Journey of my Bassman Micro build

[joulupukki]

Maybe, I am just trying to make life easy on you. If you are considering angle brackets or shims or whatever, it might not be necessary if the hum is so small that it can’t be heard. Ask @Tom Kamphuys he has first hand experience.

Yeah, I connected to headphones and could barely hear anything unless I deliberately put the OT right above the PT.

 

[joulupukki]

So far you have done great "publishing looking work" here which future builders might follow and I hope you could make them good all the way because it is very easy! Just extend the Common "ground bus" full board length and detach the power tube cathode resistor/capacitor from the chassis. So simple change make all the operative current return straightest path to power supply!

If you use layout you have post the pre amp current return thru the chassis and current "leak" to chassis at amp input which is worst possible place because symmetrical instrument cable is connected there and amplifier will amplify all of the signal there is in cable hot. Instrument cable is long so some noise will transfer to hot pair. There is a concept "signal/noise ratio" if you want to recearch it more...

The use of Chassis as a part of operative circuits, which use Mains for power, has been forbidden in commercial manufacturing at least 50 years. I had thought this is in all advanced industrial countries? If its not forbidden in USA perhaps it is because your Mains voltage is half what the Mains is here in Europe?

@2L man Do you mean like this?

Instead of this:

 

[joulupukki]

@2L man also, if I'm understanding correctly, I believe the input jack should still be grounded to the chassis, right? I'm using a Switchcraft Mono Shorting Jack (12A).

 

[joulupukki]

Whew! I just had a scary moment. I thought I had built the back panel 180° backwards. Turns out, it's really good I didn't start drilling the top! The template I've been making needs to go *inside* the chassis. I'm gonna print a backwards one so I can tape to the top and drill the top holes.

 

[mistermikev]

I've started a Bassman Micro build. This will be my 2nd build (did a Mojotone Princeton Reverb earlier this year). This time, though, I wanted to also build the amp and speaker cabs. I've built two 1x12 speaker cabs and they're all done (and a British 18w TMB amp head cab just waiting for the electronics).

All the components for the Bassman Micro are ordered and on the way here. I initially thought I'd use up some extra 1/2" Baltic Birch I had left over from the other cabs, but for whatever reason, the router I'm using (or maybe the wood) had some massive tear out this evening. I thought of glueing it up anyway and just patching it with wood putty but that'd be a LOT of wood putty. So, instead, I'll re-cut new boards and do it out of 3/4" pine instead.

Surprisingly, I was able to order all the parts from Mouser, TubeDepot, and AmplifiedParts. But, it was a lot more than I thought it'd be. It should be a great amp when it's done though.

These are the mods I'm planning to include:

1. Three-way NFB: https://robrobinette.com/5F6A_Modifications.htm#3-Way_Negative_Feedback_Switch
1. JTM45 (???? I don't yet know what value I should use here)
2. None
3. Bassman Micro (15K)
2. Tube/SS Rectifier Switch

3. Tone Stack On | Off
4. 4 | 8 Ohm Speaker Output
5. Headphone Jack

I'll include pictures along the way. Thank you all who have already answered questions!

Here’s a pic of one of the 1x12 speaker cabs I built. I’ve got a Mojotone British BV-30H 30W 12" 8 Ohm in one of them and a 12” Mojotone Greyhound 70W 8 Ohm in the other.

View attachment 983163

tell me more of this.... 'bassman micro'! that sounds really cool. I've build some pretty complex pedals... did my own layouts for ce-2, and other fairly complex circuits... and have often thought about doing a simple tube kit build... but I just chicken out for the idea of messing with high voltage. is this a mojotone kit build?

 

[Paul-T]

tell me more of this.... 'bassman micro'! that sounds really cool. I've build some pretty complex pedals... did my own layouts for ce-2, and other fairly complex circuits... and have often thought about doing a simple tube kit build... but I just chicken out for the idea of messing with high voltage. is this a mojotone kit build?

The whole story is here:

Bassman Micro

Bassman Micro Amplifier

robrobinette.com

 

[joulupukki]

I printed a template backwards to mount on the top so it's ready for top drilling. I also ... DIY'd the turrets into place. Probably not the cleanest job ever, but they will work. My cheap Weller soldering iron (no temp control) must not be good enough because wow, it takes so long to heat those turrets up to get wires to solder to them. All the connection wires that go on the back are on. It's ready for components!

Note: I think I could have probably added one more turret for V2's grid stopper so it could just go all the way to the bottom instead of having to have a wire through a hole, but this should work anyway.

 

[2L man]

@2L man also, if I'm understanding correctly, I believe the input jack should still be grounded to the chassis, right? I'm using a Switchcraft Mono Shorting Jack (12A).

Yes its that simple change! And yes it is beneficial to connect input jack Common to ground.

Also a "ground loop hum kill circuit" (I don't know real name but using thise phrases it is possible to find information) can be used. It consists four components between Common and Chassis. Two diodes which current rating is at least double what the HV fuse is and they are opposite each other. These prevent voltage stay less than about one volt before HV fuse burn. Also 10 ohm 2W resistor which short Common to Safety earth "softly". And 100nF X-class capacitor which "short" high frequencies. Also a SPDT switch NC contacts can be connected and it short when NC is selected and make this circuit operative when switched to NO.

Mains Safety Earth must always be hard wired to metal Chassis.

 

[joulupukki]

Yes its that simple change! And yes it is beneficial to connect input jack Common to ground.

Also a "ground loop hum kill circuit" (I don't know real name but using thise phrases it is possible to find information) can be used. It consists four components between Common and Chassis. Two diodes which current rating is at least double what the HV fuse is and they are opposite each other. These prevent voltage stay less than about one volt before HV fuse burn. Also 10 ohm 2W resistor which short Common to Safety earth "softly". And 100nF X-class capacitor which "short" high frequencies. Also a SPDT switch NC contacts can be connected and it short when NC is selected and make this circuit operative when switched to NO.

Mains Safety Earth must always be hard wired to metal Chassis.

Ok, that makes sense. Thank you for explaining. Yes, good point, I should hard-wire the main ground to the chassis instead of just use a solder lug.

 

[joulupukki]

Time to solder the components. Fingers crossed!

 

[joulupukki]

Ok, not too hard. My soldering got better towards the end. I used an alligator clip as a quick heat sink on most the parts. Most the resistors are soldered to the bottom of the post and the caps on the top section of the turret post.

Next up, time to drill all the holes into the top of the chassis.

 

[joulupukki]

Making good progress. Drilling the top was ... stressful! There were so many holes to drill. Haha.

This is the first look at the panels with the protective tape off. All the pots and inputs are mounted. I've got the board test-fitted in there, but am going to have to re-solder one resistor to a higher turret location so that the 4th board mounting screw can go in. Tomorrow I'll take the board out and do that and then start wiring as much as I can before putting the board in and wiring up the rest. It seems like it's close but I know wiring & soldering from here is gonna take a while. I'm hoping by tomorrow afternoon/evening I'll be able hear it for the first time.

 

[King Fan]

Wow, super nice. Great work.

Your faceplates look great too. Can I ask who/where/how you ordered those? They don’t look too thick, either.

 

[joulupukki]

Wow, super nice. Great work.

Your faceplates look great too. Can I ask who/where/how you ordered those? They don’t look too thick, either.

Thanks. Taking a break for some lunch. I’ve got a lot of the soldering done, but still need to hook up the remaining pots and the board to the tubes. I definitely don’t like soldering wires into the tops of the turrets. It’s hard to heat them up properly. So I’ve been favoring wrapping around the side when there’s room. Come to think of it, the eyelets on the PR board I did were a lot easier to solder. Oh well. Good to get experience with both.

I sent my faceplate files off to Sandy at Precision Design: http://www.precisiondesignin.com/

I’m pretty pleased with them and it’ll look a lot better than permanent marker right on the chassis … or whatever else I had started to conjure up in my head. I finally figured that I might as well do it nice to match everything else.

 

[King Fan]

I sent my faceplate files off to Sandy at Precision Design: http://www.precisiondesignin.com/

Aha, never heard anything but praise for her work, and yours show why. They're acrylic? How thick are they, if you can check?

Yeah, breaks are great. If you've made this much progress during the week, I hate to think what you'll get done on a weekend.

 

[Len058]

Beautiful work! I wish I could be that patient and meticulous.

 

[joulupukki]

Aha, never heard anything but praise for her work, and yours show why. They're acrylic? How thick are they, if you can check?

Yep, they’re acrylic. As far as I can tell, the same type of stuff you’d see on an office door or a trophy plaque. They are 1/16” thick.

All the front panel potentiometers, switches, and input jack are wired! Now on to the tube sockets and then it should be ready to rock!

Actually, I’ve still gotta figure out how to mount the chassis into the cabinet. I would use the rivet nuts but … they make the whole chassis come off the floor of the cabinet by about 1/16” (or just barely under that). I don’t want there to be a gap between the bottom of the faceplates and the amp cab. So … I may use a wood chisel to make room for those nuts. Otherwise, it’ll be some JB Weld to hold on some keps nuts. Regardless, if this this works, it’s going in the cab today if I have anything to do with it.

 

[Eddiebaby1]

Blimey mate, your work is stellar!
Massive respect for your skillset.
By comparison I fitted new pickups in my Tele yesterday. My iron is too small to effectively heat the joint on the back of the volume pot. Luckily I recognised the problem before it became a crisis, cut the lead on there to about 3/4 inch and with solder and a bit of heatshrink I just added the pickup lead and accept I'll need to go back after getting a new iron and undo my bodge.
I didn't feel too bad about it until I saw your stuff

 

[joulupukki]

Blimey mate, your work is stellar!
Massive respect for your skillset.
By comparison I fitted new pickups in my Tele yesterday. My iron is too small to effectively heat the joint on the back of the volume pot. Luckily I recognised the problem before it became a crisis, cut the lead on there to about 3/4 inch and with solder and a bit of heatshrink I just added the pickup lead and accept I'll need to go back after getting a new iron and undo my bodge.
I didn't feel too bad about it until I saw your stuff

Sounds like you’re off to a great start!

 

[joulupukki]

It’s all wired! Time for testing. Safety glasses on and light bulb current limiter at the ready. Here are some pre-test shots…