cascading gain stages

[jack2911guitarblues]

Hi there,

Wondering if anyone (or more!) can give me some information on theory of tube amps, and in particular cascade gain stages.

I have built a 1987x clone, and also just installed the "hotswitch" in it,  whereby the 2 inputs are able to be run in series in the the first tube  (V1  a & b).
I am slowly understanding how all this works and find it all immensely satisfying.   But my question is, when the pre-amp is cascaded, the "hotswitch" schematic shows the signal coming from V1 A Plate, going to one side of  the Vol. control,  and then the signal goes from the wiper straight straight to pin 7 of the 2nd stage of V1, which I understand, no problems.

Why is that another type of cascade circuit I have seen, shows the signal FIRST going through a 470k resistor and paralleld with a 250pf cap,  and THEN going  on to pin 7 of the 2nd stage of V1?
Is the cap and resistor necessary when cascading the signal from the plate on V1a, to the input of V1b??

What does this Cap/resistor combination actually do as far as gain/tone do?  and why is it used anyway??

I would appreciate some theory on this, whenever some one has the time or chance to answer my question!  Hope my question is not too confusing!!

Jack

[Willabe]

This might help you.

http://www.valvewizard.co.uk/gainstage.html


                  Brad   

[AZJimC]

What does this Cap/resistor combination actually do as far as gain/tone do?  and why is it used anyway??

There are those on this board that can give all the math involved, and I am not that guy. One way to look at it is this. The resistor forms an attenuator that passes all frequencies equally, but at a slight reduction. The picofarad cap which bypasses it would only pass higher frequencies but with less attenuation than the resistor. This would be called a high pass filter.

One reason this might be used is that LOUD bass tones can rattle a speaker, cabinet, and circuiit, particularly a low cost speaker, etc. At higher gains it is common to try and bleed off some of the bass to help shape the final output.

[kagliostro]

Here you can have a very interesting read on the matter, I think you'll find it very useful


http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used


K