First post and questions about Mojo 18w TMB Build

[gwmullins]

Hello everyone, this is my first post here. Wish I had found this forum before starting my latest build - Mojotone 18 W TMB. This build is my 6th build. The others have been various versions of Princeton Reverbs and Tweed Champs. So let me cut to the chase.

The normal channel is fine. The issues I am experiencing are with the high gain/TMB channel. When I turn on the amp if the master volume is set to anything above 0 there is a lot of loud crackling and staticky sounds. At first, it is very loud. Play the amp for 5 minutes and cracking and static sound volume become much less, but never really goes away.

This amp seems to be extremely sensitive to lead dress and I have addressed this issue eliminating squealing.

I have talked to Mojotone several times about the crackling and staticky sounds. Mostly, they wanted me to recheck the wiring to insure that I haven’t bypassed anything in the tone stack or any other coupling caps. I did this. In fact, I completely disassemble the entire amp, checked the layout against the schematic, and put it back together. During this process I found some differences in the circuit between the schematic and the layout diagram ( the tone stack wiring layout is incorrect according to the schematic, so I fixed it.) Tone stack now works as it should.

I also move some grid stoppers to the tubes and off of the turret board and onto the tubes.

I should also mention that I replaced every capacitor in the tone stack and the coupling cap between TMB channel and the phase inverter. I double and triple checked for cold solder joints throughout the entire amp. I retouched every solder joint in the amp to insure there are no cold solder joints. I moved some of the grounding points to see if it made a difference, it did not. I have changed all of the tubes, no difference.

I increased the el84 bias resistor from 130 ohms to 180 ohms to get the el84 plate dissipation down from 14-15 watts to 11.5 watts (output transformer resistance method, BTW, each leg of the OT primary was about 321/303 ohms, just seem high).

When I reassembled this amp I checked every resistor for proper resistance. Found 2 out of tolerance and replaced them.

All of the Test Point readings are within a couple of volts DC. Those readings that should be zero are within 15 mV of zero.

I know, this is getting long winded.

So, does anyone have any idea what could be causing the crackling and staticky sounds associated with the Master Volume on the TMB channel?

I am beginning to wonder if there are just too many carbon comp resistors in the high gain section for this channel to ever be quiet.

Thanks in advance for any thought, ideas, and comments.

Greg

[crgfrench]

I would use Dale CMF60 instead of CC resistors. Also maybe try replacing the MV pot.

[geoff 1965]

welcome to the forum Greg,
if you can add some good close up pics of the amps board,wiring to pots & tube sockets etc it helps.
in the meantime if you click on "downloads" then "18W technical discussion" you will see a search box,just type mojo in and all the past posts will come up and you can browse through,there's quite a few! but you might see an area with similar problems you can check out.
good luck

[JMPGuitars]

Welcome to the site!

I agree with Craig, CMF series metal film resistors are awesome, and will help with noise in general. The most important point that should be metal film is the 1M resistors on the input jacks.

The ONLY spot that benefits from the carbon comp resistors is the PI plate resistors.

With scratchy pots, I would recheck the wiring...but I'd look at the downloads section here and compare the pots to one of our schematics.

Mojotone kits are a huge thorn in our side. They're exceptionally poorly designed, and cause headaches for everybody. They can be made to work well though, so keep at it...or ditch their board and make your own based on our docs.

Lead dress is exceptionally important regardless of the amp you're building. Some photos would certainly be helpful.

Regarding grounding and soldering, please read the 2 threads in my signature.

Thanks,
Josh

[lavrgs]

The documentation from mojotone is sparce...but if you go to Stewart-McDonald they have a comprehensive set of instructions that seems to address some of the mojotone shortcomings. Good for reference if nothing else

[gwmullins]

Thanks for the quick replies/advice. I’ll get started on the suggestions.

Greg

[Bieworm]

I may have overlooked, but did you try swapping the tubes for known good ones? On my first build I experienced the same problem. It turned out to be a brand New JJ EZ81 rectifier tube. Likely not your problem for the normal channel is fine... but both preamp and powertubes will give crackling noise when faulty. I also had a problem with cheap tube sockets. One of them let one of the tube pins slip sideways and thus just touched the pin, not grabbing it.. same crackling static noise.

After having quality problems with the current generation of JJ tubes I have banned them from all of my amps. They ain't what they used to be anymore. Back in the days I was pretty happy with JJ tubes, especially when an amp sounded too bright the JJ would tame the highs.. but they are out now!

[gwmullins]

Requested photos:

[gwmullins]

A few more photos:

[gwmullins]

I’ll try a better photo of the spreadsheet.

[JMPGuitars]

Those photos are all very small and hard to see much...but I can see you would benefit from reading both of the threads in my signature regarding soldering and grounding.

Thanks,
Josh

[gwmullins]

Thanks for the comments. Recall that I completely disassembled this board and reassembled it. In the first assembly all soldering joints were wrapped as per the link. Second time assembly is when I decided to use the holes in the top of the turrets. It may not look as pretty, but all of the solder joints are solid.

I can repost the pictures in a larger format.

What in particular did you see as problematic with the grounding. FWIW, I read the articles in your link last week. One discrepancy was/is the mains share a ground point with one of the can caps. I thought about moving this can cap ground to the same grounding strip as the other can cap.

Thanks for taking a look. I’ll repost larger pictures.

Greg

[gwmullins]

Hopefully, better pics. You should be able to zoom and retain detail. Please let me know if you can’t see what you are looking for and I’ll try to get what you need to see.

Again, thanks for your time and attention. I really appreciate it.

Greg

[JMPGuitars]

The ground scheme is one of your bigger problems. Follow all the rules, including not using a ground bus across the back of the pots. That one is a big problem. Study the thread, and it will make a difference, I'm not going to repeat everything here (I'm far too lazy for that).

Next is getting those carbon comps off the input jacks. The jack resistors should be metal film, especially the 1M to ground. The grid stopper resistors should be at the tube socket side when they can be, and the wires running to them should be shielded.

Your heater wiring may or may not be contributing to the problem. Have you tried seeing if laying the heaters flat on the chassis has any effect on the noise? However, I think it's more likely that the lead dress coming from the tubes in general is more problematic. Wires shouldn't be longer than they need to be. Plate and grid wires should avoid each other, or be run at right angles. Other wires need to be positioned specifically to avoid noise.

Thanks,
Josh

[gwmullins]

Thanks Josh.

[gwmullins]

Which of the preamp section grounding layout scenarios shown in the sketch attached do you think is best/worst?

[JMPGuitars]

Which of the preamp section grounding layout scenarios shown in the sketch attached do you think is best/worst?

As drawn, #4 is the best, but #3 is okayish. The reason #3 isn't as good is because it requires a longer ground wire, which is a potential problem.

The general rule is that the ground point should be close to the higher gain channel, assuming one of the channels is actually higher gain.

However, is your TMB channel actually higher gain than the regular channel? I dunno. It's fine as drawn for #4, but I don't know if either channel is really biased hotter than the other.

Your high/low inputs on each channel are not relevant to that concept. They're still the same input entry points and share the same ground point.

Thanks,
Josh

[gwmullins]

Josh,

Thanks for your input. The TMB channel is higher gain. It figures that no’s. 3 & 4 provide the best grounding scenario. Not a lot of room between the board and the pots to drill a hole for a grounding lug without pulling the board. As soon as my metal film input jack resistors arrive I’ll make the mod to no.4.

Thanks,
Greg

[gwmullins]

Thanks everyone for your input, especially Josh for the somewhat assertive post above.

The scratchy/staticky problem is much improved. However there is still a minor issue I’ll get to in a minute.

What I have done: fixed the grounding scheme, moved grid stoppers to the tubes, replaced the signal wires with shielded wire (grounded on one end), replaced the carbon comp resistors on the jacks with metal film resistors, addressed the lead dress. This has dramatically reduced the scratchy/staticky/windy volume.

Here’s the last thing. I still get some Intermittent low volume scratchy/staticky/windy sounds from the master volume. These errant sounds will completely go away if I play through the amp for a few minutes. Leave the amp sit turned on without playing and the sounds will come back in a few minutes. Maybe also of note is that when the amp is first turned on, these errant sound will start as soon as the amp warms up and will continue no matter how long I wait to start playing. Start playing and the sounds go away.

There are no cold solder joints.

Any ideas?

FWIW, this amp is completely usable now thanks to you guys.

Thanks,
Greg

[JMPGuitars]

Glad to hear you're making progress!

Some updated photos of the build would be helpful. I'd also recommend tapping the filter caps to make sure they're okay. And of course, post your voltages.

Thanks,
Josh