Journey of my Bassman Micro build

robrob

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The wiring of the power amp master volume shouldn't matter. I made a mistake on the layout--I meant to wire it like the schematic. I thought I fixed the layout error but I'll have to double-check.
 

joulupukki

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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...
Well shoot, maybe I should be building the EF80 version. No wonder it’s being talked about so much. I guess in order to do that (besides ordering different tubes and other small parts) I‘d need a different power transformer. If the MT 18w TMB kit ever comes I won’t really lack on power option though. ;)
 
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joulupukki

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One thing I’ll have to sort out is that I think I’m not going to go with a headphone output, nor a line out jack since I have a Torpedo Captor X I can use for both of those. What I do want to do is put in a mini toggle switch for switching between 4 and 8 ohms. I found another thread that talks about this (https://www.tdpri.com/threads/bassman-micro-build-thread-w-pictures.1055088/page-5), but I’m not sure I fully understand yet the connections I’ll need to make. But, I’ve got time to figure that out.
 

joulupukki

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Or maybe since you'll have a big amp soon enough, a true Micro that gets distortion at reasonable levels would be a useful addition to your amp stable...
Exactly. I re-read Rob’s Bassman Micro site and if I’m understanding it right, the 12AU7 would be around a 3-4 watt amp, using a 12BH7 would be 4-5 watt, and the EF80 is around a 2 watt amp. Am I missing something there (different than what I stated earlier)?

From the site about the “5F6A-M Bassman Micro LTP”:
The 12AU7 with both triodes in push-pull will develop around 3 to 4 watts of output power. Swap in a 12BH7 and the output increases to 4 to 5 watts.
About the “Bassman Micro LTP EF80”:
This version of the Bassman Micro LTP uses two EF80 novel pentodes in push-pull for true pentode power tube overdrive with about 2 watts of output power.
 

Bitsleftover

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I built mine with the post PI MV and it works a charm. Maybe not right at the bottom end of the scale, but it’ll take the majority of the volume off while maintaining a good tone. I mainly use it for keeping a constant volume between playing clean or dirty. I use an attenuator to do the grunt of the work.
I also added a Diode/Tube rectifier switch and can’t tell which is selected without looking at the switch position.
I’m not sure it’s possible to use a figure that will replicate a JTM level of negative feedback vs a bassman level etc. the power sections are just too different in my humble opinion. I found a figure that made the presence control work well and did away with the switch.
Great looking head cab by the way!
 

joulupukki

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I built mine with the post PI MV and it works a charm. Maybe not right at the bottom end of the scale, but it’ll take the majority of the volume off while maintaining a good tone. I mainly use it for keeping a constant volume between playing clean or dirty. I use an attenuator to do the grunt of the work.
I’ll definitely be putting in this volume knob.
I also added a Diode/Tube rectifier switch and can’t tell which is selected without looking at the switch position.
Good to know. I won’t worry about adding the SS option and just keep it tube-based. Thanks for that info.
I’m not sure it’s possible to use a figure that will replicate a JTM level of negative feedback vs a bassman level etc. the power sections are just too different in my humble opinion. I found a figure that made the presence control work well and did away with the switch.
Great looking head cab by the way!
👍🏻
 

joulupukki

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Ok, I think I understand what’s going on in this build with the output speaker impedance switch (https://www.tdpri.com/threads/bassman-micro-build-thread-w-pictures.1055088/post-10423887). It’s switching between 8 ohms and 15 ohms with a SPDT switch easily because it shares one common wire (#4 - Yellow) between those two outputs.

For the Hammond 125B OT these are the wires for 4 and 8 ohms:

4 ohms:
#5 - Violet
#6 - White

8 ohms:
#2 - Orange
#4 - Yellow

Is it possible to switch between these two with a single output jack? If so, what kind of jack, switch, and wiring?

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joulupukki

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Ok, maybe what I ought to do is follow this same thing and switch between 8 and 16 ohms. My main goal is to be able to have the option of using both of my 1x12 speaker cabs I built (each has an 8 ohm speaker in them).

The back of my speaker cabs have two 1/4” jacks that are currently wired in parallel. What if I were to use one of the cabs and wire those jacks in series? That way if I were to plug both of them in, they’d be 16 ohms and I could switch the output of my amp to 16 ohms to match. Or, if I’m just using one speaker, I’d switch it to 8 ohm.

Assuming that would all work, is this the right wiring I’d need to do?

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joulupukki

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Actually, would I need to use a shorting jack on that speaker cab jack plate and have a dedicated Input & Output (shorting jack would be on the output)? Like this:

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joulupukki

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Burning the midnight oil today to wrap up the Bassman Micro amp head cabinet. I’ve gotta find some black screws for the handles and maybe some black screws for the back, but other than that it’s ready for whenever I can get the chassis put together.

Also, part of the electronics came today … and the rest should arrive tomorrow.

I documented most today’s steps in that Volley thread I linked to earlier in case you wanna see more details. But, here are a bunch of pics from today…

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Here’s the two amp head cabs I’ve built. This one for the Bassman Micro and the British style 18w. I hope I can get the actual amps put together soon!

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Oh, and the metal mesh is now installed on my 18w head as well (finally got some and took care of stapling it into both amps tonight).
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willholt92

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Beautiful work on the cabs, I really need to get on with doing mine for my JTM micro.
I wish I had access to a table saw that could run dado stacks, it would make finger jointing so much easier but it's not easy to get hold of them in the UK (yet..... although it might be soon with safety standards changing)

What size round over bit did you use? 1/2"?
 

joulupukki

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Beautiful work on the cabs, I really need to get on with doing mine for my JTM micro.
I wish I had access to a table saw that could run dado stacks, it would make finger jointing so much easier but it's not easy to get hold of them in the UK (yet..... although it might be soon with safety standards changing)

What size round over bit did you use? 1/2"?
Thank you. Yes I did a 1/2" roundover. I maybe should have done 3/4" but that bit that I have for my router isn't the best. I actually used the Porter Cable dovetail jig with a router to do the finger joints (just using a straight 1/2" bit). It's maybe a little slower than a dado stack, but it works just the same. I'm excited to get the electronics put together.
 

joulupukki

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My transformers and chassis came today (Hammond 1444-1372). Wow, it's a close fit, but not too tight ... whew! Also, I didn't realize the chassis would already have mounting holes drilled in the bottom. They barely miss where the cabinet feet are mounted.

Below is the front panel arrangement I'm thinking of (ignore the wrong input jack type and the fact that I don't yet have the pilot light).

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IMG_0541.jpeg

jtm-45-micro-sample-layout.jpg
 

joulupukki

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Here's the design I'm looking to do on the front faceplate. I drew it vector-based in Photoshop and now just need to figure out how to convert it to EPS so I can get a quote for pulling it off.

BHT-JTM45-ML-FRONT-PANEL.jpg


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Looks like I need to order a different pilot light. I ordered the Fender-style with red jewel light, but that probably won't work with the design I'm looking to do (so it looks more like a Marshall).

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joulupukki

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I’m also working on the layout of the innards … not done, but here’s the progress so far…

Untitled_Artwork.jpg
 
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