Effect of screen node filter on tone

[Enzo]

Yeah the choke takes the place of some capacitance.

Fred, really, install the 4.7uf make it all work. Now tack a 20UF in parallel and FIND OUT what difference it makes. Then report your findings.

[Fred G.]

Enzo, I'd love too, but I need to turn this one over. I've had it a while and need to get it back to the customer. The 4.7 uF at 500V is nearly impossible to find (in a quality brand), that's what precipitated this cap value change experiment. I really can't keep using this amp as a test bed, much as I'd like - I need to get it back to him.

What would I expect to discover, by doing this - what would the result be, sonically? He's a reasonable guy, maybe I could suggest it to him if I had something to sell him on it with. Personally, I'd like to know, myself. I'd not charge him any further, I'd just like to know.

Thanks Enzo!

[Chuck H]

Enzo, I'd love too, but I need to turn this one over. I've had it a while and need to get it back to the customer. The 4.7 uF at 500V is nearly impossible to find (in a quality brand), that's what precipitated this cap value change experiment. I really can't keep using this amp as a test bed, much as I'd like - I need to get it back to him.

What would I expect to discover, by doing this - what would the result be, sonically? He's a reasonable guy, maybe I could suggest it to him if I had something to sell him on it with. Personally, I'd like to know, myself. I'd not charge him any further, I'd just like to know.

Thanks Enzo!

Well if YOU heard a difference with the higher value cap and HE'S the amp owner I expect HE will hear a difference. So if there IS a difference it looks like you have the amp at least long enough to get the right value part in the right voltage rating. Regardless of how hard it might be.?. Unless you're saying you can't. Which you didn't. You just said it was hard. Anyway...

With this in mind you SHOULD have the amp open and on the bench with the OEM value and spec part in place. How much time would it take to tack a 20uf cap across it and listen???

[Fred G.]

No, wait - back up a minute - I can't find the 4.7 uF at 500V in a quality manufacturer. 20 uF was suggested as a viable alternative, "preferable" by that particular individual. I have a 20 uF in there now. I have a shiny new 10 uF 500V F&T e-cap I could replace it with. I was wondering how much of an effect this would have on the tone. There was a definite difference I perceived between the 20 uF vs the Original 4.7, the 4.7 was less "strident" when pumping in the cascaded preamp gain stages. The 20uF seems to give a lot more distortion right out of the gate, on the other hand, and I had to fiddle around all over again to find the "sweet spots". But I did find them.

And yes, the amp is open and still on my bench. I just completed about an hour of playing it with the reverb tank hooked up and giving it a final shake-down this afternoon. I just couldn't bring myself to button it up quite yet without investigating this further...

[Enzo]

But the original was also an old tired cap, not simply a different value, right? SO we were not comparing a fresh 4.7 with a fresh 20. 10uf, why not? I think since you did a rebuild on this amp, MANY things are different, so it will not sound the same, but hard to imagine it will all be down to the value of one filter cap. So unless you point out the whole discussion, He won't concern over THAT cap. SO if tomorrow you do find a 4.7uf cap and install it, the amp STILL won't sound as it used to. imagine you could hear a difference between the two values, but I also imagine the overall changes will overwhelm that small differnce.

[g1]

I was told by one amp company that a small screen node cap was deliberately chosen for tube reliability reasons. The screen voltage will not hold up under high drive conditions so it is somewhat similar to adding individual screen resistors when they are not present.
I'm more inclined toward the 'compression' argument, but thought it should be mentioned.

[Greg_L]

But the original was also an old tired cap, not simply a different value, right? SO we were not comparing a fresh 4.7 with a fresh 20. 10uf, why not? I think since you did a rebuild on this amp, MANY things are different, so it will not sound the same, but hard to imagine it will all be down to the value of one filter cap. So unless you point out the whole discussion, He won't concern over THAT cap. SO if tomorrow you do find a 4.7uf cap and install it, the amp STILL won't sound as it used to. imagine you could hear a difference between the two values, but I also imagine the overall changes will overwhelm that small differnce.

I think all this ^^^^ is a very good point.

The 10uf will probably be an imperceptible difference.
And like mentioned you're changing/fixing so much stuff it would be nearly impossible to pinpoint any sonic differences due to one minor cap value change.

[teemuk]

I was told by one amp company that a small screen node cap was deliberately chosen for tube reliability reasons. The screen voltage will not hold up under high drive conditions ...

Aside from Mesa's "Dynawatt" I'm not familiar with any manufacturer that would do this deliberately. Not saying it is not possible, but Mesa's design is certainly quite different, whereas screen circuits of generic amps are typically very well filtered concerning the overall low current draw and that one really has to push the power section to even get any significant current draw.

All I'm saying is that - although it would make a lot of sense - compression by modulating screen voltage isn't really that typical feature and never has been. And really, how many amps today we get to crank to volume levels where one can distinctly detect a difference of 4.7 uF vs. 10 uF screen supply filter?

[pdf64]

Mouser etc usually carry several lines of 4.7uF 500V film caps. Yes they’re more expensive than ecaps, but should a lot (eg last 10 times) longer.

[Enzo]

In fairness, modern caps last a good 20 years, so a film cap will last in that amp...200 years?

[pdf64]

I was thinking of the expected operating life figure in product data sheets.
It’s usually a worst case, at max rated voltage and temperature, typically 1k to 20k hours for ecaps. It’s not often noted for film caps, though I recall noticing one at about 50k hours.
Of course caps are rarely worked that hard in guitar amps.

[Helmholtz]

I am not surprised by the sound difference with higher capacitance.
It confirms my own experience with increasing screen filter values in vintage Fender and Marshall amps.
As the Prosonic doesn't employ global NFB, the effect will be more pronounced.

Screen compression seems to be the most plausible explanation.