Lil' Bumble-B - 'micro' Bassman with OD channel

[Littlewyan]

Bypassing the cathode on V2A would make it easier to overdrive.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

Maybe use this as an example, I've never played one in person but in videos the clean channel sounds pretty awesome.

[Lynxtrap]

Bypassing the cathode on V2A would make it easier to overdrive.

Thanks, yes I've been thinking about that and will absolutely try to put a cap back on the cathode.
I was thinking less gain into V2B when I removed it, but I guess the NFB from the unbypassed cathode raises the headroom on V2A a fair amount.

[Littlewyan]

I used to think like that, but found it does change the tone of the stage removing the bypass cap and it's harder to overdrive as you say. You'd be better off adjusting your voltage divider after V2A if you want less drive into V2B.

Edit: I also think you're cutting a lot of treble for the level of gain you've got and letting a lot of bass through. If you're concerned about gain levels I'd move the 220K grid stopper to before the volume pot to act as a small voltage divider and grid stopper and lower the first coupling cap to .0022uF. Remove the .001uF cap between the last two stages, you've already got an unbypassed 470K there acting as a grid stopper. If it's still a bit fuzzy move the .0047uF to be inbetween the last two stages (assuming you're going to fully bypass V2A).

[Littlewyan]

I have just looked at your tonestack and your 100K lead volume pot will be loading it down a lot and really affects the response. Seems to drop the bass a bit and raise the treble, could be why it sounds so trebly. Could you swap that 100K pot with a 1Meg? Or maybe even stick a resistor in series with it to act as a voltage divider to make the volume easier to control.

And as you have a 100K level control going into those other two stages, I doubt you'll be overdriving them, so why not have the OD go straight into the PI? Have a switch at the input to flick between the clean and OD channels.

[Lynxtrap]

I have just looked at your tonestack and your 100K lead volume pot will be loading it down a lot and really affects the response. Seems to drop the bass a bit and raise the treble, could be why it sounds so trebly. Could you swap that 100K pot with a 1Meg? Or maybe even stick a resistor in series with it to act as a voltage divider to make the volume easier to control.

Thanks for all of your suggestions!
The Lead Level pot affecting the response of V2B is something I had not thought of! The Gain and Lead Level pots were both 1M when I started out. I did not get much usable sweep out of them and also started looking at the pots and trimpots in the ODS cicruits (mainly #102) which are all 100K. But they don't have a tonestack in the OD section of course. I'll look into it!

And as you have a 100K level control going into those other two stages, I doubt you'll be overdriving them, so why not have the OD go straight into the PI? Have a switch at the input to flick between the clean and OD channels.

Certainly a possibility, but inserting the signal before V3 was deliberate for a few reasons. One was the option to overdrive the following stages (I'll have to investigate if that actually happens, haven't really focused on that lately). Another was to have the "main" tonestack affecting the OD sound as well, just as an additional tone shaping option.

I already have a switch, wiring it to the PI would be easy enough. But I'm not really sure I see the benefit of it...?

[Lynxtrap]

I made some changes along with some of the suggestions. Added a 2.2uF bypass cap to V2A, went down to .0047 for the coupling cap from that tube, and changed the Lead Level pot to 1M with a 330k resistor from the tonestack.

It improved things. But I've still got that "ripping cardboard" fuzzy sound on lower notes. What is that, still too much lows into V2B?

I made a quick recording, and I swear it sounds much better in the room
If anything the sound I'm talking about got more exaggerated on the recording. I started on the clean channel for reference, and I think you'll hear what I mean as soon as I flip over to the lead channel.

https://soundcloud.com/tom-forsman/bmbl ... al_sharing

[Littlewyan]

I still think you're sending way too much bass into your second stage and cutting too much treble. Start by lowering the .0047uF off of the first stage to .001uF to bring the bass down. For the treble start by clipping another 220K in parallel with the one going into your second stage. If you like this then swap in a 100K. Personally I'd also move that grid stopper to be before the volume pot, so it drops your gain a bit and still cuts the treble slightly, as I remember you were concerned about the level of gain.

[Lynxtrap]

Well, this might be it

I pretty much did what Littlewyan suggested: less bass and more treble.
I also changed the top treble cap in the James to a whopping 750pF for a mid hump around 300Hz.
I did that because it dawned on me that the nasal and grating kind of "cheap Marshall pedal" sound was partly because of a mid focus around 1-2kHz.

After experimenting with the voltage divider between V2A and V2B using a temporary pot, I ended up with 330K/100K. Not a huge change from 470/100, but that just seemed to be the sweet spot, and it got rid of some of the crackling/fuzzy artifacts on low notes.
The voltage divider ratio actually seemed to make a bigger difference in that regard than changing cathode resistors, kind of a revelation.

At that point, when I finally liked what I heard, it also (finally) dawned on me that it doesn't sound at all like the ODS type of sound I had in mind, and that I probably wouldn't be able to coax that sound out of this circuit no matter what I try.

Instead the OD channel at mid gain settings sounds more like a turbo charged version of the clean/Bassman channel, and that kind of makes more sense than trying to make a whole different amp out of it.

Turning up the "OD Level" pot to overdrive the following stages makes the overdrive smoother, and when also utilizing the main tonestack, a very wide range of sounds can be obtained. Even some "Dumble-esque" ones

Lil Bumble-B_Rev 13.png

[Littlewyan]

Glad you're pleased with it. And unfortunately you're right, you will really struggle to get an ODS sound out of this given it's a completely different gain structure. But a turbo charged bassman is not a bad sound at all!

[Lynxtrap]

I've still been thinking about how to sqeeze some more low mids out of the OD channel, and I ended up adding another pot. Had to put it on the back of the chassis as there is no room on the front.

I recently used the same topology in a pedal steel preamp: Gainstage -> mid control -> gainstage -> James.
In that case it was a Framus for scooping the mids. Here I tried a Big Muff style "tilt control" that I copped from Blencowe's preamp book ("Focus" on my schematic). It's essentially the same tone control I had at the end of the OD channel to begin with.

The values are such that there's a hump in the low mids with the knob at noon. Turning it down, it brings up the treble and cuts the bass some, and turning it up does the opposite. The curves I'm getting with TSC do not correspond to Merlin's for the bass frequencies for some reason.

I've only had time to test it for a short while, and I might still do some more tweaks, but it seems to work great!
I can now finally dial out that grating high-mid Marshall grind for a fatter and warmer sound, while still being able to bring in treble as needed.

EDIT: I changed the subject of this thread to better reflect what it's about, I hope that's not considered bad manners...

Lil Bumble-B_Rev 14.png

Focus sweep.jpg