Newby Mojotone 18 Watt Build

[TXFlatSix]

Good day fine folks. I took the precarious step of building a Mojo 18 watt British amp head. I got a good deal on the head cab so decided to get the chassis and small part kit from them. I bought the PT and OT from GDS Amplification. I built it based on the Mojo schematic and layout.

I have built a few Fender clone amps over the years and plenty of pedals...and guitars. So when I put this guy together I anticipated the need to troubleshoot but humbly ask for assistance with this one. Currently it powers up but I am getting no sound at all. I have swapped tubes also...all of them power up. I have gone thru and checked voltages and can post those.

I have checked it over so many times I am sure I am not seeing the problem. Worked the multimeter checking connections...checked the layout multiple times. Recently fed it a signal with a dummy load connected in an attempt to signal trace and no signal at the jack or pin 3 or 7 of V1 or V3 when testing that channel. It's like I am grounding the whole signal out.

I will attempt to attach some photos if I am allowed.

I am hoping that the experts here can see something that indicates why.

Dave

[TXFlatSix]

Gut shot

[TXFlatSix]

A few more for good measure.

[Daviedawg]

You have started in the right direction. You may have done some of what I suggest already of course. In your position I would have a very close look at the input wiring. Test for continuity from tip of guitar cable at the guitar end to appropriate valve connection and that it is not grounding. To eliminate external and jack socket issues.

Then I would suggest going to the output and checking for noise. Try a simple down and dirty test bearing in mind the high voltages you are tampering with.

With amp on and speaker connected stick your ear close to the speaker to see if you can detect any hum or other noise. If you can, then using your multimeter probe, tap at the anodes of the power valves to see if you can create noises. If you do hear noise then work back along the signal path until you are back at the input side of the preamp valves. If you are still hearing noises the problem will lie in the vicinity of your input wiring. If the noises disappear before that, you have at least isolated some of the circuit as passing signal. Alternatively you can insert signal at those nodes in lieu of tapping.

If you are getting no noise at the speaker then the issue needs to be searched for by comparing the schematic and layout you used in the build with what you actually built to try to identify it. Use a copy of both schematic and layout and mark them up as you progress so you can see what you have reviewed.

Your voltages in the form suggest that the amp should be making music.

Dd

[TXFlatSix]

Dd, thank you. I think you are on to something with the input jack wiring. I tested the tip of an input cable at the guitar end with the other end in the jack and I have continuity from the tip to the jack shield.

Attached is the layout I used.

[Daviedawg]

Not a fan of the input socket diagrams. The ground sleeve is the left side projection, i.e. the open end. You can see they are grounded to the pots. Check how you have interpreted the diagrams.

Dd

[TXFlatSix]

Dd, that was it. Rewired the inputs correctly and both channels work now.

Thanks so much.

Dave

[Daviedawg]

That's great. It's a good feeling figuring it out and sorting it.

Dd

[TXFlatSix]

It sure is.

Should I be concerned about the V4 and V5 voltages?

[Daviedawg]

There is nothing in the voltage readings that will do harm. The preamp and PI readings look good. But for classic 18 watt tone two targets are: close to 320v on the plate voltage of the EL84s (pin 7) and 12v on the cathodes (pin 3).
There must be a lot of previous posts on the topic of voltages in the power stage. If you need advice then post your schematic here.

Dd