Journey of my Bassman Micro build

[joulupukki]

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.

 

[joulupukki]

This is a mockup of what the amp head will kind of look like. I stitched some pics of my speaker cab together to give me an idea of the proportion. Although it’ll now be a 3/4” thick case instead of 1/2”. The faceplate on this mockup is wrong - I just grabbed a picture of an 18w TMB and Photoshopped it in. I still have to figure out what I’m going to do to make the faceplate look nice and professional. I’m checking in with a local place that does trophy plaques and engravings first before I start looking into other options. But, I kinda think I should wait until I’ve built it and have all the switches and stuff in place (and mods working). We’ll see.

 

[Paul-T]

I'll be watching and taking notes. EF80 or 12AU7?

Your speaker cab looks lovely - especially the beading.

 

[Telekarster]

Awesome man!!! Can't wait to see your build thread on this. You did a heck of a job on that speaker cab man! Looks really great. That's gonna be a sweet setup when you're done with it!

 

[joulupukki]

Your speaker cab looks lovely - especially the beading.

Thank you. I still need to re-do one of the speaker cab’s grill cloth and piping. I didn’t quite understand how to make the cloth tight or piping really straight (first time building cabs). Misting that salt and pepper “cloth” (it’s more like wood strands) does wonders in helping it be a bit more pliable. I also let the piping soak in hot water and then suspended it with a clamp and weight at the bottom to let it dry straight. That helped on the 2nd one but it’s still not as straight as I’d like.

I'll be watching and taking notes. EF80 or 12AU7?

It’ll be more like a JTM45 Micro actually because I’m planning to use 12AX7 tubes for V1 (and V2 & V3). Hopefully that’ll be the right move. I can always try a 12AY7 later just to see what difference it makes.

What’s crazy is that $-wise, the electronics for this build are only about $60 away from the Mojotone British 18w TMB kit I have on endless backorder. So I question why didn’t I just go for a higher-wattage amp, but at least this should be fun to do and it’ll be a great amp for experimenting with the different mods. …and I was able to actually order all the parts.

 

[Paul-T]

The beading on the top is next-level.

For EF80 vs 12AU7, I was wondering what your output valve/s are?

 

[Hyakuya]

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

Cab looks lovely... I've just started my first attempt at a cab, for my Champ Micro, so hoping it'll turn out good as yours.

3 way NFB.... In my Baseman Micro I used a 10k trim pot on one side of the switch and the 15k resistor on the other. I've settled at about 6k for the JTM45 side but it's tweakable.

Also, tube / SS rectifier switch probability not worth the effort....

 

[joulupukki]

The beading on the top is next-level.

Thanks. I used my table saw to cut the groove, then used one bigger piece of tolex to wrap the sides and bottom of the box, and then a piece across the top. Then I shoved the beading/welting cord in with a rounded tool handle. I’ll show how to do it in one of the videos I’m making when I build this Bassman Micro head cab.

For EF80 vs 12AU7, I was wondering what your output valve/s are?

I’m following the design & schematic here: https://robrobinette.com/Bassman_Micro.htm

The output valve will be a single 12AU7. I just realize I misspoke earlier about the alternate V1 tube. I’ll go edit my post to make it right. You can use a 12AY7 or 12AX7 in V1.

 

[joulupukki]

Ok, full disclosure, I am a software engineer who works at a company called Volley (volleyapp.com) and I swear that’s not the reason I’m putting this here, but inside that app I am documenting (with videos) this entire Bassman Micro build, starting with the work I did this morning on the cabinet for the amp head.

I don’t know the best way to describe what Volley is, but think of it like if Slack and Marco Polo had a super child…haha. It’s geared mostly for helping people work together across time and distance (like text messaging, but video-based).

In any case, I’ve created a “Guitars” space out there and inside of one of the channels I’ve started a build thread where I’m going to keep posting short videos of the different steps of my build. If nobody comes in there, that’s fine too, but I thought it might be a fun way to explain how it’s going.

Here’s a quick video explaining how to make it into the Guitars space and into my build thread: https://hi.volley.app/view?m=VGhZQpCphDhEahvpnXs0Qf-tk

And, to actually get into the Guitars space, click/tap on this link once you’ve signed in: https://hi.volley.app/land?tk=nbSVTfVRmyLhiPw8xyTzvU-tk

Again, I hope this isn’t inappropriate, but I think if you’re interested in following along and chatting/asking questions/etc. it could be fun. It’ll be a fun experiment anyway. In any case, I’m excited to hear how this amp turns out!

 

[joulupukki]

Here are a few pics of today’s progress building the cabinet for the amp head…

 

[joulupukki]

Cab looks lovely... I've just started my first attempt at a cab, for my Champ Micro, so hoping it'll turn out good as yours.

Thank you. Take your time and try to learn as much as you can online/here/etc. I was almost going to do a Champ Micro. Post in here when you’ve got it done. I’d love to see it.

3 way NFB.... In my Baseman Micro I used a 10k trim pot on one side of the switch and the 15k resistor on the other. I've settled at about 6k for the JTM45 side but it's tweakable.

Oh perfect! I was wondering what the JTM45 side ought to be. I saw pictures of one person’s build and they used a 6.8K resistor for it. Smart idea for a pot on the NFB. On my princeton reverb build I tried a couple values but decided it’s either normal or off and I don’t really need the in-between values.

Also, tube / SS rectifier switch probability not worth the effort....

@Hyakuya did you end up trying one and it didn’t make much difference? I‘m planning on tube rectification for sure but wondered if switching to SS would tighten it up at all and make it bite more with the NFB set to the JTM45 (or off) setting. It would definitely keep it more simple to not have it in there. I do have a switch and diodes that would make it work, but yeah, would love to hear what you may have experimented with already.

 

[Hyakuya]

I use an EZ81 rectifier but I recall a comment in a thread some time ago about this. I think it was @robrob (but I may be wrong) saying that the micro amps draw so little current that there's no benefit to switching between the two.

The other thing I did with my BM was loose the master volume. Partly to save space on the faceplate and partly because I found it didn't do much.... Either way, check how it's wired in the schematic because it's different than the layout (I used the schematic).

 

[joulupukki]

I use an EZ81 rectifier but I recall a comment in a thread some time ago about this. I think it was @robrob (but I may be wrong) saying that the micro amps draw so little current that there's no benefit to switching between the two.

The other thing I did with my BM was loose the master volume. Partly to save space on the faceplate and partly because I found it didn't do much.... Either way, check how it's wired in the schematic because it's different than the layout (I used the schematic).

Ah good to know. I will double-check the schematic (vs. the layout). Thank you for the tip.

 

[joulupukki]

All routed and filled all teeny gaps with wood putty. I forgot to cut the grooves for the welding cords, but I’ll do that once the top is all smooth. I use my table saw and make a 3mm deep cut across the top, front rounded edge, and back. It’s a tiny bit nerve wracking but is easier on these small amp heads vs the bigger speaker cabs.

I still need to figure out where the welting should go because the Marshall-style handle is too wide. I ordered a dog bone style handle and will measure it when it comes and then decide.

After I sand the wood putty I’ll spray the inside flat black and it’ll be ready for applying tolex. Gonna take the rest of the weekend off.

 

[joulupukki]

I couldn't wait. I sanded and prepped it for inside paint. It'll dry a flat to satin color and is just enough so that when you take the back off, you're not seeing bare pine. Just personal preference. It should have plenty of time before Monday morning. Next up, beading/welting groove cuts across the top and then on to applying the tolex!

 

[King Fan]

I use an EZ81 rectifier but I recall a comment in a thread some time ago about this. I think it was @robrob (but I may be wrong) saying that the micro amps draw so little current that there's no benefit to switching between the two.

The other thing I did with my BM was loose the master volume. Partly to save space on the faceplate and partly because I found it didn't do much.... Either way, check how it's wired in the schematic because it's different than the layout (I used the schematic).

Sorry, @robrob ... seems like I'm always tagging you, but we're talking about two things you'll know for sure. Re the EZ81, @Hyakuya , you're right, I do recall Rob saying there *likely* wasn't much sag with an EZ81 at these voltages. I'm not sure about voltage drop; on his page, Rob says about 20V IIRC. That drop might come in handy if it works that way in these amps, but I'm not sure a small 'sag' resistor on a SS recto wouldn't be just as good. So yes, I'm not sure about the value of a SS/tube switch.

The MV thing seems important: You note yours didn't do much, but Rob seems to say it should be highly useful. But it gets confusing -- there are two MV knobs on this amp, pre- and power amp. Are you talking about the *power* amp MV? Which version? Here's the 12AU7/LTP layout and schematic; both say to tap the 470K/82K junction:

Here are the EF80/LTP diagrams, where indeed the schematic and layout differ (and the layout differs from the AU7 version). Rob recently reminded us the EF80 has lots more gain than the AU7 -- I'm not smart enough to know if the change in the layout was maybe intentional. If I may, which output tube version did *you* build, and how was yours wired? (EDIT: see note below where I awake to the fact the 82Ks are now 220Ks....)

 

[Hyakuya]

I built the EF80 version and followed the schematic.

It's the power amp MV I'm refering to... I understand the idea behind this MV and I'm not saying it doesn't work for some people, it just didn't have enough impact for me to justify installing it. The pre amp master and bright volume do enough for my needs.

My Champ Micro on the other hand... has an MV in the same place (ish) and that is fab.

 

[joulupukki]

Ah, good to know. @King Fan I was just about to post very similar pictures because after studying the 12AU7-LTP schematic & layout diagram they sure look the same to me.

I also would like to hear @robrob ’s opinion of the rectifier switch in this one, whether there’d be any noticeable difference. In my case, I’m wondering if running it in SS mode would give more overdrive, combined with the JTM45 NFB (6K) vs tube rectifier and designed 15K NFB.

Also, am I understanding this right, the EF80 build would be a higher watt output than the 12AU7-LTP build?

 

[King Fan]

I built the EF80 version and followed the schematic.

It's the power amp MV I'm refering to... I understand the idea behind this MV and I'm not saying it doesn't work for some people, it just didn't have enough impact for me to justify installing it. The pre amp master and bright volume do enough for my needs.

My Champ Micro on the other hand... has an MV in the same place (ish) and that is fab.

Thanks, very helpful. So you tapped the 470K/82K junction, like the schematic. Oops, my bad, I now see this is the 470K/**220K** junction in the EF80 version. So maybe the layout version gets a less-attenuated phase signal to admix at the pot? Duh, I dunno....

Yes, @joulupukki, you're right about the EF80s -- Rob has said he recommends the 12AU7 version for folks who want a true bedroom amp, as the EF80s (nominally ?2.5W) can run SPLs in the range of some 5W amps. Ask anyone who's tried to convince a spouse their Champ is great for bedroom use...

 

[RicciAntoni_07]

Looking Ace!! Can't wait to hear it!