Band-pass in a Silvertone 1472

[SDS]

Okay folks,

I know this sounds like a bad idea but I want to add a band-pass filter on the mic channel prior to the first 12AX7 of a Silvertone 1472. Can anyone help with the cap and resistor values?

I would like it to cut below ~220hz and above ~850hz.

Here is the schematic:
http://www.prowessamplifiers.com/schema ... ne1472.pdf

Thanks!

[SDS]

Hmmm.... no takers, huh?

Okay, let me re-phrase the question.

First, my understanding of R1 and R2 are that they are grid resistors that limit the top end of the frequency range prior to the first 12AX7. 2 questions stem from this:

1. I've heard that grid resistors prevent oscillation in the amp, but the mic channel doesn't have a grid resistor... why doesn't it oscillate?

2. If decreasing the value of R1 and R2 opens up the top end, then increasing it should filter out more top end. Is there a maximum value before the filtering is traded off for an overall loss of volume? Basically, If I want to experiment with different values, what's the highest I should bother to try?

Next, I would like to add a high-pass filter after the grid resistor, prior to the first 12AX7. Having never built a high-pass filter, here's my third question:

3. Can someone suggest some basic values to start with. Even a quick schematic would be great.

Again, I'm looking for a very 'mid-range only' tone which sounds idiotic but it will be combined with another tone later. If any of this is such a bad idea that I shouldn't even try, feel free to tell me that too.

-SDS

[Leo_Gnardo]

I would like it to cut below ~220hz and above ~850hz.

Break out your cow cow cowculator and apply normal "corner" frequency calc with the resistors in place.

boiled down for easy cookin'

F = 160,000 / RC with R in ohms and C in microfarads

I got that pesky 2pi and million factor (F to uF) outa the "basement" for simpler calculations. This has worked for me many a year. Now for you.

Quick n dirty, parallel cathode & plate R's with C's.

RC pair in the cathode will determine your lo cut corner.

RC pair in the plate, the hi cut corner.

Actually you'll need to solve for C, so you know what value to parallel with your R's. so...

C = 160,000/ RF where F is your corner freq in Hz.

Punch the calc, solder up & smile. Nearest reasonable value C to your calculation should do OK. Let us know how ya make out.

Going for "telephone tone" are we?

[SDS]

Sweet...

Thanks Leo!!!

It might take a little while before I dive in and get this done but I'll let you know when I do.