality) plate chokes. Is the CCS a compromise to save money or is it even better than a plate choke? Would a really top notch plate choke that's massively overspecced be the ultimate way to load a tube?

Any thoughts appreciated.

Cheers
Doug