Journey of my Bassman Micro build

joulupukki

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All of my speakers are 8 ohm speakers. My two speaker cabs have two jacks on the back of them so when I plug them into each other (and one to the amp) they are a 4 ohm load since they’re wired in parallel.

It sounds like maybe to solve the proper voltage when I switch to 8 ohm the NFB wire needs to switch with it and maybe the NFB resistor as well?
 

andrewRneumann

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All of my speakers are 8 ohm speakers. My two speaker cabs have two jacks on the back of them so when I plug them into each other (and one to the amp) they are a 4 ohm load since they’re wired in parallel.

It sounds like maybe to solve the proper voltage when I switch to 8 ohm the NFB wire needs to switch with it and maybe the NFB resistor as well?

Ahhh… so you been using the 4-ohm position then. The NFB isn’t correct in the 4-ohm position then for the reasons I elaborated—the ground is the purple wire (not orange) and the NFB is still yellow. Have you tried it (NFB switch, presence) with a single cab on 8-ohm? My guess is you’ll get some awful squealing when you do, but who knows… if you do, you’ll have to swap the primary leads on the OT. And then the 4-ohm will be out of commission until we get the grounding sorted out.

I am assuming the design NFB values were based on an 8-ohm speaker. If that is wrong, and it was designed for the 4-ohm tap, we can move the NFB to the white wire.

NFB does not have to move when you change speaker impedance. So long as ground doesn’t move.
 

joulupukki

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I’ve been doing most of these tests in the 8 ohm position with one speaker. Here’s a shot of the 8 ohm position, right?

378A5051-C00D-4F21-A4BD-98BA614A9860.jpeg


I replaced the 6.5K resistor with the 2.6K resistor for the JTM45 setting on the NFB switch. In all positions I can hear some very subtle differences, including the presence knob. The presence knob barely rolls off some of the ice picky sounds. I can barely tell a difference between the new JTM45 position now and OFF, which, when off, it’s just barely louder. But this all depends on the volume levels. If I dime everything, no discerning anything, of course, other than some serious distortion. ;)

I haven’t noticed any squealing in any setting. Lemme grab the other speaker cab and plug it in and I’ll report back if the NFB switch and presence knob still seem like they’re doing anything on the 4 ohm setting.
 

andrewRneumann

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I’ve been doing most of these tests in the 8 ohm position with one speaker. Here’s a shot of the 8 ohm position, right?

View attachment 990176

I replaced the 6.5K resistor with the 2.6K resistor for the JTM45 setting on the NFB switch. In all positions I can hear some very subtle differences, including the presence knob. The presence knob barely rolls off some of the ice picky sounds. I can barely tell a difference between the new JTM45 position now and OFF, which, when off, it’s just barely louder. But this all depends on the volume levels. If I dime everything, no discerning anything, of course, other than some serious distortion. ;)

I haven’t noticed any squealing in any setting. Lemme grab the other speaker cab and plug it in and I’ll report back if the NFB switch and presence knob still seem like they’re doing anything on the 4 ohm setting.

Oh crap… do you have a post-PI master volume?
 

joulupukki

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Oh crap… do you have a post-PI master volume?
I don’t think so. Well, actually, maybe…let me look at the schematic again.

Here’s what I’ve found so far in my experimentation.

In 8 ohm mode with one speaker and the 2.6K resistor for the JTM45 setting…

Switching to the JTM45 setting (from OFF) makes the amp just barely quieter. The presence knob does work, but it is VERY subtle.

In 4 ohm mode with two speakers the JTM45 setting is easily discernibly louder and the presence knob definitely easy to discern though it works backwards from what it does in 8 ohm mode. I’m thinking maybe I’ll switch the purple and white wires around and see if that makes any diff.
 

joulupukki

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Ah ha! Well that’s definitely a ginormous squeal with white and purple swapped. Gonna put that back pronto!
 

joulupukki

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Ok well the whole NFB thing is defeated by a master volume that is turned down. Turn the MV to 100% and turn down the preamp volume to a reasonable level and do all your tests like that.
Ok.

Sadness. This entire build I had zero melted wires from the soldering iron and then this caught me off guard when I was putting it back. Ugh. :(

2B9EFBD0-8294-406C-995B-D9D75304971D.jpeg


No white shrink tube so this will have to do. Character.

D89967D6-412D-4CAA-A278-F60A8736D9C8.jpeg
 
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andrewRneumann

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In 4 ohm mode with two speakers the JTM45 setting is easily discernibly louder and the presence knob definitely easy to discern though it works backwards from what it does in 8 ohm mode.

The backwards operation of the presence knob is probably an indication that the phase of the NFB changes between 8-ohm and 4-ohm. I’m now surprised it wasn’t squealing in the 8-ohm setting… maybe it will once you turn up the MV.
 

joulupukki

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The backwards operation of the presence knob is probably an indication that the phase of the NFB changes between 8-ohm and 4-ohm. I’m now surprised it wasn’t squealing in the 8-ohm setting… maybe it will once you turn up the MV.
It definitely does not squeal in either 4 ohm or 8 ohm positions with how I have it wired now … even with everything dimed. With my 2.6K resistor on the JTM45 setting I can definitely tell that it makes a difference. The presence knob works as well. Presence knob is more pronounced in 4 ohm mode, but working backwards.

Not much difference between the OFF and 5F6A (15K NFB resistor) mode.
 

joulupukki

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Hmm. I know it would change the voltages, but what if I were to connect the NFB wire to the top middle terminal of the switch?
 

andrewRneumann

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Hmm. I know it would change the voltages, but what if I were to connect the NFB wire to the top middle terminal of the switch?

It would probably work the same in 8 ohm but be less effective in 4 ohm.

I feel like I’m missing something on your work. Your OT secondary is grounded by the output jacks right? That means that in 8-ohm the orange wire is a 0 volts (chassis). And in 4-ohm the purple wire is at 0 volts (chassis). Is that what you have?

To me this means that the phase of the NFB (taken from the yellow wire) changes when you switch impedance and you should get the squeal on one of the two settings. I actually can’t understand why swapping the white and purple led to a squeal because it didn’t change the phase of the NFB. Possibly it just changed the intensity of the positive feedback enough to oscillate. My guess would be you are still getting positive feedback on 4-ohm but not enough to get it to squeal.

Anyway, it’s getting late. I will think some more about this. Personally I think you need to go with a 3PDT mini toggle there and add a resistor so this thing works properly in 4-ohm and 8-ohm. I can draw something out and share it with you tomorrow.
 

joulupukki

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It would probably work the same in 8 ohm but be less effective in 4 ohm.

I feel like I’m missing something on your work. Your OT secondary is grounded by the output jacks right? That means that in 8-ohm the orange wire is a 0 volts (chassis). And in 4-ohm the purple wire is at 0 volts (chassis). Is that what you have?

To me this means that the phase of the NFB (taken from the yellow wire) changes when you switch impedance and you should get the squeal on one of the two settings. I actually can’t understand why swapping the white and purple led to a squeal because it didn’t change the phase of the NFB. Possibly it just changed the intensity of the positive feedback enough to oscillate. My guess would be you are still getting positive feedback on 4-ohm but not enough to get it to squeal.

Anyway, it’s getting late. I will think some more about this. Personally I think you need to go with a 3PDT mini toggle there and add a resistor so this thing works properly in 4-ohm and 8-ohm. I can draw something out and share it with you tomorrow.
Ok, thank you for humoring me with this. I switched the NFB wire to the middle terminal and it behaves a bit more like I’d expect it to. But everything is pretty subtle. If I play quiet chords/notes (on the looper so I can fiddle with the presence knob) I can hear the intricacies of the strums fade out as I turn the presence knob down (in either 4 or 8 ohm). I’m gonna button this up for the evening as well.

My 18w TMB kit arrived today so I’ll probably be distracted with that for the next few days, although no PT in it (Mojotone/Heyboer has a backorder on them with no ETA of arrival).
 

joulupukki

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Alright. This is how I’ve got it wired now. I can at least tell that it’s subtly working and definitely moving to that 2.6K resistor made it so the change is more discernible between the OFF and JTM45 position.

8C3956C3-4CA5-4E1B-A3DA-A06BA72F185F.jpeg
 

joulupukki

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I should have searched this first before going down this route. I can see by using two separate output jacks there’d be less confusion … and maybe that’s what I ought to be doing. I could abandon the headphone jack and the switch and maybe put a 1/8” headphone jack where the switch is. …although then my back panel isn’t gonna be accurate.

Or, as long as my current hook-up isn’t gonna destroy anything prematurely, maybe I just leave it and call it good. There are plenty of other knobs on this amp to dial in some great tone without worrying about the NFB circuit at all.
 

robrob

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Multi-ohm tapping these output transformers can be tricky. What wire numbers are you using for your output jacks?

1654257807864.png
 

joulupukki

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Multi-ohm tapping these output transformers can be tricky. What wire numbers are you using for your output jacks?

View attachment 990238
@robrob Here’s how I currently have it hooked up:

For 4 ohm:
Ground: 5 (Violet)
Tip: 6 (White)
Impedance: 22.5K

For 8 ohm:
Ground: 2 (Orange)
Tip: 4 (Yellow)
Impedance: 22K

——

One thing I could try instead in order to have a common wire between the two positions would be to use wire #4 (Yellow) as the ground. For 4 ohm, the primary impedance would be 27K. In that case I could then connect NFB to #2 (Orange) and that would stay as the 8 ohm NFB tap. Would that make the NFB circuit perform better?
 

andrewRneumann

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Or, as long as my current hook-up isn’t gonna destroy anything prematurely, maybe I just leave it and call it good. There are plenty of other knobs on this amp to dial in some great tone without worrying about the NFB circuit at all.

That isn’t going to destroy anything. There’s just going to be less feedback in the 4-ohm setting, but it should work as advertised in the 8-ohm. It’s better than your first layout because it doesn’t change the phase of the NFB when you flip the switch. And remember the MV plays a big role. If you are playing around with the MV at 50% you aren’t going to get much NFB at all. If you are happy with those limitations, just leave it at that!
 

joulupukki

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That isn’t going to destroy anything. There’s just going to be less feedback in the 4-ohm setting, but it should work as advertised in the 8-ohm. It’s better than your first layout because it doesn’t change the phase of the NFB when you flip the switch. And remember the MV plays a big role. If you are playing around with the MV at 50% you aren’t going to get much NFB at all. If you are happy with those limitations, just leave it at that!
Roger that. Strangely, when I had it only connected to #4 (Yellow) and went to 4 ohm mode (and the presence knob was working backwards) the increase in volume and effectiveness of the presence pot was unmistakably functional – not subtle at all. With the current wiring, that’s not the case.

If 27K primary impedance would be “ok” then I’m definitely curious to see how using #4 as the common wiring (ground for both 4 & 8 ohm) would affect the behavior.
 




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