Mojotone Vibro Champ AA764

[Ian R. Taylor]

Hello, 1st post here. I'm building a Mojotone Vibro Champ kit and as I kind of expected with my first guitar amp kit, I've hit a snag and need some help!

Thanks in advance to anyone who wants to chime in. This has been a learning experience for me. I've built a K12 stereo tube amp kit that I use and love, and lots of guitar cables, but nothing as complicated as this kit, even though I know it is simple compared to most of Mojotone's stuff.

I'm getting hum, even with nothing plugged into it and the volume off. When I turn it up and plug in a guitar cable and touch the end, it makes sound like you'd expect so it does seem to be amplifying the signal. Aside from the hum it appears to work. If I turn the volume max or minimum is makes some higher pitched noise. When I turn up the speed and intensity coontrols you can hear the tremolo effect,(not real loud) even though there's no tremolo footswitch plugged in. If I ground the center contact (the tip) of the tremolo pedal jack then the oscillation goes away.

So far I have gone over all the soldering connections and redone some that looked suspicious and reflowed all the others. I redid the input jacks the way they are on Triode Electronics wiring diagram and that did nothing. With an alligator patch cable I tried adding a ground at several points but that didn't make the hum any different. I have studied the wiring diagram pretty extensively and I can't see any errors but obviously I've messed something up! Help!

I have a 1970 Silverface Vibro Champ and it's amazing how long (and pretty messy) some of the wires are in it and it is a really quiet amp. If anypody has any ideas I'd really appreciate any pointers. Thanks!

[martin manning]

Is the hum 60/50 Hz or 120/100 Hz? That will identify the hum source as pre- or post-rectification. The lower frequency is what you get when you touch the open guitar cable tip.
What happens if you pull the first 12AX7 (preamp tube)?
What happens if you pull the second 12AX7 (trem oscillator tube)?
The trem oscillator should be running unless you ground the foot switch jack tip i and turn it off, so it's working as it should.

[pdf64]

A voltage survey may be helpful.
VDC to the chassis common at every anode, grid and cathode, and the can cap terminals.

[Ian R. Taylor]

Is the hum 60/50 Hz or 120/100 Hz? That will identify the hum source as pre- or post-rectification. The lower frequency is what you get when you touch the open guitar cable tip.
What happens if you pull the first 12AX7 (preamp tube)?
What happens if you pull the second 12AX7 (trem oscillator tube)?
The trem oscillator should be running unless you ground the foot switch jack tip i and turn it off, so it's working as it should.

The hum sounds an 8va higher than when I touch the tip of an open guitar cable tip, so it must be the 120/100 kind.

When I ran it without the 2nd tube (the trem tube) the trem oscillation stopped, which would make sense with your info that it should be on when there's no foot sw plugged in. I didn't know that and will make up a shorting plug.

Today was nuts and truthfully it was probably best for me to take a day off from working on the amp.

I really appreciate your comments - thanks a lot!

[Ian R. Taylor]

A voltage survey may be helpful.
VDC to the chassis common at every anode, grid and cathode, and the can cap terminals.

Tomorrow the plan is to work on the amp, so I'll post the voltages as suggested, as well as trying the amp without the preamp 12AX7 installed as Martin stated.

Thanks very much, I really appreciate the advice!