Micro Bassman Build

[vigilante398]

12au7 in preamp and 12ax7/12ay7 in power.

Methinks you swapped those around. Per Robinette's design 12AY7 and half of a 12AX7 is preamp, 12AU7 is the power tube. 12AX7 doesn't make a great power tube.

12AU7 is rated per the datasheet for an absolute max 2.5W output, but I'm not sure the Hammond 125B is the best transformer to take full advantage of everything you can squeeze out of it. I mostly see amps like this labeled as everything from 1/4W to 1W, never higher.

 

[Nic]

Methinks you swapped those around. Per Robinette's design 12AY7 and half of a 12AX7 is preamp, 12AU7 is the power tube. 12AX7 doesn't make a great power tube.

12AU7 is rated per the datasheet for an absolute max 2.5W output, but I'm not sure the Hammond 125B is the best transformer to take full advantage of everything you can squeeze out of it. I mostly see amps like this labeled as everything from 1/4W to 1W, never higher.

Well, you are right about those tubes, my mistake, 12AU7 IS the power tube and the other two are preamp. I'll edit my post because they are in the right place, I've made sure about that.

Guess the OT would be what's keeping this at room volume then.

 

[Locrian99]

Well, you are right about those tubes, my mistake, 12AU7 IS the power tube and the other two are preamp. I'll edit my post because they are in the right place, I've made sure about that.

Guess the OT would be what's keeping this at room volume then.

@vigilante398 what would be an ot that could be used for a little higher wattage?

 

[Nic]

@vigilante398 what would be an ot that could be used for a little higher wattage?

I saw Rob says that the 125C would but that he chose the 125B because it did a better job for the original bassman sound.

 

[Harry Klippton]

Any chance of us getting a sound clip?

 

[Nic]

Any chance of us getting a sound clip?

Will need to get the wife on it, once I get a speaker

 

[Nic]

She plays a lot better than I do

 

[MBFX]

W O W !

That is a beautiful build. I love the color. I would absolutely buy one of these!

 

[Nic]

W O W !

That is a beautiful build. I love the color. I would absolutely buy one of these!

Thank you, really appreciated !

 

[cdwillis]

I missed this when you first posted it. This is awesome. Purple is my favorite color, coincidentally.

 

[Feral Feline]

Great job.

Makes me want to follow suit, and I have a transformer for it, IIRC. Gotta check if it's B or C, and check what it was originally earmarked for.

 

[Nic]

I missed this when you first posted it. This is awesome. Purple is my favorite color, coincidentally.

Thank you !

Great job.

Makes me want to follow suit, and I have a transformer for it, IIRC. Gotta check if it's B or C, and check what it was originally earmarked for.

I still have some plans for this. I modded my rumble to be able to use it as an external speaker. I'm planning on playing with the bright cap and putting an NFB switch. I also want to check the voltages to see if everything checks out.

 

[eurobailey]

Great work! I’m looking at the deluxe micro at the moment.
Can i ask how you drilled out the square hole required for the transformer ?

Was a step drill enough for the valve holes?

 

[jimilee]

I love this, I’ve been looking for this exact thing. A 4x10 or 2x12 is way too much audio and too many heavies.

 

[Nic]

Great work! I’m looking at the deluxe micro at the moment.
Can i ask how you drilled out the square hole required for the transformer ?

Was a step drill enough for the valve holes?

The transformer sits on top of the enclosure. There is only two holes with rubber grommets for the wires to go in the enclosure and four small holes to screw it on.

Yae, my step drill was enough fot the valves.

I think that enclosure is aluminum, so drilling was easy, for the plug i drilled a few holes and enlarged the opening using a file.

 

[Nic]

Bad news is something is wrong with this build. At first I said it was really not powerful for the supposed wattage compared to my wife ac4 playing at 1/4W... but now it's silent. I can get a signal if I use a preamp pedal in the fx return jack but thats about it. I need to take some voltage measurement and do some audio probing... when I feel brave/confident enough.

 

[eurobailey]

The transformer sits on top of the enclosure. There is only two holes with rubber grommets for the wires to go in the enclosure and four small holes to screw it on.

Yae, my step drill was enough fot the valves.

I think that enclosure is aluminum, so drilling was easy, for the plug i drilled a few holes and enlarged the opening using a file.

Ah interesting - most of the chassis I’ve seen has a hole in the top to fit “part of the transformer” and then bolted on. Interesting that you were able to just bolt on top .

 

[owlexifry]

Bad news is something is wrong with this build. At first I said it was really not powerful for the supposed wattage compared to my wife ac4 playing at 1/4W... but now it's silent. I can get a signal if I use a preamp pedal in the fx return jack but thats about it. I need to take some voltage measurement and do some audio probing... when I feel brave/confident enough.

do some probing and post a voltage chart.

 

[Nic]

do some probing and post a voltage chart.

That's the plan, just not a priority right now. I've slowed a lot. I have 4 boards on the bench with resistors waiting to be soldered. It's just been a busy summer!

I already have a mapping of what voltages to expect at every point, just need to take the time to do this.

I'll sure let you all know when I'll have some values.

 

[owlexifry]

That's the plan, just not a priority right now. I've slowed a lot. I have 4 boards on the bench with resistors waiting to be soldered. It's just been a busy summer!

I already have a mapping of what voltages to expect at every point, just need to take the time to do this.

I'll sure let you all know when I'll have some values.

understandable.

sounds like you know where to get started on the troubleshooting.
good luck