Replacing signal caps during "blackface" mod

[jelle]

I would say that within the context of this thread, a silverface fender, it is very likely to have scratchy pots due to DC on the pots. This DC typically does not come from leaky caps but from a conductive paraffin coated fishpaper board.

Also around the phase inverter this conductivity can cause unpredictable bias issues. What happens is that the paraffin takes up water over time, which in turn increases conductivity of the board. It is called Fenderitis among techs and mostly affects the later silverface amps.

[mr_hankey]

I would say that within the context of this thread, a silverface fender, it is very likely to have scratchy pots due to DC on the pots. This DC typically does not come from leaky caps but from a conductive paraffin coated fishpaper board.

Also around the phase inverter this conductivity can cause unpredictable bias issues. What happens is that the paraffin takes up water over time, which in turn increases conductivity of the board. It is called Fenderitis among techs and mostly affects the later silverface amps.

Why doesn't that happen to earlier amps? They use the same type of board (I think).

[jelle]

Hee een Hagenees! Yes, it has the same board material: fish paper of phenol/paper board. The difference: also see my comment earlier in this thread, the later amps have paraffin coated boards.

For examples see below in these two pictures that Google kindly provided:

[rp]

Always wondered about that ugly wax. So they did it to make the boards more water-resistant but it actually made things worse?

[jelle]

Short term improvement.... Long term it was actually much worse.

[212Mavguy]

To the OP question of leaving or replacing those blue molded tone caps...

I have a cousin of yours, a '68 Blackline Super Reverb redone by my tech to blueprinted and sometimes matched standards resistor values, NOS AB CC's, F&T 'lytics, he yanked out the blue moldies that I really liked the sound of and put in Russky k40y-9 PIO's at my request from the PI forwards through the reverb/trem channel. We ;eft the blue moldies in the normal channel. A few other mods were done at the same time to the trem and reverb sections. Stuck in some vintage tubes all round and woooeee!

I lke the sound of the PIO's more "liquid" type of smoothness and sweetness all up and down the volume/gain settings on this non master amp. Cleans are sweet, Distorted tones are to die for, nice bloom, very articulate. Attack definition is tight, sustain is nice. I also bought a 1/15 baffle and replaced the stock 4/10 setup with a JBL g135 in the new baffle...Got the head home from the tech, mounted the new baffle and speaker, hooked all up and fired it for less than two minutes and then packed it to gig with that night, it roared sweetly then and ever since.