18Watt.com

I bought a Mojotone 18w TMB combos off Reverb - builder unknown. The TMB channel was crazy bright and spiky so I opened her up and traced everything to see if I could find the issue. I found two things - see the diagram below.

1 - Two of the tone caps were swapped. Swapping the 10nf and the 22nf caps was simple and made a night and day difference.

2 - One of the silver mica caps was missing. What is the function of that cap in the circuit? Obviously the amp powers up and works, but I'm assuming it serves a purpose there...

The amp works well and sounds great (now that the tone caps are in the right place) except on the TMB channel with high volume. When I have the gain at say, 25% or higher and the MV is turned past about 50% it starts to sound like crackle and break up in a really nasty way. Is that related to the missing cap? Or do I need to look elsewhere to track that down?

Thanks!

hello,
the capacitor is a "snubber" for the phase inverter,it connects between the phase plates and it's purpose is to remove very high "ice pick" frequencies and aids amplifier stability.you can sometimes see them across the power tube plates,don't know if that is the cause of the crackle/breakup though but it would be easy to patch one across and see if it helps.
find the correct schematic for the amp and trace through the wiring and component values.

Killer, thanks for the input. I don't have one on hand so it's in the mail. I'll pop it in and give it a shot.

I traced the board layout and found those two issues, but I didn't check all of the resistor values. I suppose that will be the next step if this doesn't fix the breakup issue.

okay,
with the amp being an "unknown self build" it pays to go through double checking component values and wiring to the pots & tube sockets etc.
keep us posted with your progress and if you can add some good close up pics it sometimes helps.
good luck

Here's the board as I got it. I already swapped the two caps. Getting the 47pf cap today and installing. Played it with the guys last night and it really sounds good now.

I'll take some more close ups when I have it open again. Overall, it looks pretty well buttoned up.

Got the cap installed and that totally addressed the high-volume crackling & distortion. This thing sounds amazing now. Thanks for the help!

Hi, i have the same problem, when I up the volume and the master volume (the treble pot also too) i have a awesome fuzz sound, very compresed and broke, but the problem is that a high noise like parasite noise or ''broke'' amp noise appears. I installed gridd stoppers on the preamp but the problem continues.
Today i'm going to try this capacitor beetween the plates in the phase inverter valve... But in the electrical schemas of ceriatone TMB (trinity amps also) this cap doesn't exist.

Thank you

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