18Watt.com

Suit yourself. It's your amp. I wouldn't want that in an amp, even if it works.
I don't see the missing ground on the volume pot. If you mean the normal channel, the ground is on the middle lug of the tone pot.
Get it cleaned up first is my advice

Something lost in communication in this thread:

Cleaning up the wiring in the amp, is very likely part of the problem. Noise is often caused by bad lead dress. Additional noise is caused by bad ground, and all are often made worse by the mojotone layout.

Besides that, bad solder joints and burnt wires should be replaced.

Oh no, Mojotone strikes again!

Im done messing around with he trem channel. Does anybody know a simple way of removing it entirely? TIA Bob

I would post my own thread I dont see a way to. Bob

now this is messy...

rockon1 wrote: ↑

Mon 02/28/22 7:10 pm

I would post my own thread I dont see a way to. Bob

Please do start your own thread. Click the forum link above that goes to: viewforum.php?f=4

Then click the "New Topic" button:

Bieworm wrote: ↑

Mon 02/28/22 12:27 pm

Suit yourself. It's your amp. I wouldn't want that in an amp, even if it works.
I don't see the missing ground on the volume pot. If you mean the normal channel, the ground is on the middle lug of the tone pot.
Get it cleaned up first is my advice

Ifollowed the layout and the normal channel had no 0volt reference- the layout doesnt show one...

rockon1 wrote: ↑

Tue 03/01/22 4:26 pm

Bieworm wrote: ↑

Mon 02/28/22 12:27 pm

Suit yourself. It's your amp. I wouldn't want that in an amp, even if it works.
I don't see the missing ground on the volume pot. If you mean the normal channel, the ground is on the middle lug of the tone pot.
Get it cleaned up first is my advice

Ifollowed the layout and the normal channel had no 0volt reference- the layout doesnt show one...

Always use the schematic as your main guide.. the layout is there to put the parts in the right locations. When I something feels weird I always turn to the schematic. This is what's always in front of me when building:

Bieworm wrote: ↑

Tue 03/01/22 4:31 pm

rockon1 wrote: ↑

Tue 03/01/22 4:26 pm

Bieworm wrote: ↑

Mon 02/28/22 12:27 pm

Suit yourself. It's your amp. I wouldn't want that in an amp, even if it works.
I don't see the missing ground on the volume pot. If you mean the normal channel, the ground is on the middle lug of the tone pot.
Get it cleaned up first is my advice

Ifollowed the layout and the normal channel had no 0volt reference- the layout doesnt show one...

Always use the schematic as your main guide.. the layout is there to put the parts in the right locations. When I something feels weird I always turn to the schematic. This is what's always in front of me when building:
16461702135378235794532314425824.jpg

Thats like asking some one to use sheet music assuming they no how to read it. Not everyone is on the same level.

Again.. I didn't know sh#t about reading schematics 2 years ago.. I learned. It's not hard at all.. the logic comes with practice

Im sure its not that difficult....most things arent....Again I may not have that kind for time left and I want to prioritize my focus on things I deem important ...Bob

Hi All,

I had this same issue with the Mojotone TMB 18. (squeal and hum). This my 5th amp build and when I started running into issues I was reading on your to forum educate myself and fix the problem. I'm no expert, been using BYOC clone stuff which is step by step instructions. I'm trying to branch out and be flexible with different kits with less instructions.

I built the amp per the Mojo diagram.

1. Too eliminate the squeal I put a metal plate in the head box to enclose the bottom of the chassis. Squeal gone, but still squealed on the bench. So shielding problem there.

2. I to rewired the Gain, Master, Treble and input jack, no more squeal on the bench, but still a loud hum with the master up halfway and gain off.

3. About ready to redo the ground as per this forum and I have the "a duh" moment. (noise seems more serious that a ground issue unless I'm missing a ground point) So I checked all the ground points. Maybe I put a 820K resistor where an 820 ohm should be. Nope. Start swapping preamp tubes with know good. Found the second tube bad that caused the issue. (with running amps it's the tubes or the filter caps and soldering on new amps, right )

Gain channel is as quiet as the clean channel. Very nice sounding amp. I may build the next more from scratch, but I do like the wiring diagrams. I use the schematics to debug, but that doesn't help with parts placement.

Thank you, thank you!!!! I'm glad I could resolve by just reading the forum and logic. I'll be back for advice on my next build.

Tubeamp wrote: ↑

Fri 10/28/22 12:26 pm

Thank you, thank you!!!! I'm glad I could resolve by just reading the forum and logic. I'll be back for advice on my next build.

Hi, glad you could resolve the issue. If you're going to do an 18W or 36W build, I would suggest looking in our downloads sections for layouts/schematics. viewforum.php?f=25

Thanks,
Josh

Hi Josh,

Thanks, I'll be looking over the schematics and see what looks like what should be next.

Thanks!

I just started the build of the mojotone 18 watt TMB. So I don't have a completed amp. One thing that I noticed on the layout after reading this thread, is the wire lead coming from the 16uF filter cap for the preamp stage runs across the 470K resistor that connects the gain pot wiper to the grid of the first gain stage of the high gain section. This will cause hum. I will move the resistor to the tube socket and run a shielded wire from the gain pot. Unfortunately, I have already installed all of the components on the board, but are not soldered, so I need to take some sections apart. I should have checked this out before I started.

I am now reviewing the entire layout and circuit diagram to see if there are any other issues. Also, I noticed that a few people on this site are not happy with mojotone kits. All I can add at this time is that there are some issues with the build documentation and will let mojotone know. This is my first mojotone kit and so far I am disappointed with the layout and documentation. Hopefully I can make this a great sounding amp!

Mark

MSVguitar wrote: ↑

Sat 11/05/22 5:59 pm

I just started the build of the mojotone 18 watt TMB. So I don't have a completed amp. One thing that I noticed on the layout after reading this thread, is the wire lead coming from the 16uF filter cap for the preamp stage runs across the 470K resistor that connects the gain pot wiper to the grid of the first gain stage of the high gain section. This will cause hum. I will move the resistor to the tube socket and run a shielded wire from the gain pot. Unfortunately, I have already installed all of the components on the board, but are not soldered, so I need to take some sections apart. I should have checked this out before I started.

I am now reviewing the entire layout and circuit diagram to see if there are any other issues. Also, I noticed that a few people on this site are not happy with mojotone kits. All I can add at this time is that there are some issues with the build documentation and will let mojotone know. This is my first mojotone kit and so far I am disappointed with the layout and documentation. Hopefully I can make this a great sounding amp!

Mark

I'm certain from your questions and diligence thus far that you should come out with a fine amplifier, once completed/troubleshooted.

There are always issues with the mojotone documentation. That and their weird layout is why there's so many issues with their kits. But don't worry, as I said, I'm confident you'll be able to work through all that, just take your time.

Thanks,
Josh

If you're building a Mojo kit, I recommend downloading the Stewmac documentation for it. Stewmac sells Mojo kits but has created their own documentation and IMHO the Stewmac docs are better. Disclosure: I have never built from a kit, always scratch built. But I have reviewed a lot of online instructions for kits.

crgfrench wrote: ↑

Tue 11/15/22 8:56 pm

If you're building a Mojo kit, I recommend downloading the Stewmac documentation for it. Stewmac sells Mojo kits but has created their own documentation and IMHO the Stewmac docs are better. Disclosure: I have never built from a kit, always scratch built. But I have reviewed a lot of online instructions for kits.

Thanks for the info, I knew that Stewmac sold Mojo kits but never checked out their documentation. I looked for the 18 Watt TMB on the Stewmac site but didn't find it. I should good with the Mojo documents. I've built kits as well as a scratch build for a Silvertone 1483 circuit. I learned a lot on that one. So much more to learn.

Thanks,
Mark

The amp chassis if finished. I made all of the changes for the grounds, added shielded wire on the inputs, gain, master volume and treble pots. I am getting a loud hum when I max the gain pot and set the master volume at noon. When I increase the master close to max, the hum is very loud and the amp starts to squeal. I used a scope to check the grids and plates of each tube, starting at V1B, to look for any noise with no signal at the input of the TMB channel. I set the Gain and Master at maximum and used an 8 ohm dummy load on the speaker output. Everything was clean until I checked the plate of V2B (pin 1) where I found a noise signal of around 200 mVpp. V2B (pin 1) is the node that attaches to the tone stack and then the master volume. Then I checked the grid and plate on the phase inverter. There was a noise signal on the phase inverter plates of 2 Vpp. Also, I checked the B+ for the preamp and phase inverter for noise and they were clean. I included a picture of the pots and a file that shows all the voltage checks.
Any suggestions on what to do next would be greatly appreciated.

Thanks,
Mark