Don't worry about the slight increases in resistance. Those old carbon comp resistors are typically +/- 20% so they can be 20% under or over the value and not cause any problems.

I checked the data sheet for the 6V6 and the maximum resistance for the grid load resistor (the 220K) is 500K, so at +/- 20% you're well below the max.

The main problem with carbon comps is that they drift in value over time compared to metal film, metal oxide, or carbon film resistors. Sometimes when they drift just right the amp sounds very good. Other times, it doesn't doesn't work out so well. That's partially why some vintage amps sound great and others don't.

They can be noisy in certain positions, such as the plate load (100K). But some think that there is some magic/mojo if you put carbon comp resistors in those positions. I have not tried it myself, but I think it would be hard to tell the difference in a double blind test.

Quote Originally Posted by red
I've decided to stick to carbon comp resistors just because Victoria uses them and I love the way the 518 sounds. They're probably overkill to use all over the place, but it makes my life simpler, and if the noise they add won't be more than what the 518 puts out, I'll be happy.

I've sourced some new carbon comps (still waiting for a couple of 100k ones that turned out to be harder to get) and started wiring the board and soldering them in place. This time no surprises with the resistivity rating after soldering.

But - the two 68k grid resistors came at 71-72k and turned 73-74k after soldering. And the 220k power tube grid load resistor came at 250k and went to about 254k after soldering.

I don't think that either drift is spectacular or particularly important for the well-being of the finished amp, but as CCs will do, they will most likely continue to slowly drift upwards. The two "68k"s won't make a noticeable difference, but I'm worried about the "220k" - what if it goes to 300k? I'm assuming it won't affect the tone but will affect the power tube "loudness", and the power tube itself.

Hmm, maybe that's actually a good place for a metal or carbon film. If I understand it's purpose correctly, no signal actually goes through that resistor, except maybe to "go away" to ground.