Amp Gurus - what will changing these values do?

[johnny q]

In a late 60's marshall plexi circuit:

V1B coupling cap changed
from .0022 to .022.

Adding a 100pf brite cap (previously, no brite cap installed)

Thanks all

JQ

[bob-i]

In a late 60's marshall plexi circuit:

V1B coupling cap changed
from .0022 to .022.

I thought it WAS a.022uF. The .0022uf will pass less bass.

Adding a 100pf brite cap (previously, no brite cap installed)

Thanks all

JQ

Um... add brightness :jo

Across the volume pot it will add brightness as the volume is turned down but make little or no change as it's cranked up.

[donnyjaguar]

Often the coupling cap values are chosen to keep low-frequencies out of the power amplifier because of the lack of inductance and magnetic permeability in the output transformer. You have to factor in the value of the pull-down (or bias) resistors with respect to the reactance to get the filter shape. But of course the reduced reactance of the coupling capacitor at low frequencies is largely off-set by the negative feedback anyway.

[rooster]

Correct me if I'm wrong, since I'm pulling this out of memory:

The V1B coupling cap is the coupling cap between the "bright" input and the volume control. By switching it to .022, you now have no really bright channel anymore. You kind of have one, since the cathode of V1B is set up with high-frequency emphasis, as opposed to V1A which is flat out full spectrum gain. Don't know why you'd want to do that, since a good chunk of the "magic" of a JTM amp is the ability to patch both inputs together and bring up both volumes to mix them, giving you the bassy smoothness of the "dark" channel with the chime of the "bright" channel.

rooster.