Effect of screen node filter on tone

[Fred G.]

I'm working on a Fender Prosonic, doing a cap job. I was advised by a tech on another forum that I could take the original 4.7 uF filter up to around 20uF if I wished. I wonder what the effect of this is on the tone of the amp? I used the 20 uF, and it seems like the amp is a little "harsher", maybe? I'm curious what taking it down to 10 uF would do? What kind of effect on tone? Thanks in advance!

[The Dude]

I'd love to help, but no schematic and you don't say where the cap is or what circuit/gain stage it is in.

[Helmholtz]

The value of the screen filter cap affects time constant and amount of screen compression.
A lower value should result in a somewhat softer attack.

Why did you change the original cap?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Delta362]

I have one of these in for a check-up now as well (bad 100uF/100V cathode bias cap on the bias board on mine). I also noticed the small cap in the doghouse and wondered if it was a typo on the schematic as the board looks like it can easily fit the 47uF (stencil almost appears as if was expecting a much larger sized cap).

[Fred G.]

The cap is in the screen node of the filter section, right after the choke.

The schematic is ubiquitous and can be found easily.

I replaced the original 4.7 uF because I was doing a cap job - original e-caps, late 1990's.

I swapped to a 20 uF at the suggestion of a tech on another forum. The 4.7 uF 500V is nearly impossible to find in a quality brand.

I am really mostly interested in what effect the different value has on the overall tone and character of the amp.

Thanks!

[Helmholtz]

I am really mostly interested in what effect the different value has on the overall tone and character of the amp.

As said above.

BTW, it's good practice that the OP provides the schematic - saves all others time and guesswork.

[Fred G.]

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Greg_L]

Why would they use such a small cap value for the screens though? Most high voltage 6L6GC Fenders have much larger caps for the screen supply don't they?

[Helmholtz]

Why would they use such a small cap value for the screens though?

I guess the designer preferred the softer attack with the smaller value.

[Fred G.]

That's the answer I was looking for! Thanks! Does a minimal value have any negative effect, other than to alter the attack?

[The Dude]

Nope. No negative effect. In fact, some might consider it to be a positive effect, depending on your tastes.

[nosaj]

Cause Bruce zinky was a little eccentric. The prosonic is a very cool Amp which can dial in a variety of sound.
nosaj

[Helmholtz]

Too small a value would increase ripple hum and might give rise to motorboating.

[The Dude]

The amp is likely going to be a bit "spongier" and less chunky/tight sounding. If that's not something you like, you can experiment with higher cap values.

[mozz]

Well, there is a choke, which many amps lack.