My 18w TMB Kit Build

[mountainhick]

White looks heckofalot better.

 

[joulupukki]

White looks heckofalot better.

Yeah. I was trying to match something like these. I think I got pretty close for everything else but will just settle for plain white. It'll still be good.

 

[joulupukki]

Here's how the logos turned out. I used hot glue for the amp heads (probably wouldn't again) and an interesting trick for the speaker cabs. Here's a video showing what I did:

 

[willholt92]

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.

View attachment 993834 View attachment 993835 View attachment 993836 View attachment 993837 View attachment 993838 View attachment 993839

As with your other thread (Micro Bassman) your work on this TMB is exemplary.
I did actually build a new cab for my micro last week just not had time to write a post haha.

For your logo and gold lettering work, could you print the logos with the top face offset inwards by 1mm around the perimeter and offset above the top surface by the same. I'd then use a foam roller to just hit the top surface in gold. Foam rollers give a better finish as well, I use them for clear coats on furniture.

The 1mm offset should hopefully keep the gold nice and clear.

Other option would be to pad print it (I would expect this is how Marshall do it on the Reissues)

Again, absolutely killer work.

 

[joulupukki]

As with your other thread (Micro Bassman) your work on this TMB is exemplary.
I did actually build a new cab for my micro last week just not had time to write a post haha.

Thanks man. These have been fun to work on. I'm wondering now what do I do? Of course, I need to learn how to play better. There are still a couple of mods I want to try out with the TMB ... and I have a thermistor arriving tomorrow to prevent the current inrush issue at mains power up. I'd love to see your micro when you're ready to show it.

For your logo and gold lettering work, could you print the logos with the top face offset inwards by 1mm around the perimeter and offset above the top surface by the same. I'd then use a foam roller to just hit the top surface in gold. Foam rollers give a better finish as well, I use them for clear coats on furniture.

The 1mm offset should hopefully keep the gold nice and clear.

Other option would be to pad print it (I would expect this is how Marshall do it on the Reissues)

Ah, that could work. Definitely worth a shot. Maybe even only 0.5mm raised would be enough. At some point I'm going to try to figure out how to actually print a full on taper so it's skinnier at the face of the font and I can just paint that gold and it may look closer to the originals. For now, the white doesn't look too bad. Hehe.

 

[willholt92]

As with your other thread (Micro Bassman) your work on this TMB is exemplary.
I did actually build a new cab for my micro last week just not had time to write a post haha.

For you logo and gold work, could you print the logos with the top face offset inwards by 1mm around the perimeter and offset above the top surface by the same. I'd then use a foam roller to just hit the top surface.

The 1mm offset should hopefully keep the gold nice and clear.

Other option would be to pad print it (I would expect this is how Marshall do it on the Reissues)

Again, absolutely killer work.

Thanks man. These have been fun to work on. I'm wondering now what do I do? Of course, I need to learn how to play better. There are still a couple of mods I want to try out with the TMB ... and I have a thermistor arriving tomorrow to prevent the current inrush issue at mains power up. I'd love to see your micro when you're ready to show it.

Ah, that could work. Definitely worth a shot. Maybe even only 0.5mm raised would be enough. At some point I'm going to try to figure out how to actually print a full on taper so it's skinnier at the face of the font and I can just paint that gold and it may look closer to the originals. For now, the white doesn't look too bad. Hehe.

They look amazing, haha that's always the problem when a project finishes, what next? And I have all these parts left?
I'll keep an eye out for any new project you start, your threads have lit a fire under my arse to try and get on with mine again.

My thread is here, ashamedly it's been going on for over 2 years now but having a 2 year old means most of my free time is taken up at the moment.
https://www.tdpri.com/threads/2nd-build-robs-jtm45-micro.1027897/

What CAD program do you run your printer from?
If you can add draft to the part in CAD this will work (I am a product designer/product development manager and we use Solidworks at work) Draft is critical in manufacturing a plastic injection moulded part to allow the part to be extracted from tooling. This is obviously not a requirement for 3D printing but if you added a few degrees draft to your part before printing this will mean the lower section of the logo will naturally be wider than the top and will give the look of the gold being "framed" by the white. Just be sure the draft direction is correct, I'd say 2 degrees should give enough draft.

 

[joulupukki]

They look amazing, haha that's always the problem when a project finishes, what next? And I have all these parts left?

Thanks. Of course ... that's actually how my Bassman Micro project started. I ran out of tolex for finishing my speaker cabs and realized I had to order more ... and there was plenty leftover to build another small amp head. Haha. What's crazy is that the micro amp easily cost me more than the Mojotone 18w TMB kit. I guess I could have bought cheaper components, but figured I would just make it really good (including the custom faceplates).

If I could build a Marshall-style amp that's powered half-way between my micro and the 18w that'd probably be about perfect. I love the size of the micro head. I definitely feel most accomplished from that one since it's a from-scratch build and not one from a kit. I'll probably end up trying a few mods on the 18w to make it more usable in my small bedroom studio. The Captor X works pretty well, but sometimes a pain to have to connect it. It'd be nice to have a bit more control of the master volume instead of from 0 to 60 with the slightest turn. I may try that post phase inverter master volume to see how that works.

I'll keep an eye out for any new project you start, your threads have lit a fire under my arse to try and get on with mine again.

My thread is here, ashamedly it's been going on for over 2 years now but having a 2 year old means most of my free time is taken up at the moment.
https://www.tdpri.com/threads/2nd-build-robs-jtm45-micro.1027897/

Excellent looking build so far! Interesting you also saw that there's less hum with the OT slightly tilted. When I installed it, I didn't bother to actually implement the tilt and it's actually super quiet. I mean, I could probably get it more quiet, but it's performing really well.

What CAD program do you run your printer from?

Not sure. I'll have to ask my friend who printed it for me. I designed the artwork with vector art in Photoshop, exported it to an .ai file so I could open it in Inkscape to convert it to an SVG that I then opened in tinkercad.com to make the .stl file.

If you can add draft to the part in CAD this will work (I am a product designer/product development manager and we use Solidworks at work) Draft is critical in manufacturing a plastic injection moulded part to allow the part to be extracted from tooling. This is obviously not a requirement for 3D printing but if you added a few degrees draft to your part before printing this will mean the lower section of the logo will naturally be wider than the top and will give the look of the gold being "framed" by the white. Just be sure the draft direction is correct, I'd say 2 degrees should give enough draft.

I couldn't find anything in tinkercad that would let me add draft. I imagine some pay-for CAD software could do that, like you suggest. I didn't know what feature to search for, so thank you for mentioning that it's called, "draft."

 

[joulupukki]

I’ve now got an Ametherm SL15 22102 thermistor (220 Ohms @ cold start) installed on the amp as a current limiter. I first tested a SL22 12102 (120 Ohms @ cold start) but didn’t like the voltage drop after the amp warmed up. It affected the bias to run quite a bit higher on the power tubes.

The fine folks at Ametherm helped me calculate that the minimum safe cold-start resistance would be about 85 Ohms. At about 90 seconds after power off the thermistor cools down enough so that it is about 85 Ohms. That’s to keep it within “safe” start up point.

Watching the 120V pilot light you can visibly see the slow startup because it slowly lights to full brightness.

I measured between 200 - 450 mA of max spike at startup. The voltage drop on the mains line is about 3.2 VAC across the thermistor. Once the amp is turned on in standby it draws about 100mA. Once standby is off and the filter caps are all charged it draws around 220mA.

Plate dissipation on V3 is 110% and on V4 is 100% just about like before adding the thermistor. Unless I measured something wrong, the plate dissipation values with the SL22 12102 were 121% and 105%.

Here’s a little video I made during testing…

Here’s my temporary setup for testing. It always feels a little sketchy doing stuff like this but at least with a breadboard it’s not just freely moving around.

 

[joulupukki]

I came across this video explaining a type 3 post phase inverter master volume which, as far as I understands it, can lessen the MV by phase cancellation. I’ve got a handful of board-mounting potentiometers and tested it this morning to see what kind of affect it’d have and how it’d sound.

I started with a 100K potentiometer and I could barely turn the knob before it seemed at full blast. Eventually I worked down to a 5K pot and at the 5K setting the amp isn’t putting out full volume, but the range I get out of it is definitely home/bedroom worthy.

It seems like if I could find a 5K switching pot, that might be the best option. That way, I could mount it on the back panel (plenty of room back there) and switch it off for original stock capability and with it on the TMB’s MV and the normal channel’s volume knobs are much more usable at home.

Anyone know of a good option there? I guess the other option could be to use a on/off switch in combination with a non-switching potentiometer. Or even an On-Off-On switch with two fixed resistors for a couple of different values.

Edit: Or…I just plug in the external attenuator. Ok, that’s much easier and probably a better sound. Just sometimes a pain.

 

[Len058]

Anyone know of a good option there? I guess the other option could be to use a on/off switch in combination with a non-switching potentiometer. Or even an On-Off-On switch with two fixed resistors for a couple of different values.

Edit: Or…I just plug in the external attenuator. Ok, that’s much easier and probably a better sound. Just sometimes a pain.

I really don't miss the regular MV. I only have post PI volume and see no reason for switching to a regular one.

Is the attenuator that good? I feel that using the PPIMV lets the amp breathe more. An attenuator compresses more and diminishes some harmonics. To me an attenuated amp has the same feeling as a modeller. Sounds great but I like the PPIMV more. Maybe I'm fooling myself but if I am I'm ok with that.


I just used the 1M pot that was in the Madamp and the 1M from Rob's design. I can get a dirty tone at just above TV levels. I watch youtube lessons and play along.

 

[andrewRneumann]

I came across this video explaining a type 3 post phase inverter master volume which, as far as I understands it, can lessen the MV by phase cancellation. I’ve got a handful of board-mounting potentiometers and tested it this morning to see what kind of affect it’d have and how it’d sound.

View attachment 995261 View attachment 995262

I started with a 100K potentiometer and I could barely turn the knob before it seemed at full blast. Eventually I worked down to a 5K pot and at the 5K setting the amp isn’t putting out full volume, but the range I get out of it is definitely home/bedroom worthy.

It seems like if I could find a 5K switching pot, that might be the best option. That way, I could mount it on the back panel (plenty of room back there) and switch it off for original stock capability and with it on the TMB’s MV and the normal channel’s volume knobs are much more usable at home.

Anyone know of a good option there? I guess the other option could be to use a on/off switch in combination with a non-switching potentiometer. Or even an On-Off-On switch with two fixed resistors for a couple of different values.

Edit: Or…I just plug in the external attenuator. Ok, that’s much easier and probably a better sound. Just sometimes a pain.

I preach a dual gang pot for PPIMV if you have the space.

At 5K, I would start to worry about the power rating of the pot. If it is a 0.5W pot, then the rated rms voltage across 5K is 50VAC. Not out of the realm of possibility for a long tailed pair if on a relatively high voltage supply. Plus you want some buffer in there... 2x to 3x more power capacity then regularly demanded e.g.

 

[joulupukki]

I really don't miss the regular MV. I only have post PI volume and see no reason for switching to a regular one.

Is the attenuator that good? I feel that using the PPIMV lets the amp breathe more. An attenuator compresses more and diminishes some harmonics. To me an attenuated amp has the same feeling as a modeller. Sounds great but I like the PPIMV more. Maybe I'm fooling myself but if I am I'm ok with that.


I just used the 1M pot that was in the Madamp and the 1M from Rob's design. I can get a dirty tone at just above TV levels. I watch youtube lessons and play along.

That’s cool. I guess the one benefit of using an attenuator is that you can overdrive the power tubes. Both ways are probably good. I mean the whole point is just to not have ear-bleeding volumes and enjoy playing.

I preach a dual gang pot for PPIMV if you have the space.

At 5K, I would start to worry about the power rating of the pot. If it is a 0.5W pot, then the rated rms voltage across 5K is 50VAC. Not out of the realm of possibility for a long tailed pair if on a relatively high voltage supply. Plus you want some buffer in there... 2x to 3x more power capacity then regularly demanded e.g.

Yeah, admittedly this is the part I don’t quite understand. Robinette reports that the type 3 works really well on his 5F6A amp, but maybe because it leans more to the clean territory vs. the 18w? He says to use a 1/2W pot and describes it in his 5F6A and 5E3 notes.

 

[andrewRneumann]

Yeah, admittedly this is the part I don’t quite understand. Robinette reports that the type 3 works really well on his 5F6A amp, but maybe because it leans more to the clean territory vs. the 18w? He says to use a 1/2W pot and describes it in his 5F6A and 5E3 notes.

I'm probably off on my chicken scratch analysis of power in that potentiometer. I don't love the type 3. It changes the load on the driving tube. It's a band-aid for those who want a MV but don't want to rewire a chunk of their amp. Just two wires and pot--pretty simple, but not the best.

 

[Len058]

I preach a dual gang pot for PPIMV if you have the space.

At 5K, I would start to worry about the power rating of the pot. If it is a 0.5W pot, then the rated rms voltage across 5K is 50VAC. Not out of the realm of possibility for a long tailed pair if on a relatively high voltage supply. Plus you want some buffer in there... 2x to 3x more power capacity then regularly demanded e.g.

Do you mean the Lar/Mar type? Maybe I haven't found the right version buta lot of the options for ppi-mv's are for fixed bias. If I'm not mistaken the 18w and the Bassman-micro are Cathode bias. How would you implement a post phase inverter MV on those?

 

[andrewRneumann]

Do you mean the Lar/Mar type? Maybe I haven't found the right version buta lot of the options for ppi-mv's are for fixed bias. If I'm not mistaken the 18w and the Bassman-micro are Cathode bias. How would you implement a post phase inverter MV on those?

On Rob's Trainwreck pages, search for the Type 2 master volume. Yes, according to Rob, a version of it is called the Lar Mar version. It can be used with fixed bias or with cathode bias a like. Just respect the maximum resistance between cathode and grid of the power tube in use. (This would include the cathode resistor for cathode bias, and any resistance in the bias circuit for fixed bias.)

For cathode bias, nothing special is needed. The lower leg of the pot is grounded and the wiper goes to the grid of the power tube. For fixed bias, we just apply bias voltage where ground would have been.

 

[Len058]

On Rob's Trainwreck pages, search for the Type 2 master volume. Yes, according to Rob, a version of it is called the Lar Mar version.

Thanks got it. I should have thought of it myself. Next time ordering parts, I'll add a dual gang pot to the list.

 

[joulupukki]

Interesting. So after trying to learn more about this it looks like the master volume option that the Bassman Micro layout uses is type 3, phase cancellation, right?

Loading…

robrobinette.com

 

[Len058]

Interesting. So after trying to learn more about this it looks like the master volume option that the Bassman Micro layout uses is type 3, phase cancellation, right?

Loading…

robrobinette.com

Yes

 

[joulupukki]

With the La-Mar Type-2 PPIMV you normally take out the grid leak resistors and use a 250K stacked potentiometer. However, this amp uses 470K grid leak resistors. I assume then that if I were to add this I would use a 500k/500k potentiometer with a 7.87MOhm resistor across the terminals of each of the pot lugs to effectively have 470K with the potentiometer turned all the way up. Correct so far?

Next question, what are these 5.1K resistors (circled) in the screenshot? Normally grid stopper resistors go to ground in series with the grid leak resistors. But these don’t. Is this just a different way of using grid stoppers? Would I leave those in place and connect the PPIMV wires at same turrets where the green wires (green wires would be removed) connect?

 

[andrewRneumann]

I assume then that if I were to add this I would use a 500k/500k potentiometer with a 7.87MOhm resistor across the terminals of each of the pot lugs to effectively have 470K with the potentiometer turned all the way up.

If it were me, I would just use a 500k and not worry about adding 7.5MOhms. I can tell you are a by the book kind of guy though. If the wiper fails, you are going to know something is wrong and investigate before too long. (Hum, loss of volume, distortion, no clean headroom, etc.) The 7.5Mohms is hardly a "safety" compared to the 1M max on the datasheet. It's cathode biased, so you have some safety there.

Normally grid stopper resistors go to ground in series with the grid leak resistors. But these don’t. Is this just a different way of using grid stoppers? Would I leave those in place and connect the PPIMV wires at same turrets where the green wires (green wires would be removed) connect?

Those look like regular grid stoppers to me. Unfortunately, they will have to be relocated for the Type-2. The best place is right on pin 2 of the EL84's. Some people just float them free as they are strong enough to support a wire. Some heat shrink will give it even more support and protection from inadvertent short. Or you can install an isolated lug to solder them to. Not recommending using pin 1 as it's "IC" on the datasheet (internal connection).

The green wires will stay in place, but they will have the grid stoppers on the other side. The turrets holding the 10nF caps will wire to the top of the pots. The wipers will come down to the end of the green wires. And the bottom of the pots will go to ground.