# Tweed Man Bass only - without the switch



## KarenColumbo (Oct 22, 2019)

Lords and Ladies, since I'm a Bass Player I'd like to omit the "Secret Gain" switch completely, since I don't need this gain for the perfect grumbling bass sound. Now there's 4 positions at which there's switching going on:
1. Local feedback at the first gain stage
2. Bypass cap at Q1
3. "Slope" resistor at tone stack and
4. Treble cap in the tone stack

I guess I need:
1. activated, 2. the higher value (since local feedback reduces gain), 3. the lower value (more treble shift) and 4. the lower value (higher treble frequency)

Is that correct?

Of course, I just bridge the solder pads @ the switch on the PCB, but I want to understand what happens when I switch those. Can someone tell me?


----------



## Chuck D. Bones (Oct 23, 2019)

Almost...
The switches in the schematic are all shown in the Bass position.
1. Local feedback at the first gain stage engaged
2. Bypass cap at Q1 set to 22uF (makes more gain at low freq where you need it for bass)
3. "Slope" resistor at tone stack set to 56K
4. Treble cap in the tone stack set to 220pF

Here's a Fender Tweed 5F6 schematic for reference.  
Catalinbread copied the tone stack verbatim with one obvious deviation: R14.  The output impedance of a cathode follower is on the order of 1K, so why the 33K?  I can tell you from playing around with Duncan's Tone Stack Calculator that R14 does make a difference.  But I digress...
Catalinbread reduced the source bypass cap from 250uF to a sane value.
The local feedback is a construct to bring the 1st stage gain in line with the gain of a single 12AX7 (or 12AY7).  

Personally, I'd set the treble cap to 470pF so the mid notch is lower in freq, but that's just me.


----------



## KarenColumbo (Oct 24, 2019)

Thx! That makes some things very clear. I guess that a genuine SRPP (like in this case, because of the 1k resistors in the middle of the respective jfet pairs) likes to drive a bit of impedance in the next stage, maybe the 33k is a remnant from experimentation with this?

Never noticed the uuuge source cap there - thx for pointing that whopper out. Curious. 

I set the treble cap to 330p - it was what I had left in the tiny-cap drawer. And you're right - sounds a bit throatier now. Thank you!


----------

