What's This Cap For?

[FourT6and2]

This schematic shows the Phase Inverter (V3) and power section of a single-channel, high-gain, Marshall-ish amp. One thing I'm wondering about is C40. What is going on there? I haven't seen a cap used in this position before.

Schematic: http://img06.deviantart.net/d3db/i/2...-dbbrwsd.jpg?1

[SoulFetish]

This schematic shows the Phase Inverter (V3) and power section of a single-channel, high-gain, Marshall-ish amp. One thing I'm wondering about is C40. What is going on there? I haven't seen a cap used in this position before.

Schematic: http://img06.deviantart.net/d3db/i/2...-dbbrwsd.jpg?1

That cap couples both grid signals together at frequencies above 72kHz. Because each grid signal is out of phase with each other, those signals cancel each other out, creating a low pass filter. The caps location on the schematic, is right near the grid leak resistors, that may be significant or not but , it's there to suppress high frequencies from causing instability.

[FourT6and2]

That cap couples both grid signals together at frequencies above 72kHz. Because each grid signal is out of phase with each other, those signals cancel each other out, creating a low pass filter. The caps location on the schematic, is right near the grid leak resistors, that may be significant or not but , it's there to suppress high frequencies from causing instability.

So it helps reduce high-freq. oscillation in the power amp? At 72KHz, that's well beyond the range of human hearing. But what would the end result be if there was some sort of oscillation at that range, without that cap there? I mean, you wouldn't hear it. But maybe the tubes would red plate?

[rjb]

At 72KHz, that's well beyond the range of human hearing. But what would the end result be if there was some sort of oscillation at that range, without that cap there?

Maybe a plane could crash?
https://en.wikipedia.org/wiki/Decca_Navigator_System

-rb

[Jazz P Bass]

So it helps reduce high-freq. oscillation in the power amp? At 72KHz, that's well beyond the range of human hearing. But what would the end result be if there was some sort of oscillation at that range, without that cap there? I mean, you wouldn't hear it. But maybe the tubes would red plate?

What the oscillation 'does' is robs available power.

[pdf64]

What the oscillation 'does' is robs available power.

And will likely cause the amp to sound bad.
Free running oscillation at max power is the worst case scenario but easiest to identify.
The basic issue would be a low to non-existent margin of stability; that may also manifest in parasitic oscillations, weird freq response, perhaps only with new tubes of a certain manufacture or when plugged into particular cabs/cables.

[rjb]

What the oscillation 'does' is robs available power.

And will likely cause the amp to sound bad.

:silly:
Good, sound priorities.
Who cares if you crash a few planes?
:/silly:

-rb

[Old Tele man]

Those "in-the-know" know that uncontrolled ultra-sonic oscillations can result in severe "red-plating" for seemingly no reason at all...and...ultimately meltdown of grids and plate.

[FourT6and2]

Those "in-the-know" know that uncontrolled ultra-sonic oscillations can result in severe "red-plating" for seemingly no reason at all...and...ultimately meltdown of grids and plate.

This was my first thought. This schematic is for a "hot-rodded" Marshall. I'm thinking the builder encountered some sort of issue like red-plating power tubes and this cap was the fix. And it made its way into the schematic someone made.

I'm building a similar amp and I was wondering if it was part of the inherent design. But probably not.

[SoulFetish]

This was my first thought. This schematic is for a "hot-rodded" Marshall. I'm thinking the builder encountered some sort of issue like red-plating power tubes and this cap was the fix. And it made its way into the schematic someone made.

I'm building a similar amp and I was wondering if it was part of the inherent design. But probably not.

You using global negative feedback?

[FourT6and2]

You using global negative feedback?

Yup. 39K on the 8ohm tap.