HOW DOES THE MOJO 18 W TMD LAYOUT LOOK TO YOU? EXPERT OPINIONS NEEDED.

[ViperDoc]

I built up a Mojo 18 Watt TMD combo a couple months ago and it sounds great on the clean channel, hisses like a pissed cobra on the 800 channel when the master is turned up, and it smells like something is burning slowly. I thought I'd post the layout ans schematic and see what y'all think. Here they are:

https://www.mojotone.com/site/TT_SB2_Fi ... 14%202.pdf



Here's a link to the build manual: https://www.mojotone.com/site/TT_SB2_Fi ... :05:14.pdf

I think this might need some work, but what should I do? Thanks!

[drew]

Post high resolution photos and voltage measurements. Builder error is far more likely than “something is wrong with Mojo’s layout” error.

[ViperDoc]

Post high resolution photos and voltage measurements. Builder error is far more likely than “something is wrong with Mojo’s layout” error.

Fully agreed, drew. I learned from a fellow TAG member that several other 18W TMD builders using the mojo kit had the exact same results as reported on the 18Watt forum. I'll have to break out the amp and photograph it after my current project is done, but that's a good idea.

[pompeiisneaks]

Post high resolution photos and voltage measurements. Builder error is far more likely than “something is wrong with Mojo’s layout” error.

Fully agreed, drew. I learned from a fellow TAG member that several other 18W TMD builders using the mojo kit had the exact same results as reported on the 18Watt forum. I'll have to break out the amp and photograph it after my current project is done, but that's a good idea.

Yeah from what I read on a thread recently I'd say there's a high likelihood there are errors on the mojo stuff

~Phil

[Colossal]

There definitely have been some errors discovered in Mojo layouts of late. And some of them can result in significant struggle for the builder!

[sluckey]

I've just compared the schematic and layout. The schematic is correct. And the layout is in agreement, with two very minor exceptions... The tmb input jack is wired just slightly different from the schematic, but it is of no consequence. The tmb bass control is wired just slightly different from the schematic, again it is of no consequence. I don't like the fact that the preamp ground connects to chassis via all the pot bodies, and I really don't like the fact that the reservoir cap ground is attached to the mains safety ground rather than the PT HV center tap.

The tmb channel should hiss, especially when comparing to the normal channel. All components contribute to noise (hiss), especially resistors and tubes. Just the way it is. Noise is directly proportional to gain, and the tmb channel has a lot of gain. You can greatly reduce the hiss by wiring V2B as a cathode follower. Then you would have a tmb channel that resembles the plexi structure. Still gonna sound good, just not like the later high gain JCM 800 structure.

Mojo clearly states that their tmb channel is based on their high gain jcm 800 preamp. OTOH, the popular 18W TMB amps developed over at 18W.com are based on the lower gain plexi or JTM/JMP series amps. I suspect that the people complaining about a hissy mojo tmb amp just aren't aware of these facts.

I've attached the popular Richie TMB schematic so you can see for yourself...

[drew]

I learned from a fellow TAG member that several other 18W TMD builders using the mojo kit had the exact same results as reported on the 18Watt forum.

Including the part about smelling like something is burning?

[Colossal]

I learned from a fellow TAG member that several other 18W TMD builders using the mojo kit had the exact same results as reported on the 18Watt forum.

Including the part about smelling like something is burning?

I think that may have been 'startup smell' of a PT cooking off some varnish, but several guys have been having a very tough time with TMB kits specifically with squealing and trem channel issues. Mojo did not specify or included shielded wire when it is needed, poor ground scheme, missing or erroneous parts, and other QC issues.

[ViperDoc]

I've just compared the schematic and layout. The schematic is correct. And the layout is in agreement, with two very minor exceptions... The tmb input jack is wired just slightly different from the schematic, but it is of no consequence. The tmb bass control is wired just slightly different from the schematic, again it is of no consequence. I don't like the fact that the preamp ground connects to chassis via all the pot bodies, and I really don't like the fact that the reservoir cap ground is attached to the mains safety ground rather than the PT HV center tap.

The tmb channel should hiss, especially when comparing to the normal channel. All components contribute to noise (hiss), especially resistors and tubes. Just the way it is. Noise is directly proportional to gain, and the tmb channel has a lot of gain. You can greatly reduce the hiss by wiring V2B as a cathode follower. Then you would have a tmb channel that resembles the plexi structure. Still gonna sound good, just not like the later high gain JCM 800 structure.

Mojo clearly states that their tmb channel is based on their high gain jcm 800 preamp. OTOH, the popular 18W TMB amps developed over at 18W.com are based on the lower gain plexi or JTM/JMP series amps. I suspect that the people complaining about a hissy mojo tmb amp just aren't aware of these facts.

I've attached the popular Richie TMB schematic so you can see for yourself...

There's hiss, and then there's all out screaming. I have the latter when the master is turned up. Like needle in the devil's ear squealing.

Regarding a V2B cathode follower, yeah, I have no idea what that is. WELL, I know where the V2B cathode connection is, but how would you change the circuit?

Thanks for the other TMB SCH. I'll take a look tonight.

[sluckey]

Look at the schematic I posted. It would be very easy to convert your hissy mojo to a growling "normal" 18W TMB.

[sluckey]

Here's an easy way to convert using your existing layout. This converts V2B into a cathode follower...

convert4.png

[ViperDoc]

Here's an easy way to convert using your existing layout. This converts V2B into a cathode follower...

convert4.png

WOW, thanks for putting that together. I have several questions, but I'll check out the three drawings and let it soak in a bit. Much appreciated!

[ViperDoc]

Here's an easy way to convert using your existing layout. This converts V2B into a cathode follower...

convert4.png

One of these days this may, perhaps, become somewhat intuitive. So the cathode follower sends the unfiltered V2A plate voltage straight into the V2B grid. I noticed also the V2A cathode resistor is much lower in value than the original circuit. How does that make the cathode follower "low gain"?

[sluckey]

It's all simple math...

V2B is the cathode follower. A cathode follower has a gain that's less than 1, typically .8 to .9. Converting V2B from a gain stage to a no gain stage makes the overall gain of the preamp much less.

Let's say for example that the orig mojo circuit has a gain of 20 (not actual measured numbers) for each of the three triodes. Total gain would be 20 x 20 x 20 = 8000.

Now change one of those stage gains to 0.9. The total gain is now 20 x 20 x .9 = 360. This is the the basis idea.

But in reality the stage gains of the original mojo are a bit difference. In fact the stage with the 10K cathode resistor is called a starved gain stage so the stage is probably on the order of 5 to 10. So, let's say two of the gain stages are 20 and one is only 5. (Again, these are not real measured numbers). So the total gain is 20 x 5 x 20 = 2000.

The mods I propose make the preamp look like a plexi preamp. The second gain stage has been changed from a starved gain stage to a normal gain stage (10K cathode resistor changed to 820Ω) and the third gain stage has been changed to a cathode follower. Now the total gain is 20 x 20 x .9 = 360. Much lower.

As illustrated, the mod is very easy to do. And it's just as easy to undo. I'm betting that if you do the mod you will not undo it.

[sluckey]

I may have jumped the gun on suggesting to change the mojo hi-gain preamp to the plexi style preamp. If you really like the mojo as is except for the hiss and screaming, then try this very simple mod. Add a 470K and optional 470pf (500pf) in series with the top of the volume control. The real JCM-800 2204 has this to tame gain a bit. I believe the mojo circuit would benefit from having these components also, and still maintain that hi-gain overdrive. It's a 5 minute mod, very easy to do, and easy to undo. Take a look.