Mojo760 PT definitely needs a ground reference

[sthebluesman]

This is an update on my hum issue. One of my YouTube friends gave me the fix on my video of me playing the Two-Stroke but pointing out the extra hum. For some reason the Mojo760 PT does not have a center tap that goes to ground like the Weber PT does in Dave’s book. To stop the hum, you will need to solder a 100 ohm, 1/2 watt resistor on each green wire to ground for the 6.3v heater supply. At first, I was going to put these on the 6V6 socket which is very easy but I decided to place them from the ground buss on the PT to the pilot light connections. The amp sounds great now.

[Robin]

Congrats, thanks for the update

[Andy]

Thanks for this! Good info.

I have switched to the Weber transformers for the kit. I do think that the Mojo transformers, made by Heyboer are probably the better made transformers, but the Webers will be better in the kit. They take longer to get though.

[sthebluesman]

That will make it simpler hopefully. The amp sounds amazing. I can’t stop playing it and I have a bunch of vintage Fender amps. I feel like building another one for my harp player and voicing it for harp. I’ll wait and see how he likes this one first.

[Andy]

I emailed Mojo to see if we could get them to make some 760s with the CT. I really love the Heyboer transformers that they offer. Apparently, they removed the CTs from their whole line when they added EU versions. They mentioned that the external balancing is more accurate, which is probably true but its certainly more difficult.

If we use Mojo transformers in the kits, we’ll make sure to add the additional instructions. I’m sure Hammond makes a nice transformer that would work as well. I’ve got a few Weber power transformersmon order but they take a few weeks to get them so it’s hard to get when needed.

[sthebluesman]

Yes, just put it in the instructions and use the Mojo. The only thing I was not sure of was if there was a preferred placement of the external ground. At first, I was going to just add the resistors to the octal socket and ground them to the grounding lug for pin 1 but then I thought it would be better to have it grounded closer to the PT. I don’t know if it makes any difference. Also, after talking to Gerald Weber, he said he has built amps both ways and he preferred the brass grounding plate because it was quieter. I ordered the plate from WeberVST and will eventually put it in as an experiment to see if it makes any difference.

[ChasO]

Hello,

I am currently putting my kit together that I bought here this summer just when Bluesman’s posts started. So I have one of these PT, and I am to the final stage of building the amp (ie everything is now prep’d and I am about to start the final build). I have the 2- 100 ohm 1/2 watt resistors and I was planning on soldering them directly to the top and bottom light terminals and then running to ground lines from them to the ground terminal on the PT lug. Is this all that I have to do? I will check back this afternoon before I start it up.

Thank you,

Charles O.

PS attached is a pic of the cab I built for this amp.

[Andy]

That should be it. I usually place 1 100Ω resistor on each of the Lamp terminal posts where the green filament wires attach. Then the other side of the resistors I mount to the 3 post ground terminal that attaches to the PT bolt.

I took some pictures on the last build so that I could do a full tutorial. Here is one. See image for clarification. Will save a thousand words

[ChasO]

Thank you for the info and the picture! I was wondering if I could just wire the resistors directly to the ground or if I was going to need to use some wire. Looks like they can reach the ground lug directly, that will be easier than splicing in some wire.

One last item, I just noticed something on your wiring diagram that is not in the book. On the input jack you show a wire from the center tap to the ground tap (common actually?) or the one that the center setting of the boost switch attaches to. Do I need this? It looks like the center lead is the one that disengages when you plug the patch cable into the input for your guitar so I am guessing it should go somewhere to likely reduce noise when you do not have an instrument plugged in? I will follow your newer schematic, but I am just curious why Mr. Hunter did not have that…

Thanks again for the pic that is a great help and makes me feel less nervous about doing something not on the drawing .

Charles O.

[Robin]

The input jack illustration in the book is misleading (but sorta correct if you know how to interpret it). The correct hookup for the input jack is: tip lug goes to pin 2 of V1, the shielding for that lead goes to ground on one end of the wire only (to avoid a ground loop), tape or heat-shrink the other end so it does not touch anything. The Ring lug wires to ground (the same one as the pots and preamp grounds) and the other lug wires to the Ring lug (so the input is sent to ground when nothing is plugged in) for prevent noise. The 1M resistor goes between the tip lug and the ground lug on the input jack. The 68k resistor between the input jack and pin 2 is optional, the Two Stroke design does not need it, but it won’t hurt anything the leave it there.

[Andy]

I think the schematic and the layout show the same thing, but in a different way. I could have made an error though, I do that a lot.

The center of the input jack gets wired to the sleeve art of the input jack which goes to ground. You are correct in that this is to ground the input when there is nothing plugged in.

The center of the boost switch also need to be grounded. It is convienent to use those pins on the input to ground it. Its confusing because it makes it seem like those parts are intricately related but they really are not. The center to ground on the boost switch is really the ground path for the cathode of the first tube stage. Since the switch is center off, you are either grounding the 1.5K resistor alone, or the 1.5K resistor and 25uf capacitor in parallel, or the 1.5k resistor and the .47uf in parallel.

The input wiring always seems to be a bit confusing. I’ve attached a couple more pictures to show how it is supposed to wire up. The second image shows the 1M resistors to ground for the pop supression too.

[ChasO]

Robin,

Thank you. Now that I am looking at Mr. Hunters drawing a bit closer I think I see what you were indicating about having to know how to interpret it. I am pretty good with mechanical and civil drawings but electrical drawings always make me a bit nervous probably because I don’t work with them too often.

So far this has been a fun project the only part that I have found a bit unnerving was actually putting the tweed onto the cab (I once had a bad experience with a formica top and this part reminded me of that experience ). I will post a new topic either late tonight or Thursday on how the final build went.

Thanks to everyone here at TAN for the help and all the info to look at on the site.

Charles O.

[ChasO]

Andy,

Thank you for the pics. Of course now I want to redo the wire to the boost switch to look like yours . I think though that since I have everything set for final assembly I will just add a short piece from the ring ground to the terminal with the boost wire seperatly, it might be a tight fit there now with two wires there already but I will wait to fix that until the other ends are connected to the board so they will stay in place better.

Thanks again everyone!

Charles O.

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