Mojotone AA764 Vibro Champ hum problem

Ian R. Taylor

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I got the Mojotone kit assembled today and it makes the right sound when I touch the end of a guitar cable plugged into it, but there's a hum problem. It is there when there's nothing plugged into it.

I tried reflowing the solder connections. I tried swapping the speaker polarity, using another speaker (I have the amp setup for an 8Ω speaker and was using a 4Ω speaker), and used alligator patch cables to add a ground to different places. I rewired the input jacks to the way Triode Electronics has on their wiring diagram. I redid several wires in hope of getting rid of the hum but I need some help from experts.

You can hear the tremole softly when there's nothing plugged into the footswitch jack. That goes away when I ground the center (tip) contact on the footswitch jack.

I've built a K12 sterao tube amp and lots of guitar cables but this is my first guitar amp build. I'm really looking forward to hearing this amp! I built a cabinet and have a Celestion G12H for it.

I've read everything I could find online, including some very good threads here of similar Vibro Champ builds. If anybody has any pointers I'd really appreciate it.

Thanks!
 

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Wrench66

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I've heard that single ended amps, such as the champ, will hum a little when turned up. My 5F1 hums a little when turned up. Could this be what you are hearing? How does it sound when playing?

There are many people here with more knowledge and experience than me that will chime in here I am sure.

Edit: Check the power tube bias. The 470 ohm cathode resistor isn't enough for modern wall voltage. I remembered mine was biased to hot and the hum subsided somewhat when I cooled the bias.
 

Ian R. Taylor

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Thanks Wrench66.

The hum occurs even when the volume is full off and nothing plugged in. I am running through a variac that I set on 90V when I play, but the hum occurs at 60-120V, it just gets louder the more I turn the voltage up of course. For all my testing I'm running the variac at 120V.

I don't play guitar, I play keyboards - but not in the way you're probably thinking. So since it's a synth, the signal is super clean. In my studio, my 1970 Silverface Champ is so silent you can't hear if it's on, even at 10. I usually play at 9 1/2, the last half doesn't get any louder, just harsh. The SF Champ has lots of original caps, it badly needs a tune up and there's way too much extra wire everywhere, so it's amazing it's silent. I can't tell if it's on by hum.

I'm using new JJ tubes in the kit plus I tried all new EH tubes as well, all work fine in the SF.

I bought a Klein MM325 meter for $36 at Home depot yesterday and will attempt to follow a D-Lab video to check the voltages.

One thing I should mention is that I did run out of the supplied yellow cloth covered solid wire, so I substituted a piece of yellow stranded wire for a ground wire , a remnant from the twisted yellow pair that goes to the rectifier. I've attached a pic, the yellow arrow points to the stranded, substitute wire. It seems unlikely that that would cause hum but I'm trying to think of anything I might have messed up. The green twisted pair wires look like they are touching the red twisted pair but they aren't, I just moved them for the photo. I tried moving anything I could with a wooden stick to see if it changed the hum but didn't find anything.
 YELLOW WIRE.jpg
.

Thanks, I appreciate your comments.
 

Wrench66

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I would check voltages and post those here. I’m no expert, but the only thing I can see thats problematic would be the 500 ohm cathode resistor. That is probably the stock value from when the amp was originally in production and the wall voltage was around 110 VAC. Modern wall voltages are now routinely in excess of 120 VAC and would run your power tube a little to hot. I would recommend checking the bias, if you know how. There are plenty of youtube vids that describe the procedure.
 

Ian R. Taylor

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Also, can’t see it in the pics, are the input jacks grounded at all?
I had the input jacks wired re the Mojotone wiring diagram, and then switched it to the wiring diagram that Triode Electronics has. I've attached a pic, the red arrows point to a brown wire that runs to a ground rail soldered to the backs of the pots. The rail and the chassis have good continuity to the ground wire on the power cable. I guess the ground rail is extra grounding as the pots are actually grounded to the chassis by how they are mounted.

I always use a variac outputting 90V when I play, but for testing/building the amp I set it to 120V. The hum is the same at 60V or 120V, it's just louder at 120V. When I played through it for fun I can drown out the hum (sometimes) but it isn't just a bit, it's a "something's wrong" hum. My worn out '70 SF Vibro is dead silent. I can tell that other than the hum the amp sounds very good. I've tried various cables, I use Mogami 3 wire with the shield lifted on one end so it's not cables or tubes. And because I don't play electric guitar (quick -duck!) my synth signal is very clean.

Thanks for your help!
GROUNDING.jpg
 
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Wrench66

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Definitely sounds like something is amiss. Typically, hum is caused by bad filter caps or grounds.

We’ll just have to wait for more knowledgable folks chime in.
 

Ian R. Taylor

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Yes, it's something I messed up. It was so hot and muggy today I couldn't work on it, my workshop is outside but covered so hopefully it won't be raining tomorrow.
I'm going to spend some time on this, a great video from Terry on measuring voltages:
 

D'tar

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Humm... Whats the frequency Kenneth? :naughty: 60Hz/120Hz?

I don't see a heater center tap. Can we see the whole circuit? Using your ohm meter here is a quick check to see if the inputs are functioning as designed.

Edit: I see the 1st post has the whiole circuit photo.. Thanks.... Still cant see the heater center tap.

1660744097052.png
 
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Soul Tramp

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Try these steps:
  • Pull V2 (vibrato 12AX7). Is hum still there?
  • Ground pin 2 on V1 (input jack 12AX7). Is hum still there?
  • Ground pin 7 on V1. Is hum still there?
  • Ground pin 5 on the 6V6. Is hum still there?
 

Soul Tramp

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I don't see a heater center tap.

If you don't have a virtual CT for the heaters, you should. Whatever you have for instructions should address this. It reduces hum.
 

Ian R. Taylor

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Humm... Whats the frequency Kenneth? :naughty: 60Hz/120Hz?

I don't see a heater center tap. Can we see the whole circuit? Using your ohm meter here is a quick check to see if the inputs are functioning as designed.

Edit: I see the 1st post has the whiole circuit photo.. Thanks.... Still cant see the heater center tap.

View attachment 628571
Thanks Don, here's what I've got:

The frequency I believe is 120Hz, it's an 8va higher than the lower sund I hear when I touch the end of aguitar cable plugged into it.

With the resistance tests you posted the inputs appearing switched, I get the following Ω readings (with new Klein MM325 meter):

tip input 1 to 1st 12AX7 pin 2 = 64k
tip input 2 to 1st 12AX7 pin 2 = 33.8k
tip input 1 to chassis = 118k
tip input 2 to chassis = .947k
sleeve input 1 to chassis = 0Ω
sleeve input 2 to chassis = 0Ω
 
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D'tar

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tip input 1 to 1st 12AX7 terminal #2 = 64k
tip input 2 to 2nd 12AX7 terrninal #2 = 33.8k
tip input 1 to chassis = 118k
tip input 2 to chassis = .947k
sleeve input 1 to chassis = 0Ω
sleeve input 2 to chassis = 0Ω
Those are all good results.

I use my android tuner app to determine frequency to be sure. Can you verify you have a heater center tap installed? You should have 2x100ohm resistors... one from each filament leg to ground. Shown mounted to the pilot lamp on the mojo layout.
 

Ian R. Taylor

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Yes, there are two 100Ω resistors as you described from the green filament leg wires to a grounding lug on the power transformer. Initially I wired it as in the Mojo diagram and in an attempt to rid the hum changed it to the wiring Triode has for their kit which seemed simpler and still has the two mentioned resistors.

Here's some test results from what Don posted (thanks!):

pulled V2 = hum got louder and wider bandwith (more treble), I did this with a dummy shorting plug in the trem jack. When I pulled the shorting plug out the tremolo speed and intensity seem to function, as they did when I played my synth through the amp. There's a high pitched buzzing sound when the intensity knob is maxed, and it's best when the intensity if full off.

There was no improvement when I grounded pin 2 of V1, pin 7 of V1 or grounded pin 5 of the 6V6.

Here's Mojotone's schematic: https://www.mojotone.com/Amp_Kits/Blackface/Vibro_Champ_SCH.pdf

Mojotone's wiring diagram:

Triode's wiring diagram (how I wired my input jacks):

I don't recall seeing anything about requiring to add a center tap in Mojotone's documentation. Are the two 100Ω resistors from the filament legs to the grounding tab the center tap? I'm guessing they are.

Next I will start going through D-Lab's video and take notes...

Thanks everybody who has posted, I really appreciate your time.
 

Ian R. Taylor

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Yes, I believe it's done as in Mojo's diagram. I will be using an 8Ω speaker when put in the cab. For testing I'm using the 4Ω speaker from my SF Champ. I've tried swapping the polarity on the speaker to see if made a difference. The yellow wire, heatshrinked and tucked under the jack, is for 4Ω operation. So I do have it hooked up for an 8Ω and I'm testing with a 4Ω speaker... I've heard of lots of people doing that and never heard of problems. I've tried moving the yellow wire out of where it's tucked and it made no difference.

Thanks, and here's a pic of the speaker jack:

SPEAKER JACK.jpg
 

Soul Tramp

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It looks like you don't have the speaker jack properly attached to ground. Try running a wire from the jack ground lug to the chassis ground next to the power transformer.

Since grounding the 6V6 grid didn't eliminate the hum, it is being produced in the poweramp/powersupply circuit.

Do you have another 6V6 to swap in?
 

Ian R. Taylor

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I just tried swapping a JJ 6V6 (less than a month old) from my working SF Champ into the kit amp and that didn't do it. Unfortunately running a ground wire from the power transformer ground lug to the speaker jack didn't do it either.
The kit came with Electro-Harmonix tubes, which I'd wanted to save and use the JJ's that I have in my working SF. When I had hum I put all the new EH tubes that came with the kit in. I just tried the NOS RCA 5y3 from the working SF Champ in the kit to no avail. So I have done a complete tube swap from a very quiet working Champ so it wouldn't appear to be a tube problem.

Thanks for your help.
 

D'tar

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Try connecting the unused 20uf tab on the cap can to the 20uf B+1 tab. This will put the 2x20uf parallel for 40uf and increase filtering. Can be temporary jumper. Any change in hum?
 

Ian R. Taylor

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Thanks D'tar. I need to clarify because the supplied Mojotone cap can does have a marking called "B+" In the pic below, the tab circled in blue would be the unused tab... do I run the alligator jumper from that tab to the tab with the red "2" written on it?
CAP.jpg
 

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