Fixing a Mojotone TMB kit before I even start

[cmatte82]

Hello all. Please don't shoot me. I have bought a Mojotone TMB 18-watt kit. But it hasn't even arrived yet. So let me back up a bit.

I've always wanted a rocking Marshall style amp. I've also always loved working on electronics. Aside from my tinkering with guitars, amps, pedals, and live sound gear for the last 20-25 years, I also spent some semesters in college taking a few basic EE classes. I know enough to not get killed, but to also know how little I really know. That said I've also spent the last 20 years or so as motorcycle mechanic. And while I can work on any part of a bike, the electronics has been my specialty. I have built wiring harnesses from scratch for several custom bikes. So I know my way around a soldering iron. Although I will admit the videos JMPGuitars posted about soldering did teach me a few things. So you're never done learning for sure. All that said though, I think I can make this kit work, instead of trying to return it and going a different direction. I think.

Also I've built a couple of guitars. And have the woods to build some more. But I think building my own amp to go with them would be really cool too. I should probably build a few pedals, but I'm not really much of pedal guy. I like to keep things simple.

So why did I pick this kit? Well somehow in my research before buying it, I somehow missed people having issues building it. I did try and research reviews and comments and such before buying it. Lots of great reviews. Lots of amazing sound samples. I had almost bought a much cheaper chinese kit. But decided it was worth it to buy a "better" kit. Only after payment had gone through did I find this place. So it has made me rethink it. However I like the more aggressive higher gain tones it has, more 1987x to 2204/2203 tones, verses more JTM sounding amps like most of the other 18 watters seem to be. I want more of a hard rocker. This seems to be the ticket.

So what I want to do is fix the issues before I run into them. At least as many as I can.

Knowing all that, I have also found JMPGuitars fantastic "Tremolo TMB" guide. Way better than mojotone's "instructions". So I definitely plan on following it. I've tried looking through the schematics here, but I'm not sure which one would be best for more of a hard rocking tone. So let me know which ones to look for. I mean if all I do is use the chassis and transformers, I won't feel to bad about it.

People say the layout of the mojotone kit isn't very good, but I can't seem to find anyone saying exactly why and what needs to be changed about it. Let me know what I can to change to make things better off.

Carbon comp resistors. I don't like them either. I plan on sitting down tomorrow and figuring out which values I need. So I can order some better ones. Unless there's maybe someone that sells them in a kit? Does anyone have something they really like to use?

Grounding. I totally understand how much proper grounding matters. I can't tell you how many times erratic problems in motorcycles are a bad ground. I have chased down bad grounds for a long time now. I'm game to go over the top to make sure everything is properly and well grounded. The only question I have there is the recommendation to not use a buss for the tone pots. Why exactly? As long as they still ground to only one point, I don't really see the difference. Don't get me wrong, I'm going to do as suggested and not use a ground buss. I'm just not sure what that changes. I guess it keeps the shells from being grounded, but wouldn't that help reduce noise? Just trying to learn something here.

The master volume. I know this amp is loud. I have had a Zvex nano that was 1/2 watt clean and it would easily drive a 412. So I get that 18 watts is a lot. That said I don't see the need for one really. And I've seen people say they actually hurt more than they help. So what about getting rid of it, and having a hi and lo input for the TMB channel. I think that would be better. Then maybe I could even jumper to the "clean" channel and get some cool sounds going?

Anyway my point for starting a thread like this, is maybe people will see it before starting and know what needs to be done to upgrade their kit before buying. And maybe help people make a better decision about whether or not they should even bother with this kit.

So what questions didn't I ask? And if you just had to build the mojotone kit, what changes would you make right off the bat? And I don't expect this amp to be perfect. My first guitar wasn't either. I'm sure this is just the start of it, and long as I can gain some experience and learn a few things, and ideally at least end up with a moderately useful amp. I will be happy.

Thanks in advance.

[Bieworm]

hi there!

I would not recommend the Tremolo TMB for hard rock sounds.. it's a medium gain amp that I just loooove, but it's nothing near high gain.
There's an 18W TMB schematic and layout in the downloads with documentation you can use.. but no tutorial or stuff like that. OTOH the Tremolo TMB build guide holds a treasure of great info that is also very useful in general.

viewtopic.php?t=24433

there's also a great building tutorial at Tube depot IMHO. But it's for the classic 18 Watt.. maybe get some great side info there too

look under 'technical documentation'
https://www.tubedepot.com/products/clas ... p-kit-head

or maybe Josh has some other building docs somewhere in his library .

[JMPGuitars]

If you can cancel the order, you're better off. Their chassis and cabinets are fine, and their Heyboer transformers are probably okay too. However, I much prefer the GDS/Heyboer transformers from gdsamps.com.

If you want to build the classic circuit, I suggest looking at the Modern Classics in the Downloads section. If you only need a single channel amp, you should look at the Superlite TMB. It's a little hotter, has a nice TMB, and it's a great sound.

If you want something higher gain, then look at the Xtra TMB series in the Downloads section.

I like CMF metal film resistors for the ones you'd be replacing the carbon comp with.

Thanks,
Josh

[colossal]

If you can cancel the order, you're better off.

+1

Completely agree with this. Source your own transformers and components. Anything worth doing, is worth doing right.

And get ready to rock.

[cmatte82]

Hello. I’ve got the kit. And against your good advice I’ve started putting it together. So far it is going fairly well. The back panel is way off from the chassis. So I’m going to contact them about that. But for me, it’s not a huge deal.

Also they didn’t send carbon comp resistors. But carbon film resistors. Maybe not metal film, but imo they’ll work for this first amp.

However this has lead to my first issue. Coming off v1, pin 3 there is a 680nF cap and a 2.7k Ohm resistor in parallel. However they sent a 27k Ohm resistor. I’ve been diligently checking every resistor. And so far they’ve all been within spec. So I’m going to see about sourcing a new one. I have an assortment of 1/4w carbon film resistors. I’m sure I have the right one in there. But is 1/4w enough? They sent either a 1/2w or maybe even 1w. I don’t remember right now.

I’ll probably go ahead and order an assortment of 1/2w metal film resistors. Just to have. I’m enjoying this. So might as well start getting a few parts for my next one. but I’m just seeing if I can go ahead and use one I have, or need to wait on a new one?

Thanks

[Bieworm]

In that case I hope you don’t run into much trouble.. but whatever.it’s all fixable..
An assortment of 1/2W MF resistors is pretty useless IMHO. There will be a lot of values you will never use. And for B+ related stuff you need higher wattage resistors.

[cmatte82]

Thanks for replying. I like to build random electronics. That’s why I have an assortment of 1/4 watt resistors. You never know when you might need one. And the assortment is not exactly a costly item. But I’m still wondering if a 1/4 watt resistor will work in that spot? Or should I wait and get a 1/2 watt one. Or I guess maybe I could parallel two 1/4 watt resistors around 5k? At least temporarily?

And I have the transformers wired up, and most everything installed in the chassis. I have the turret board done. I still need to wire the controls. And then install the turret board and finish up the wiring between everything. I also need to build a current limiter. Then double check everything another couple of times before I hook it up to the current limiter. I may even post some pics before I plug it in. Just see if there’s anything obvious I’ve done wrong or missed. Then hook it up, and if it passes the smoke test, start checking voltages. Anyway I’ll report back tomorrow with where I’ve gotten too.

[Bieworm]

Thanks for replying. I like to build random electronics. That’s why I have an assortment of 1/4 watt resistors. You never know when you might need one. And the assortment is not exactly a costly item. But I’m still wondering if a 1/4 watt resistor will work in that spot? Or should I wait and get a 1/2 watt one. Or I guess maybe I could parallel two 1/4 watt resistors around 5k? At least temporarily?

And I have the transformers wired up, and most everything installed in the chassis. I have the turret board done. I still need to wire the controls. And then install the turret board and finish up the wiring between everything. I also need to build a current limiter. Then double check everything another couple of times before I hook it up to the current limiter. I may even post some pics before I plug it in. Just see if there’s anything obvious I’ve done wrong or missed. Then hook it up, and if it passes the smoke test, start checking voltages. Anyway I’ll report back tomorrow with where I’ve gotten too.

There's no harm in parallelling 2x 5k half watt resistors there. I don't think those cathodes are very hard on the resistor. But it's always better to use higher wattage resistors. I try to use minimal 1 watt. Higher wattage will reduce noise...
if you want it absolutely safe you could parallel the 27k 1/2W with a 2.7k 1/4W and get in the ball park (2.45k). When the 2.7k comes to fail you will end up with 27k, which is very cold bias.. but it's still biased.

as for building.. I always use a heat sink (alligator clip) when soldering the parts to the turrets. And I always solder the wires to the controls and the tube sockets before I solder the components. That way the components only get heat just 1 time while soldering them. Some components can't take a lot of heat and are being treated nicer this way. IE the silver mica caps are notorious for failure due to overheating.

[cmatte82]

Man I knew I was forgetting something. I have an alligator clip that is actually meant to be a heat sink. I have pretty nice iron that’s nice and hot. So I can solder pretty quick without too much heat build up. Hopefully it’s all okay. If not I’ll cross that bridge.

And thanks for the tip about higher wattage resistors. I ordered a pack of 1w metal fill. For my next build I’ll set down and order a supply of really nice ones. But it will be handy to have these around. And I’ll just wait on the new resistors to get here before I test it out. I’m not rushing this.

And I just bought all the stuff to make a current limiter. So I’ll put it together. And finish the wiring the pot and inputs. Then the resistors should be here Saturday. But I’ll be busy with my boys. So it may be Monday before I get it all put together.

[cmatte82]

Well this doesn’t have much to do with the amp, but while building the current limiter, I realized the plug in the shop I was using had the neutral and hot flipped. Not sure exactly what affect that would have had on the amp, but glad I got it sorted. And before I put the limiter together, tested it, and possibly thought I had wired it backwards instead of realizing it was the wall plug. Anyway it’s fixed. The limiters together. And the resistors should be here tomorrow. So at some point I’ll get the pots and rest of the chassis wired up and ready for the turret board to be installed and everything ready for testing.

[cmatte82]

IMG_0387.JPEG

IMG_0388.JPEG

I have more photos. It hasn’t been plugged into power. I haven’t went through the actual schematic and verified everything. I’ll do that next. Just wondering if you guys seen any problems or things I missed right off the bat. Before I try to power it up.

I replaced the input jacks with proper UK cliff jacks. I moved the resistors off the clean channel input jacks to v1. I used shielded cable in the places I believe it was important. I didn’t use a ground buss on the pots. I did move where they grounded to right by the high gain channel input. That’s the channel I plan to use the most. I made sure the ground wire from the power jack is on its own terminal to the chassis by itself.

Unless you guys see something way off. My next step is to verify everything through the schematic.

Plug in a cab. Plug it into the current limiter I made. No tubes. Then go through the voltage checks in the instruction manual.

Anything you’d specifically check right off the bat that the manual may not cover? Or is especially important even if the manual does cover it?

Thanks in advance.

[cmatte82]

Okay. Well I got a little impatient. I double checked the layout. Tried checking the schematic, but I’m a little out of practice of doing that. So I went ahead and followed the instructions for checking voltages as I installed the tubes. Step by step. I wish it was a little plainer in what the voltages needed to be. I wrote down what I measured as I went. Nothing ever went sideways. Finally once it was all installed, the instructions said to let it warm up 30 mins. Then check for anything loose. Again I might have rushed that a bit. But I turned up the volumes and heard that nice subtle hiss of a tube amp. Went and grabbed a guitar. Plugged in.

And I think I did a thing!!!! Sounds great. It’s a little “cleaner” than I expected. The tone controls don’t seem to make a huge difference. I can crank the gain to 10 on the second channel. No problems. The MV on the gain channel does squeal when it’s around 8 and above. I wouldn’t mind remedying that if possible. But not a huge deal. As expected it’s loud by the time it gets to 2 and by 4 or so it is about as loud as I will probably run it. I didn’t test the clean channel pushed to “10”. Again I don’t plan on using it that way. But I jammed for 1/2 and hour or so and it just rocks. Exactly what I was wanting.

[chaliapin]

Glad it works! Im going to preempt the guys who know what they're talking about and recommend you print the relevant voltage sheet from the download section, fill it in and post it here.

[cmatte82]

Voltages pen.jpg

Here's my voltages. Everything seems to be in the ballpark. My v2 is the TMB channel and v3 is my PI on my amp. The one that seems the furthest off is v2b. My v2b voltages seem to be close to the other input stages on my amp, however compared to v3b (the tmb channel on the chart) they're considerably different. Not sure why that is. I also wasn't sure where some of the voltages on the bottom of the chart were supposed to be taken. So wrote down the voltages on the large caps. The first two are the side mounted cap, and the next two are the cap mounted outside the amp. Lastly I wrote in the voltage on the 16uF cap on the board. These are voltages the manual said to check. Let me know what else I need to check. But I feel things are at least pretty close. This amp is a lot of fun to play. But I want to make sure nothings way off before I keep rocking out on it and something bad happens.

[cmatte82]

Oh and fwiw I have a KLD amp kit that should arrive thursday. It's their DC30 style kit. I actually ordered it months ago, but am just now receiving it. We'll see how it goes.

And then I think I need a tweed style amp in my life. Any suggestions on good kits? Or good websites like this with lots of info about them? If I get the KLD amp going, maybe it will be time to just build one from scratch?

Oh and once I know the voltages are good, or they're not and what I need to do to fix them, I'll post some clips of my 18w TMB. It rips apart the DSL50 I inherited from my dad.

Thanks again

Matt

[cmatte82]

Anyone??? These voltages look okay? Any recommendations or suggestions?

[cmatte82]

So one last check in. I’ve been hesitant to play this much until someone confirms the voltages seem to be okay. It seems alright from a “tone” stand point, but I’ve also heard some amps sound really good right before they go. And I’d rather not experience that. But if I don’t get a response I guess I’ll go with it. Thanks again.

[JMPGuitars]

You need to get correct voltages for feedback.

On my charts:
A = the DC voltage coming from the rectifier (or directly after Zener diodes if being used). This is typically the first filter cab.
B = the 2nd filter cap.
C and D are following the B+ line after dropping resistors. Your amp may or may not have both. These voltages go down. Typically starting from around 345VDC, unless your B+ is too high.

Rather than crossing out the values on the tubes, just switch V2 and V3 ID numbers, and the rest should follow.

You need to calculate your output tube dissipation. I don't know what your cathode resistor value is, so I can't do it for you. You can use this calculator to figure it out: https://robrobinette.com/Tube_Bias_Calculator.htm

Thanks,
Josh

[cmatte82]

Thank you so much!!! I do have a few questions. The voltage of the rectifier tube is 336v. It goes to one side of a large capacitor and on to the OT. I believe this is the B+ voltage?

Then there’s a 2w 2.7k resistor going to the other side of the capacitor. Not ground, there’s two caps in the same can, and you probably know this. Just making sure. At that side of the resistor the voltage drops to 299v.

Then there’s a 10k resistor feeding the the first side of the second cap can. Voltage there is 259v.

Then another 10k resistor goes to the other capacitor and voltage is 234v.

Are those the right voltages you’re wanting?

Then I follow pin 3 back from the tubes. They’re tied to a 5w 130 ohm resistor with a 25 uF cap to ground. Voltage on the ground side is 0v. It 10v on the other side. This is the cathode resistor correct?

If so, from the instructions on the page you linked to, I measured voltage between pins 3 and 7 on the el84’s. It was 316v. So input that with el84 12w tubes. Calculated. Then I put the 10v and the measured 128 ohms. It shows I’m at 102.5% of max dissipation. Is this okay?

I wouldn’t mind trying to maybe lower it a little. Mostly for tube life. Whats the best way to lower it?

And let me know if there’s anything else I need to measure. I surely will.

[JMPGuitars]

Your rectified (DC) voltage is low. In that range, you can expect less headroom, and possibly some muddiness. What's the AC voltage on the second of the PT (the AC wires that connect to the rectifier)?

We target 85% tube dissipation. You would need to raise the cathode resistor value. Replace it with a 150 to 180 ohm resistor, depending on whether or not you're able to raise your DC voltage.