Silverface Deluxe Reverb trem cap

[Randall]

I am re-capping an all original AB868 DR, and I notice the second side of the trem oscillator tube has a 5uF/25v cathode bypass cap instead of the blackface 25uF/25v. The schematic confirms this, which I had previously thought may have been a typo, as in an omitted "2". Looks like the rest of the trem circuit is unchanged. Anyone know why, or what it is intended to achieve?

[Structo]

Interesting, but I bill bump the post so the Fender experts can chime in.

[Milkmansound]

If I recall the second triode in the tremolo circuit is just there to drive the neon lamp. Might have something to do with that

[ToneMerc]

I am re-capping an all original AB868 DR, and I notice the second side of the trem oscillator tube has a 5uF/25v cathode bypass cap instead of the blackface 25uF/25v. The schematic confirms this, which I had previously thought may have been a typo, as in an omitted "2". Looks like the rest of the trem circuit is unchanged. Anyone know why, or what it is intended to achieve?

You answered your own question, it's an oscillator generator. Thus, the cathode cap value is not all that the relevant in that role.

TM

[martin manning]

It doesn't require any more than 5uF to bypass the 100k Rk down below 1Hz. My guess is that a 25uF was used in the earlier versions to eliminate a part number. At 25uF it is the same value as all of the other low-voltage electrolytics.

[Aaron]

This makes an interesting read. The cap does play a fairly important roll.
http://www.clarkhuckaby.com/NewVibe/CloseLookVibe.html
Cheers,
Aaron

[martin manning]

Interesting read indeed, thanks Aaron! I'm not surprised that the bypass cap affects the pulse shape to some degree, but I wonder if there is any noticeable difference in the sound of the tremolo with 5uF vs. 25uF?

[Aaron]

I used 100uf in a build and find the tremolo sound more "rounder".
You could wire in a DPDT switch temporarily and try both out.

[martin manning]

It doesn't require any more than 5uF to bypass the 100k Rk down below 1Hz. My guess is that a 25uF was used in the earlier versions to eliminate a part number. At 25uF it is the same value as all of the other low-voltage electrolytics.

Looking at this a little closer, Fender switched the cathode resistor from 56k to 100k going from AA763 to AB763 (ca. 1964), and did not change the cap value. The switch to 5uF was a later CBS modification, so that may have actually been a cost move.