Mojo Champ Kit

[SWA]

Hi.

I'm new to the forum & have just completed a Mojo Champ Kit. It went together easily, all continuity checks were positive & the test volts are all within tolerance albeit slightly high which I put down to the Sovtek 5Y3 supplied.

There is gentle transformer noise without valves or speaker connected but otherwise I have a BIG PROBLEM.....

1. Speaker connected but no valves = loud hum
2. Speaker + 5Y3 connected = unchanged loud hum
3. Speaker + 5Y3 + 12AX7 connected = unchanged loud hum
4. Speaker + 5Y3 + 12AX7 + 6V6 connected = even louder hum
5. As @ 4 with guitar connected = hum as 4. & zero guitar sound
6. As @ 5 but increasing amp volume = no increase in hum & no guitar

I've scratched my head for long enough & trawled through numerous internet debugging pages; any top tips before I de-solder all the board connections & start again?

Thanks in anticipation of help.

[Steve Conner]

Sounds like you've got the wires to the transformers mixed up... double check them... better still get someone else to check them (post a picture here?)

[SWA]

Mojo Champ Problems

Hi & thanks for the quick response.

I have attached 4 photos......

1. An overview of what the whole thing looks like

2. Detail of the bundles from the P/T
a)twisted green pair to pilot & onwards for 6.3v heaters.
b)twisted yellow pair to lugs 2 & 8 for 5.0v filament on 5Y3
c)braided reds-pure reds to lugs 4 & 6 of 5Y3 +yellow/red to ground
d)black & black/red for 240v primary-continental taps insulated.

3. Detail of the P/T to 5Y3 socket connections
a)note red wire to lug 8 from cap/red OT junction on board

4. Detail of the O/T to 6V6 socket, board & 5Y3 connections.
a)note blue O/T primary to lug 3

Hope all that makes sense when viewing the pix.

Thanks for your advice.

[tubeman]

I don't see any center tap for the filament winding on the PT. If you don't have a center tap on the filament windings, then you will need to create a artificial center tap. Use 2 100Ω resistors (one for each heater wire). You can use the pilot lamp as a mounting point. Solder one resistor on each lug of the pilot lamp, then twist the other 2 ends together, then connect that to ground. Get that fixed, then we can move on.

[Bruce / Mission Amps]

If it is not the NFB wire, am I seeing things or do you have the black and yellow secondary wires from the OT soldered to the same lug on the speaker jack?

[SWA]

Mojo Champ Problems

Hi guys & thanks for the help.

6.3v Filament Wiring

I created the usual 2 resistor virtual centre tap on the 6.3v filament wires but placed it at the other end - lugs 4/5 & lug 9 of the 12AX7 to a single ground solder tag attached to one of the tube socket/shield mounts. The continuity of the connection is good. I can use to the pilot lamp end if that is a better place.

Speaker Jack Connections

The 2 connections to the speaker jack are ....
1. the O/T + secondary
2. the negative feedback connection from the + end of the 22k resistor.
3. the O/T ground (bare)wire is factory soldered to the unit frame so grounding is direct to the amp chassis - continuity is good.

I've been re-checking everything above the populated board & I've also re-checked the continuity of the under-board 'jumpers'.......

1. from the V1a & V1b plates to the + side of the 3rd power cap
2. linking the - side of the V2 cathode bypass network to the - side of the 3rd power cap (for grounding)
3. linking the - side of the 1st & 2nd power caps (for grounding)

Still struggling so please keep the thoughts coming!

[Enzo]

An ungrounded heater line can cause hum in the preamp. But the problem here is that he gets hum WITH NO TUBES AT ALL. Clearly that is not the fault of heater grounding.

To get hum out the speaker without tubes, we either have current flowing through the OT from somewhere. or we have coupling magnetically betwen PT and OT. And looking at the photos, it appears the PT is sitting on the chassis, laminations horizontal and the OT would be outside standing on end, so I doubt we have enough magnetic coupling for trouble.

Which leads me to the OT. With no 6V6, there should be no current path through the primary. SO potentially, a short from primary to frame or to secondary would be a path for B+ current to ground, and that would result in a lot of ripple current making hum in the speaker.

SO what is your B+ voltage at the power supply first filter cap - the red wire of the OT? Then what is the B+ voltage at pin 3 of the 6V6 socket - the blue wire? They ought to be about the same. If the blue wire is substantially lower, there is a problem right there.

[Steve Conner]

But he said it hums even without the rectifier tube! I'm stumped by this one

[SWA]

Mojo Champ Problems

Hi & thanks again.

This afternoon I have 238 incoming UK volts going to the correct 240 volt, black/red Mojotone 760EX P/T hook-up.

B+ at the 1st cap/red O/T lead is 408 volts.

B+ at the V2 plate/pin 3 with blue O/T lead is 391 volts.

17 volts difference - what's the verdict? Are we getting closer?

Cheers everyone.

[MWJB]

If it hums without the rectifier, it's likely down to your lead dress around the PT.

I would move the 2x100ohms to either the power tube socket or the pilot assy.

Run the rectifier secondaries direct to the tube socket, in twisted pairs. Ground the B+ centre tap at the top right PT bolt, along with the main B+ filter cap.

Pull the twisted pair heater wiring out from under the board, secondaries run direct to pilot assy, wires from pilot assy to 6V6 heaters run straight down from the pilot. Why aren't the heaters twisted together after the 6V6?

Many of your wires are much too long. Keep the 6.3VAC wires away from grid wires & plate wires, keep grid wires (12AX7 pin 2 & 7, 6V6 pin 5) short as possible (allowing for a little flex in the board).

Checkout the pictoral build guide at tubeampdoctor.com.

[SWA]

Mojo Champ Problems

MWJB thank you.

I did not appreciate that lead length & dress could go so far as to stop any amplification of the guitar signal; I thought that it would usually result in hum plus degraded guitar but I'm getting zero guitar & no change in anything when increasing amp volume.

I'll move the 6.3v virtual tap to the pilot assembly & get the twisted runs a bit tighter & out into the open as per the TAD photos.

I've currently got the B+ centre tap tagged with the incoming mains ground on the lower right P/T bolt at the moment. Is it a better convention to leave the mains ground on it's own?

I'll check, & re-route as necessary, sensitive leads & make a number of reductions in cable length - this would go some way to explain the meagre quantity supplied by Mojo!

I'll post an update in due course but in the meantime thanks again.

[MWJB]

Whoa! I don't think that the lead dress is responsible for the complete lack of sound, just for the hum/hum with no tubes installed.

Common errors resulting in no sound are wiring errors at the input & speaker jacks.

Red wire to pin 22 of the 12AX7? Isn't that a plate wire from the 100K, it should go to pin1. I can't see where the grid wire (blue?) from the 2x68K from the input jack goes too, should go to pin 2.

Please read voltages at pins 1, 6, 3, 8 of the 12AX7. Pins 2 & 7 shoulld read 0vdc.

[SWA]

Mojo Champ Kit Problems

Hi again MWJB

I think my choice of wiring colours might not be traditional! This is what I've done at V1......

Pin 1 - Red - plate V1a to 100k
Pin 2 - Red - grid V1a to 68k pair
Pin 3 - Yellow - cathode V1a to 1.5k
Pin 4 - Green - heater
Pin 5 - Linked to pin 4
Pin 6 - Red - plate V1b to 100k
Pin 7 - Blue - grid V1b to lug 2 volume pot
Pin 8 - Yellow - cathode V1b to 1.5k
Pin 9 - Green - heater

I'll recheck the volts on the pins tomorrow & post the results.

Bye for now & thanks again.

[Enzo]

OOps, you are right, Steve. Without the rectifier, my B+ to ground theory doesn't work.

[MWJB]

Connections to V1 look OK from your description, keep to one colour for grids, another for plates, another for cathodes - you have grid wires that are blue & red, it makes visual troubleshooting difficult.

Also, having the wires come out the back of the eyelets makes routings hard to follow, next time you build, have the wires to tube sockets exit the front of the eyelets, so it is clear where they connect.