Quick question about cap function

[Wes]

As an example: The Peavey 6505

What is the function of C66, C67, and C75? It seems that most amplifiers do not have this.

I have seen where caps are paralleled with the diodes of a bridge. Is it a similar function?
Is it a result of no CT for that winding?

Prowess Amplifiers - Misc - Schematics - Peavey 6505 Plus 112 Combo - Peavey 6505 Plus 112 Combo Schematics and BOM

[Jazz P Bass]

How about a hint?

Where on the schematic.

[Dave H]

Where on the schematic.

Page 1 bottom left on the inputs of the diode bridges.

[Enzo]

Hi Wes. Just a suggestion: mention the full model name in the title.


It is a similar function in that the diodes help prevent noise. Caps across diodes are there to reduce the noise the diode adds as its junction slams off and on.

The ones across the windings are there to prevent noise on the power lines from getting into the power supplies.

This has nothing to do with a center tap or lack of one.

[Wes]

Sorry, P Bass and Enzo about the ambiguity. I will be more specific next time!

Thanks, Enzo- that's what I figured it was for; a similar function to caps paralleled with the diodes.

Now- would caps paralleled with the diodes not help filter line noise as well as the diodes' switching noise?

[Enzo]

No, the noise can flow through them. When they are in shunt like they are, they essentially short the noise signal across. At 60Hz they have a much higher impedance and so do not conduct much at all there.

[jpfamps]

The main purpose of these diodes is to prevent HF switching noise from the rectifier diodes coupling back into the mains through the capacitance between the primary and secondary windings.

Modern EMC regs often require these diodes depending on where you are in the world. (You also see caps across the live neural mains input as well which does the same job).

A beneficial side effect of these snubbing caps is that less recifier switching noise is coupled back through the mains transformer and onto the filament supply, where it can further couple into the audio (in a valve amp).