Mojotone 18 watt TMB first build, need voltage measurement help please!

[ed333]

Hello all,

I've been lurking and trying to absorb information for a short while, but this is my first post.

As my first ever amp build, I just recently completed a Mojo 18 watt TMB kit and it seems to be working fairly well. There is some buzz and hiss on the TMB channel when the gain and/or MV is turned up, and a little less on the normal channel, but overall it sounds great.

I finally got around to filling out the voltage chart, but I have a question about where to take the A,B,C, HV1, and HV2 readings, as well as the best way to check the AC voltages going in to the EZ81 and PT.

I am attaching some pics and the voltage sheet as it is now.

I seem to have a major problem in that the TMB pre tube has severely low readings on pins 7 (pin 7 is 0.2) and 8 (pin 8 is 2.3)!!!

I would greatly appreciate any comments or suggestions on how to make this amp better/quieter. Also, any clue as to why I am getting so low voltage on the TMB tube?

Thanks so much, this community is an amazing resource!

-Ed

voltage chart.jpg

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[MikeyV]

I just built one of these same kits. I also get very low voltage on those two pins.

While I've had some issues with squeal, hum, feedback, etc, I've worked to get it pretty good, and it works pretty well.

So what I'm saying is that you finding matches mine. I think that's OK.

I believe that voltage chart we are referencing is for a regular TMB amp, not the pumped up JCM800 style that we are building with the Mojo kit.

[ed333]

Ok, that makes sense, thank you!

I just finished swapping shielded cable (grounded at one end only!) for the original non-shielded wire for both the TMB input lead and TMB gain pot to 470K grid resistor lead...I'm hoping this will help cut down on some noise.

I also shortened a lot of the longer leads that were cluttering the thing up.

I'm about to take out the tubes and go through the start up process again. Hopefully I didn't screw anything up!

Cheers!

[colossal]

The voltages shown for V3 (pins 6, 7, and 8 ) on the voltage chart are based on that stage being wired as a cathode follower to drive a tone stack, hence, the high grid and cathode voltage. You guys have them wired as ordinary gain stages. Clearly, the Mojo amp is different to what is referred on this forum as a 18W TMB, and this explains the results you are getting. Please post a schematic.

[ed333]

Here is the schematic that I found from the Mojotone website:



Thanks!

[colossal]

That is basically a JCM800 preamp and a plate-driven tonestack.

This is what is referred to as a 18W TMB here. You can see that there is cathode follower driven tone stack:

18W TMB Layout.pdf

[ed333]

Ok, thanks for the info. I'm very new to this world, and this amp was my first ever build.

So, my voltages are Ok? It sounds good, until I turn the MV up past about 7 or 8, and then it gets what I can only describe as a buzzy distortion. At lower volumes, I can dime the gain on the TMB channel and it rocks!

[MikeyV]

I did the same two shielded cables, and I also moved the grid resistors right onto the tube.
Took teh 68K off of the input jack, and took the 470K off the board and mounded right on the tube pin.

This made a HUGE difference.

Mine's pretty good, but it goes all feedback-y when I get the master up over 6 or 7, that's with the gain on 10.

Let us know if the shielded cables help.

Colossal! Thanks for the explanation about the difference we see in voltages. I had figured out that the JCM800 style was different than a regular 18Watt TMB, but I didn't know why, or what to call it. Thanks for chiming in.

[ed333]

I will try moving the grid resistors to the tubes and see if it helps, thanks!

Will report back after I get around to doing it

[colossal]

I did the same two shielded cables, and I also moved the grid resistors right onto the tube.
Took teh 68K off of the input jack, and took the 470K off the board and mounded right on the tube pin.

This made a HUGE difference.

Excellent! Also, make sure sensitive grid leads (12AX7 pins 2 and 7) stay AWAY from plate leads (pins 1 and 6). Almost every Mojo amp I've seen posted had poor lead dress and this can and will cause instability and oscillation. An amp with these issues will often howl or sound thin, edgy, and strident/harsh. When the leads are properly arranged, an amp will sound quiet at idle and wide, harmonically rich, with that almost 3D reverb-without-having-reverb quality at volume. You will know when you have arrived!

[MikeyV]

OK, 2/7 away from 1/6. That makes sense.

I will try messing with this. Wouldn't be surprised to see this being a problem.

It definitely doesn't sound thin or anything, It's super ballsy.

But something is going on. At max volumes I get feedback, hiss and some hum.

I'm going to check over the power grounding to see if I can lick some of that hum too.
Thanks!

[MikeyV]

I cleared the wires, moving the grid wire away from the plate wire. V1 was pretty good, V2 I had them close. Made minimal difference.

The grid/plate wires are close on both sides of the PI tube. I need to do a little more work getting these away from each other. Not too easy, as they run quite a ways from the board terminal to the tube pins.

I'll report back soon.