Need help with diagnosing a Mojotone 18w build

[_ej_]

I have a Mojotone 18watt TMB kit I built. The core tone sounds excellent, I've wired V2b as a cathode follower and prefer the gain level to the stock circuit quite a bit.

I'm having two issues, a lot of noise/interference particularly on the clean channel and a static sound that comes though when playing.

I understand that moving the grid stoppers from the input jacks closer to the tube sockets may help with the radio noise and interference. Any other tips would be appreciated. I've already tried improving the ground scheme and shielding the wires from the input to the tubes.

The static/buzz is a mystery. It's particularly noticeable on the decay of chords/notes. Its really hard to capture on mic but its apparent in the room. Happens with all my guitars/cables/cabs. I thought it was happening on both channels but I can't really hear it on the clean channel right now. Once I have a chance to crank the amp I'll try to recreate it at higher volumes.

Any thoughts on what it could be? Here are a few clips. The first one is from a while back before the mods to the preamp and the noise is a lot more noticeable. The second is from tonight, I'm running a compressor so you can hear the interference from the clean channel clearly and you can still hear the buzz and bit on the gain channel.
https://on.soundcloud.com/K83aD2H8nAkjQe6UA

https://on.soundcloud.com/ktpXezAAySaNeeCq8

Edit: here are better gut shots




So the buzz sounds like a tube rattle or buzz that blooms as the chords fade out.

[Stevem]

There’s a few possibilities here for that issue, but the first one that comes to mind is due to your change to a cathode follower.

With this mod now the PI section is getting / reviving the wrong phase of input signal to it.

There two ways to check this out and deal with it.
Either swap the outputs of the PI section to the primary of the OT, or swap the output of the OT ( secondary ) at the power tubes.

If I am wrong on this there will be no harm to the amp, but when you power it up it may scream like a Banchee out of the speaker so be ready for that possibility.

Post back how it goes.

[_ej_]

I'll give it a shot but it's had this problem since I first built the stock circuit.

[Mac Dillard]

I would check for bad solder joints.

[pdf64]

Schematic, amended with your mod?

Better photos are needed, the lighting is good but the resolution is kinda blurry, it doesn't support zooming in well.
I suggest to get 1/2 the chassis in each shot, with at least 2 of each half, taken from from different angles.

Are the speaker output socket shield lugs referenced to the chassis?

[Stevem]

Remove your input jack from the chassis to get it as far away from that first pot and the circuit components as possible, does that help at all?

[cdemike]

How is your bias set for the output section? The way you described it in the first post makes me think of crossover distortion.

[Stevem]

It’s a cathode biased output stage in this kit.

[cdemike]

It’s a cathode biased output stage in this kit.

“Set” is a bad choice of word; so to rephrase, is the bias condition within appropriate range?

[alnight]

I second checking the solder joints. Also, any chance you bumped one of the coupling caps with the iron during assembly? From your vid it sounds like some sort of connectivity/continuity issue to my ear.

[_ej_]

Thanks for the input guys. I'll take some better pics and proper clips tonight.

I've been over the amp previously looking for bad solder joints and didn't see any issues but I'll give it another check.

Here's some more information that I probably should have included in my first post.

Firstly these are the voltages. They were measured before I performed any modifications. My B+ voltage is a bit high at 354V. Heaters are 6.5V.

V1:
1 - 125V
3 - 0.7V
6 - 132V
8 - 0.6V

V2:
1 - 195V
3 - 0.7V
6 - 180V
7 - 30V
8 - 65V

V3:
1 - 195V
2 - 33V
3 - 65V
6 - 180V
7 - 30V
8 - 65V

V4:
2 - 0.09V
3 - 11V
7 - 341V
9 - 312V

V5:
2 - 0.08V
3 - 11V
7 - 343V
9 - 312V

Here are the mods I've done to the circuit so far:
-I swapped the input wires to shielded wires.
-I changed preamp pot grounding to a star ground.
-I added a 220k grid stopper to v2b (I probably should have removed this when I did the cathode follower mod and likely will remove it tonight).
-Zener Clamp Mod to the power amp to "prevent bias excursion" (Another hobbyist amp builder friend of mind recommended this).
-Finally I've converted v2b to a cathode follower to reduce the noise and gain and make the TMB channel usable.

I've attached the schematic as it stands today with all the mods.

2024-10-14 09_57_31-British_18W_TMB_SCH.pdf - Nuance Power PDF Advanced.jpg

You can find the original schematic here:
https://www.mojotone.com/British-Style- ... lifier-Kit

[R.G.]

I've had better luck just remelting every joint with a whiff of rosin cored solder. It's mindless, true, but highly likely to eliminate any issues.

[pdf64]

Is the chassis properly grounded?
I can't see anything connected to the centre lug of the mains inlet, hence the request for more and better photos.

[_ej_]

Is the chassis properly grounded?
I can't see anything connected to the centre lug of the mains inlet, hence the request for more and better photos.

Everything should be grounded, I've been through it a few times. I'm fairly confident that mains ground pin is connected directly to the chassis via the screw right next to it. I'll double check when I'm back home.

[dorrisant]

Check the connection at the wire that goes from the common of the impedance switch to where it connects to the output jack. I don't see solder there.