Journey of my Bassman Micro build

[Lowerleftcoast]

So why doesn't the 5F6A schematic have the same thing?

Quick and easy... cheap way. Leo's way.

The popular mod is to change to what you are playing with.

 

[Len058]

Hehe. Got it. So why doesn't the 5F6A schematic have the same thing?

As I understood, the Marshall JTM45had scratchy presence pots because this design.

 

[joulupukki]

Ok, makes sense. Thanks for helping me understand this stuff more. I guess at this point I'll be on a quest to determine if there is a way to make the presence knob and NFB switch actually do something audible. I don't necessarily care to stay true to the original Bassman sound/design, but it would be nice if these components weren't just eye candy. Haha.

 

[Lowerleftcoast]

Look at it this way... there are two voltages to be thinking about in this part of the circuit.

There is VDC *power* for the PI tube. (It has to make it from B+ through the tube (load) and back to B- (let's call it ground)).

The NFB is VAC. This is *signal* voltage from the secondary of the OT. It is introduced to the signal voltage in the PI to cancel some signal. The NFB resistor (15k) and the *tail resistance*(4.65k in parallel with the 25k pot) form a voltage divider. The 15k is the top resistor and the [4.65k || 25k pot] is the bottom resistor. The junction of those two resistances determine the NFB voltage introduced to the PI.

There may not be enough difference (15k vs 6.5k) *top resistor* to make a noticeable audible difference. I assume Rob has heard a difference with the 15k/4.7k voltage divider on the Micro... but what do I know.

 

[Len058]

I just tried. Mine does add just a little chime. It get's a tad brighter. I did the stock value, Rob recommended. I don't know if it did something before I changed the OT, though.

 

[Len058]

Oh yeah, I use a Celestion greenback. It can be very bright. I noticed your speaker is all midrange if I'm not mistaken.

 

[joulupukki]

Oh interesting, yeah. That could be part of the issue. I'll try it later plugged-in to the Jensen in my PR and see if that makes it more apparent. Or, I guess I could plug it in to a Torpedo Captor X and hook it up to a speaker impulse response and measure it with an analyzer just to put my mind at ease. Good thinking on the speaker. I'll try it and let y'all know.

 

[Len058]

Sorry, remembered another thing. Mine is on the 16ohm tap so it would be JTM spec if Rob's work is correct.

 

[andrewRneumann]

Another thought—the effect of the NFB will be maximized when the presence is all the way down… IE maximum resistance through the .1u cap. If you turn it all the way down, does the NFB switch make a more apparent change?

 

[joulupukki]

Sorry, remembered another thing. Mine is on the 16ohm tap so it would be JTM spec if Rob's work is correct.

With the Hammond 125B the 8ohm and 16ohm tap are the same, aren't they? 8ohm uses pins 2 and 4 (4 is the yellow wire). The 16ohm combination would be 1 and 4 (still yellow). That's where I've got it connected. To be honest, this OT is a little confusing because it doesn't have a common wire with dedicated taps.

 

[joulupukki]

Another thought—the effect of the NFB will be maximized when the presence is all the way down… IE maximum resistance through the .1u cap. If you turn it all the way down, does the NFB switch make a more apparent change?

I have it hooked up right now with a 50K mini pot between the OT yellow wire and the presence knob using alligator clips and a breadboard for testing (the mini pot drops right into the test board). If I set the pot to 1K and then, with my looper running, connect the wire to the presence knob (with the presence knob turned all the way down), I can tell a very small difference. It's a teeny bit louder and maybe a little more raw with it not connected vs. when I have it connected. The difference is so subtle. I do hear barely a difference with the presence knob as well. It just barely rolls off some of the highest end treble. Doing this test with it plugged into the PR's 10" speaker.

So, I think I'll take out the 6.5K resistor I have for the JTM45 setting and just change it to a 1K resistor and then call it good for now. I have to solder the shielded wire back on to the OT anyway.

 

[Len058]

With the Hammond 125B the 8ohm and 16ohm tap are the same, aren't they? 8ohm uses pins 2 and 4 (4 is the yellow wire). The 16ohm combination would be 1 and 4 (still yellow). That's where I've got it connected. To be honest, this OT is a little confusing because it doesn't have a common wire with dedicated taps.

I treated the yellow as the common wire so the nfb is connected to 1. I found a topic where Rob explained how to do the 16 ohm. It's described nowhere else but I stuck with it.

But now it's a regular OT so a true 16 ohm tap

 

[andrewRneumann]

With the Hammond 125B the 8ohm and 16ohm tap are the same, aren't they? 8ohm uses pins 2 and 4 (4 is the yellow wire). The 16ohm combination would be 1 and 4 (still yellow). That's where I've got it connected. To be honest, this OT is a little confusing because it doesn't have a common wire with dedicated taps.

Which OT wire connects to ground/chassis and which wire to NFB?

 

[joulupukki]

Which OT wire connects to ground/chassis and which wire to NFB?

On mine, I used the first number (wire) from the chart to connect to ground and the second to the speaker output tip. In this case, yellow (#4) is connected to the tip (NFB) and orange to ground. Is that right?

 

[joulupukki]

I treated the yellow as the common wire so the nfb is connected to 1. I found a topic where Rob explained how to do the 16 ohm. It's described nowhere else but I stuck with it.

But now it's a regular OT so a true 16 ohm tap

Nice. I know between 8 ohm and 16 ohm yellow can be used for both so yeah, that kinda makes sense, but between 4 and 8 it’s not shared. I dunno.

 

[andrewRneumann]

On mine, I used the first number (wire) from the chart to connect to ground and the second to the speaker output tip. In this case, yellow (#4) is connected to the tip (NFB) and orange to ground. Is that right?

It’s not wrong! There are different ways you could go about hooking up NFB and ground. Just wanted to make sure you picked a valid setup. You are taking NFB from the 8-ohm tap, which is fine.

 

[andrewRneumann]

Can you post a clear photo of your output impedance switch and speaker jacks from the inside? You have a selector (DPDT switch) for 4-ohm and 8-ohm, right?

 

[andrewRneumann]

You have been testing this using an 8-ohm speaker with the switch on 8? Are you sure you have the switch wired properly—when 8-ohms is selected the yellow and orange wires make contact with the red and black wires? Sometimes people get confused by which way the throw goes on these switches.

 

[andrewRneumann]

Sorry for the post storm. I'm digging through this extremely long thread trying to pull out information and I'm only making it worse by posting 4 times instead of once.

If I am understanding your wiring correctly, you have a DPDT that switches the speaker output from OT wires 2-4 (8-ohm speaker) to 5-6 (4-ohm speaker). First, as I mentioned above, check you have the throw wired properly on this switch. Verify that the 4-ohm setting is actually taking the signal from wires 5-6 and not 2-4, and vice versa. Second, the problem with this is that the ground connection (which is made by the jack sleeves/chassis) changes when you throw the switch. This is no big deal as far as the OT and speaker go, but it changes the voltage (and phase) in the NFB circuit when you throw the switch. So that's got to change if you want NFB to be the same with 4-ohm speakers as with 8-ohm speakers.

We get so many questions about this OT that I've really learned to dislike it. It is not optimal for switching impedances when you have NFB.

 

[joulupukki]

It’s definitely not my favorite either. Hehe. So in order for it to be an ”active” output I’m assuming it must be that one side is connected to chassis/ground and the other side to the tip of the speaker out?

In any case, I believe my wiring on the switch is correct. With the switch thrown to the right, the connections run from the middle terminals to the terminals on the left … and vice versa, right?

Here’s a couple of close-up shots…