Ampeg B15NC

[Walrus]

The schematic inside the lid of our B15 NC doesn't match the actual amp circuit in that the amp doesn't have a screen grid decoupling capacitor as shown on the diagram. But a quick search shows that their are schematics out there that don't have the capacitor.
What would be the benefit or not of adding one?

 

[Walrus]

Other schemo....

 

[panman]

It is just an additional filter-cap. It does not seem to me of any real significance, because the screen-voltage is already sufficiently smoothed out. It may add to stability though or another slight effect, as we know the screen-voltages do affect the function of the tubes, but I do not have enough knowledge of that.

 

[abbey road d enfer]

The schematic inside the lid of our B15 NC doesn't match the actual amp circuit in that the amp doesn't have a screen grid decoupling capacitor as shown on the diagram. But a quick search shows that their are schematics out there that don't have the capacitor.
What would be the benefit or not of adding one?

Not bypassing the screens makes the tube work a liitle "triodishly". It means the internal resistance will be a little smaller and there will be a little more Miller effect. But its actually very minute. It is certainly measurable but not audible.

 

[CJ]

from a DC point of view it makes no difference.

when you crank the amp, there will be AC on the screen side of the resistors,

since this AC voltage from the push-pull tubes will be 180 degrees apart, they should cancel if the tubes are somewhat matched.

looks like they were getting cheap by using 1 screen resistor and getting cheaper by ditching the 20 uf cap.

now if you use two 1 K resistors for each screen and no cap, there will be a negative feedback effect, i think.  ???