Preamp volume pot placement?

ledvedder

ledvedder

Well-known member
I'm seeing some schematics (Friedman designs I think?) where the preamp gain pot is placed between v1b and v2a, instead of the usual 800 placement between v1a and v1b. In those circuits. I see a 68k/68k divider between v1a and v1b and 33k into each v1 grid. Then the standard 470k/470k divider after the pot into v2a grid. Also, looks to be 220k and 100k/500pF on v1a plate. What differences does this make to the tone?
 
That’s something you just have to try out. In my experience it’s just a way to affect the feel and overall gain character of a given circuit. Just like moving the master or tone stack, ect. I like one pot in the standard 800 location and one feeding v2. Gives a nice usable guitar volume taper with tons of range.

The 220k with a snubber is very annoying sound, but worth a try. Sounds like fuzz to me.
 
Ones a gain stage in front of a plexi, I.e. the Jose mod, the other is the 2203. You have to try it.
 
If you replaced that 68k/68k divider with a 470k/470k divider (which would be similar to a 1M pot set mid-resistance) then you would likely get a much more farty distortion. Similar to needing a resistor to ground when increasing gain in the JEL mod. It changes the loading on the gain stages (both the one preceding and the one following in this case).

@burger and/or @RedPlated could tell you more about a 220k+100k//470pF vs. a 330k//???pF values for plate resistors and snubbers. Friedman does/did that a lot. Maybe bypassing the full 330k is too intrusive to the top end or something.
 
I've done the plate snubber both ways, with a cap over one resistor with a second in series, and then with a cap over a single plate resistor. For me, I prefer the latter method, just one plate R with a bypass cap. Cameron is another builder I can think of that commonly used the one plate, one cap method.

I'm hypothesizing that Friedman only bypasses the 100k plate with a bare 220k plate in series to reduce the treble cutting effect for one or more reasons. Firstly, he uses a 500pf cap, which is fairly large (1000pf have been used by others). Secondly, he uses multiple snubbers in his amps, such as the V2a plate resistor, and v2b 100k cathode resistor, both also 500pf. So I'd assume he's limiting the effect of the 1st stage plate snubber due to the others also in circuit.

I use a snubber on the 1st stage plate in most of my amps. Smaller than 500pf, though. I've experimented at length in this position and it almost always sounds better with the snubber. It 'cleans up" the scattered top end and makes the amp sound more put together. I don't realize any significant degradation of the top end with the cap over a single plate resistor. It's one of the best places to put a snubber, from my experience.

There's lots of room to tweak. You can go to a large value cap, but only snub one resistor out of two in series, or you can go the other way, using a smaller pf cap with just one resistor. So your effecting frequency and the amount of effect of that frequency by adjusting these values and layout.
 
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