First Build Blues: Mojo JTM45

[loudthud]

Lead dress can be a issue. Make sure that it's not a bad tube and that each side of the heater measures about 3.15VAC to ground. Does the transformer have a center tap for the heater winding or are there balancing resistors?

[jazisded]

I have tried a few different 12ax7's in the 3 positions and was never able to resolve the buzz. I do have a funky tube socket in v1 where the pins are not manufactured correctly. The tube does not seat as well as it should - on one side the tube does not sit as flat as as the other. I messed with the pins, but can't get it sorted out. Should i just replace that socket? It's been bothering me since i realized it wasn't right (of course this was when i first went to power it up).

The jtm45 does have a center tap for the heaters.

Tony

[big_teee]

I have tried a few different 12ax7's in the 3 positions and was never able to resolve the buzz. I do have a funky tube socket in v1 where the pins are not manufactured correctly. The tube does not seat as well as it should - on one side the tube does not sit as flat as as the other. I messed with the pins, but can't get it sorted out. Should i just replace that socket? It's been bothering me since i realized it wasn't right (of course this was when i first went to power it up).

The jtm45 does have a center tap for the heaters.

Tony

On my 2204 build I ended up taking the Heater center Tap ground loose and using the dual 100 Ohm Res. to float the Ground.
That helped, The next thing that really helped was redoing the grounds.
I had ground loops. This conducts the AC.
You might experiment with the PreAmp Grounds.
Terry

[plexitone]

Hello everyone! I'm new here and just wanted to add this regarding your bias range: You can change the 47k to 56k or 68k in the bias circuit to give you more range to run the tubes cooler.

[jazisded]

I actually ended up changing the bias resistor tonighy to an 82k and it swung the voltages too far the other way. I then tried a 68k and it was just right. The amp sounds fantastic now. I jammed with it for the first time with a friend and did notice that my other amps are buzzing a bit - maybe it is my outlet and i just never noticed it before. The normal channel on this amp was as quiet as the fender we used tonight, but it was more prevalent in the bright channel. I'm going to try it out tomorrow in another room and see if it quiets down a bit. Getting there!

[jazisded]

last issue is going to be tracking down this buzz. I plugged into another outlet and the same noise is there, so it's probably lead dress. I'll try to get some time later in the day to check it out.

[jazisded]

I've been playing the amp on and off all day today. It's sitting on the table near my laptop. I can hear the interference from my laptop being picked up by the amp. When I scroll through a page with my mouse, i hear electrical noise through the speaker that coincides with my mouse moving.

Could this just be a shielding issue? I don't have any shielding on my cabinet. I did create a makeshift shield out of tin foil, but it didn't make a difference.

[jazisded]

I also only hear this getting picked up through the bright channel. I don't hear the same noise through the normal channel. Perhaps the frequencies are just too high.

[plexitone]

Does the shield touch the chassis when installed in the head cab? It should.

The bright channel will naturally be more sensitive to HF interference because of the bypass caps (bright and mixer bypass) allowing more high frequencies to be amplified. However, if there's a drastic difference between channels, one of those caps may have a problem. Maybe try different caps?

If you haven't already, try a different tube in V1. Also try moving the bright channel's V1 grid wire around (pin 7). It looks like it's very close to the plate wire on pin 6. Try lifting the grid wire up away from all other wires, especially the heaters and plates.

[jazisded]

I spent some time this morning and prodded around v1 with a chopstick. Crackles and pops everywhere. I swapped out the tube socket in v1 - It was definitely bad as i had previously thought (the tube was not seating all the way down). I did not have an exact matching socket so i used a ceramic one instead. The amp is quieter now, but the bright channel is still not as quiet as I would like. After the swap, prodding with the chopstick did not induce any more noise.

Mojo didnt include sheilds for the preamp tubes - i swapped some from my fender onto v2 and v3 but didn't notice much of a difference (the socket for v1 has not retainer for one). Should i acquire them for the preamp tubes?

I think i also need to make a proper shield for the bottom of my head cabinet. Should I cut a piece of sheet metal to fit the bottom of the chassis and drill it for the screw holes, or just use aluminum tape on the bottom of the cab?

[jmaf]

I spent some time this morning and prodded around v1 with a chopstick. Crackles and pops everywhere. I swapped out the tube socket in v1 - It was definitely bad as i had previously thought (the tube was not seating all the way down). I did not have an exact matching socket so i used a ceramic one instead. The amp is quieter now, but the bright channel is still not as quiet as I would like. After the swap, prodding with the chopstick did not induce any more noise.

Mojo didnt include sheilds for the preamp tubes - i swapped some from my fender onto v2 and v3 but didn't notice much of a difference (the socket for v1 has not retainer for one). Should i acquire them for the preamp tubes?

I think i also need to make a proper shield for the bottom of my head cabinet. Should I cut a piece of sheet metal to fit the bottom of the chassis and drill it for the screw holes, or just use aluminum tape on the bottom of the cab?

Most these shield tests can be done before you change anything at all. If you've got some pieces of scrap iron lying around, or even non-ferrous metals, you can put them in place, touch them to the chassis(careful so you're not touching HT with it at the same time) and move them around to see if it changes. Diode noise, heater wire noise and so on will quiet down immediately.

I use a thick aluminum foil for simple tests. For transformer noise, you'll need a magnetic conductor, such as scrap iron. I have tons of small pieces of brass sheet in a box, I've quieted an amplifier by bending one into an L and soldering the L foot to the chassis.

Noise can come from everywhere: yesterday I got my butt kicked by an amp. I'd touch a coupling capacitor and it'd be quiet as a desert. If I removed my hand it was a grinder. I thought it was a grounding issue, then after 2 hours changing capacitors, checking grounds, resoldering and checking for cold solder, I decided to take the chopstick and move the wires around. Just by re-routing the PI output to the power tube grid it was the quietest amp I've ever seen.... Sometimes it's not shielding, but just lead dress.

[jazisded]

I started troubleshooting tonight by removing v1 and powering up. Most of the noise was gone and all of the really shrill stuff was gone. Removed v2 and it made a slight difference, but not as much as v1.

I then did some tube swapping in v1 and v2 but I couldn't make a difference.

At that point, i started playing around with the lead dress in the amp. Using a chopstick, i moved around as many wires as i could to see if i could induce a change. No dice. Even with the slight improvement from changing the tube socket out a few nights back, it's still really not playable on the bright channel. Its AC noise and it's definitely 120hz.

The last 10% of any project is always the most difficult! I am so close and so frustrated!

[jmaf]

I started troubleshooting tonight by removing v1 and powering up. Most of the noise was gone and all of the really shrill stuff was gone. Removed v2 and it made a slight difference, but not as much as v1.

I then did some tube swapping in v1 and v2 but I couldn't make a difference.

At that point, i started playing around with the lead dress in the amp. Using a chopstick, i moved around as many wires as i could to see if i could induce a change. No dice. Even with the slight improvement from changing the tube socket out a few nights back, it's still really not playable on the bright channel. Its AC noise and it's definitely 120hz.

The last 10% of any project is always the most difficult! I am so close and so frustrated!

Some ideas:
- Check coupling capacitors for the presence of DC voltage, one may be leaky.
- Could be a ground loop, though your images show a good wiring job on your part(I assume you've got the star ground near the reservoir cap where the potentiometer buss wire goes).
- Excessive power supply ripple and a bad reservoir capacitor are a possible source of 120 hz hum.

Hope you get the last 10% completed soon!

[scole]

hi jaz, heres how i track noise to see if its a ground problem. take a jumper wire with aligator clips, clip one end to the chassis and touch the other to each ground point in the amp. the one with the problem will go away or change sound. just use one hand, make sure your other hand is not touching the amp.

[jazisded]

I'm still working on this, just slowly. I was able to make the noise change by moving the wires around that lead to the bright volume cap, so i shortened those as much as i could. I also removed one leg of the bright cap from the volume control and that really settled the noise down. I'd like to keep the bright cap, so i am going to work on the lead dress on the input side of the amp tonight and see if i can improve this further.