6v6 JTM build noise/hair on notes

[bhyatt1]

So, I just got back on to let you guys know... Today I had an idea. I plugged in a 5u4gb that I had just for shits and giggles. Noise is gone. I have tried swapping power and preamp tubes and have basically gutted this amp multiple times just to try and fix the noise and never thought of noise in the rectifier tube. I am so relieved and so amazed at my stupidity because sometimes the solution is one of the simplest. It just never crossed my mind. Now to rebias witht he 5u4 and I should be good.

[bhyatt1]

Alternate OT hooked up and no difference. I swapped the rectifier tube from a 5y3gt to a 5u4gb just because it was something I was playing with anyways... Rebiased and ALMOST thought that the rectifier tube fixed it, it seems less noticeable but I believe it was only because I seem to be getting a little more drive (I thought higher plate voltage did the opposite ) but either way I've about given up. With the amp at the driven tones I was wanting (Eric Clapton/Hendrix--y tones) it isn't as noticable. When i want clean marshall I have a 50w 1987 clone for that stuff. Kinda like just dealing with the ghosting and what not on old jtm45s. I've heard those things over the years on recordings and never had a problem with it... Why start worrying now?
That said, I am still open to suggestions...

[danman]

Sorry that you haven't been able to track down the issue yet. Have you tried posting this problem over at the Marshall Forum? Many of the members here are active over there and they may have some fresh ideas to try.

[bhyatt1]

The problem with a scope is that I do not have an isolation transformer... Which I've been told was an issue.

[bhyatt1]

Oh and before I was warned about that, I did find that the only place I noticed any thing strange in the signal was at the phase inverter... I will recheck today when I get home

[bhyatt1]

UPDATE... When presence pot is bypassed completely my problem goes away. Checked solder joints... Maybe a bad pot?

[danman]

Possibly. I think that I mentioned early in the thread about how sensitive the wire leading to the presence pot can be. I believe that you have already tried moving the wire around a bit but it couldn't hurt to look at it again. It can definitely add noise into the circuit if it couples with some of the sensitive wires leading to the tonestack. Good luck!

[bhyatt1]

I attempted using shielded wire on the presence pot, but there was no change in the issues.

[danman]

It may be the higher frequencies from the presence circuit that is causing your noise issue. You can try bypassing the nfb resistor on the board with a small cap. Try 100pf across the resistor. You can go slightly lower if the amp gets a bit dark or higher if the noise is still present.

[bhyatt1]

I have noticed something weird about the layout provided in the metro JTM kit... On the turret board at the end where the bias pot is located there is the 220k resistor and a 1n4007 diode, however I do not see anywhere that wires are connected to these turrets... Nor do I see a jumper that connects these to the circuit? Did I make a mistake here...? It seems I have but I cannot see anywhere that shows where to connect these.
I'm not saying that this is my problem, but it's something that I just noticed and want to get fixed.

[neikeel]

You will have to have the bias wire from the HT to the 220k bias feed resistor and the diode to the 15k resistor or there would be no bias.
Please take a photograph of your board and post it showing which turrets you need.

[bhyatt1]

I was mistaken... I did in fact have that connected. Once I pulled the turret board I saw where I had attached the wires.

[bhyatt1]

The capacitor added did not get rid of the problem. I'm still working with the placement of the wires, but with no luck yet. This is with a 200MHz sine wave

[bhyatt1]

https://imgur.com/rh6dpZg

[bhyatt1]

Bump before I throw this thing in the trash