Putting a cathode bypass cap across that 1.5k resistor is a common mod. I believe that some 5F1 Champs came out of the factory with it, stock, although it was not on the schematic.

You can use values from .68uF to 22uF. The practical range is from 2uF-4uF for humbucking pickup guitars and 4uF-10uF for single coil equipped guitars.


Quote Originally Posted by red
Another noteworthy piece of information I've come across is that Victoria and the Weber 5F1 kit use a 25uF capacitor in parallel with the 1K5 resistor coming from pin 3 of the 12AX7.

Here's the Weber 5F1 layout:

http://taweber.powweb.com/store/5f1_layout.jpg

And a short film from Victoria humorously illustrating a 5F1 build:

http://www.youtube.com/watch?v=1xj6d9MLCpo

Supposedly this is a bypass cap, added in order not to screw up the 12AX7 bias as far as DC goes, but boosts the mid and low frequencies (or rather cuts some of the high ones?) in the AC current.

In any case, I don't have a quality electrolytic capacitor to add there, and I'm fed up as it is with waiting for parts so I'll just try the 5F1 I have as soon as I can, and if there'll be any problem with the tone I'll make that adjustment then.