My 18w TMB Kit Build

joulupukki

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Just thought of an easy mod you'd probably enjoy. Change the MV to a PPI master. It's an easy mod which will let you enjoy the grit at lower volumes. And/Or you'd might want to build this: https://www.marshallforum.com/media/attenuator-m-190110.10430/
Cool. Yeah, I could do that mod. I’ll have to look into it. If I really want it at non-ear-splitting volume, I do have the Torpedo Captor X reactive attenuator and it works pretty well.

I notice that a lot of the layout diagrams on 18watt.com show using 500K volume knobs instead of 1M volume knobs like Mojotone uses. I wonder if those would give a better control of volume? I think part of the reason it goes from 0 to really loud is that between about 0 and 1.5 those 1M volume knobs are basically connecting ground (measured with my continuity meter). It’d be nice to have that part of the potentiometer make things a little quieter.
 

Len058

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I don't know if a 500 pot would help. I have built a kit like the TMB which as I said modded to be an EF80 version. At first I kept the normal MV until I understood the PPI-MV. I was very happy after I changed it. A lot of the nice overdrive comes from driving the PI. A normal MV prevents that.

Here's the Plexi-Micro PPI at 4 ish over a backing track.
 

joulupukki

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I don't know if a 500 pot would help. I have built a kit like the TMB which as I said modded to be an EF80 version. At first I kept the normal MV until I understood the PPI-MV. I was very happy after I changed it. A lot of the nice overdrive comes from driving the PI. A normal MV prevents that.

Here's the Plexi-Micro PPI at 4 ish over a backing track.

Is that pre or post phase inverter MV? Cool demo!
 

joulupukki

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I think I might be convinced there's sometimes a power surge that comes from using the toroid power transformer. I went down to play a little bit on the amp and when I flicked on the mains the fuse blew. I never did put in that arc reduction contraption I tested. Would that help at all? I turned it on and off a number of times yesterday (and earlier this morning) and a few times now this evening and the fuse didn't blow. Confused.
 

joulupukki

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I'm also curious why all the modern schematics on 18watt.com show using a 2A fuse instead of 1A. I wonder if that could also be something I should be changing?
 

andrewRneumann

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Interesting. If I'm calculating correctly, 1A should easily handle the load requirements of this amp. So I still think it's likely in need of a current inrush limiter.

I would agree. If the PT is oversized for the job, inrush can be more of an issue. Sure the PT won’t run at its limits during normal operations, but when you power up that first capacitor looks like almost dead short and the cold heaters have very low resistance. I think I read somewhere that the exact moment you flip the switch can affect it too. Depending where you are in the 60Hz mains waveform can change things. (How’s your reaction time?) An oversized PT gladly supplies the excess current for start up….at the expense of the mains fuse. I seem to remember this toroidal had more VA than the stock PT?

NTC thermistor is the solution. Doesn’t work if it’s already warm, so you need to allow some cool down between restarts. I don’t know how much time. Other Shock Bros can provide the details.
 
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joulupukki

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I would agree. If the OT is oversized for the job, inrush can be more of an issue. Sure the PT won’t run at its limits during normal operations, but when you power up that first capacitor looks like almost dead short and the cold heaters have very low resistance. I think I read somewhere that the exact moment you flip the switch can affect it too. Depending where you are in the 60Hz mains waveform can change things. (How’s your reaction time?) An oversized PT gladly supplies the excess current for start up….at the expense of the mains fuse. I seem to remember this toroidal had more VA than the stock PT?

NTC thermistor is the solution. Doesn’t work if it’s already warm, so you need to allow some cool down between restarts. I don’t know how much time. Other Shock Bros can provide the details.
I'll have to time my startups better with the waveform. ;)

Mojotone doesn't seem to publish all the details about their normal power transformer, but here's an 18w replacement from Hammond that shows their values: https://www.hammfg.com/files/parts/pdf/290PAZ.pdf

From what I can see, the AnTek I'm using (https://www.antekinc.com/as-1t300-100va-300v-transformer/) is a little more beefy than that.

I'll get a thermistor and then test it out and measure how long it takes to cool down to build enough resistance to be useful again. Probably between 30 - 120 seconds: https://www.ametherm.com/blog/inrush-current/cool-down-time-inrush-current-limiter
 

joulupukki

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Ok, just doing some testing here of some 3D-printed logos. These are all holding on with a tiny bit of double-stick tape. I think the sizing is right for the 18w larger head, but the size for the speakers and micro amp needs to come down. This also needs to be done:

1. Figure out how to "age" these so they aren't stark white or ... I need to paint them so they match more of the vintage style.
2. Have to sand them so the 3D layer lines aren't seen
3. Need to position the speaker logo down a bit

For reference these are all 7" wide. The newer ones for both the micro amp and speakers will be about 5.25" wide.

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BigDaddy23

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These look great!

With regard to finishing and fixing them - I fume mine with acetone to remove the layer ridges. I have some solid sections in the letters where I can then put a screw through to fix to the cabinet face. The centers of the "O's" in yours would work for that or you have to build in a couple small holes somewhere in the design. The dot on the 'i' and the curve on you 'M' may work.

From a colour point of view, you could use black plastic then paint to the colour you want - leave the solid screw sections black and use a black screw to fix.
 

joulupukki

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These look great!

With regard to finishing and fixing them - I fume mine with acetone to remove the layer ridges. I have some solid sections in the letters where I can then put a screw through to fix to the cabinet face. The centers of the "O's" in yours would work for that or you have to build in a couple small holes somewhere in the design. The dot on the 'i' and the curve on you 'M' may work.

From a colour point of view, you could use black plastic then paint to the colour you want - leave the solid screw sections black and use a black screw to fix.
Thanks! This has been a super fun project!

These are printed with PLA and I don't believe fuming with acetone will smooth it, but I have a couple of other ideas. One other thing we (my neighbor and I) are going to try is to print the top 1/2 mm with a gold filament. That way, I may not even need to paint or age any of it and it'll just be a matter of smoothing it out.

I guess adhering the logos with an adhesive or superglue isn't really recommended? Probably wouldn't really stand the test of time vs. screw attachment? I guess I hadn't thought that far ahead. Not sure how well it's going to attach to the speaker grill cloth either. I guess that's probably why a piece of wood is left in the baffle (for screwing down the logo). Hmm. I may have to go back to the drawing board on that.
 

BigDaddy23

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Ah you are using PLA - I use ABS for mine.

With your cabinet, you can always add the small piece of wood to the baffle to support the logo without taking the grill cloth off. You have to remove the speaker but you should be able to clamp it with a small spring clamp. Either that or use super glue and hold it whilst it goes off. Just make sure you paint the wood black first.
 

joulupukki

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I’ve used 1A slow-blow on both of my 18-watt builds. 500mA for a second HT fuse (not all have this).
Yeah, I think the 1A should work fine. I've chatted with an engineer at Ametherm and they sized up this project and are recommending this thermistor: SL22 12102


I'll bet with that installed, it'll work perfectly. I've now read a number of reports of toroid power transformers having this issue and am 99% sure this will solve the problem.
 

joulupukki

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Ah you are using PLA - I use ABS for mine.

With your cabinet, you can always add the small piece of wood to the baffle to support the logo without taking the grill cloth off. You have to remove the speaker but you should be able to clamp it with a small spring clamp. Either that or use super glue and hold it whilst it goes off. Just make sure you paint the wood black first.
Makes sense. I may end up either trying clear silicone caulk or hot glue. These logos are really light and shouldn't need much to hold them, nor should they make the grill cloth sag. A wood backing on the cloth isn't a bad idea though. Thank you for the suggestion.
 

joulupukki

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I've been playing around with different sizes of logos and also trying to see if I could make some that are gold. But, I think they are turning out way too splotchy and so I am probably just gonna go with plain white. Ideally if I could figure out how to taper/bevel the edges in the STL 3D artwork I would just have it print that way and with a small flat spot on the top I could paint that gold. But, I have seen pictures of Marshall amps where the cab has the gold piping and it still uses white lettering for the logo. They seem to look pretty good still.

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