6v6 JTM build noise/hair on notes

[bhyatt1]

After some research, I decided to attempt upping the filtering from 32uF total, to 50uF (utilizing a good NOS 5y3gt)... No dice. I still think I'm looking for a bad solder joint. Unless the 1Mfd capacitor thing might still work.

[bhyatt1]

As of today, I have completely removed the board and am planning on reflowing all turret joints thoroughly before replacing and resoldering the board back in. Through deductive reasoning I have come to the conclusion that it must be a bad joint SOMEWHERE.

[danman]

The only cap that you need to be careful of going to high on would be the first one. The following caps have some resistance between them which limits the current draw and protects the rectifier tube. The first cap is like a dead short to the rectifier until it begins to charge up and can place excessive strain on the rectifier if you go to large. All of the caps that follow will have a dropping resistor between them which limits the flow of current. You have probably ruled it out with your test of the 50uf main cap though.

Another area of the circuit that some builders have mentioned causing your problem is the filament wiring. I have never experienced it myself but I have read of noise radiating into the preamp from the filament wiring. If you used the standard Marshall style filament wiring scheme then I would doubt that this is causing your problem. It couldn't hurt to try making some slight adjustments and see if it helps though. I also wonder if there may be an issue with the particular OT you used. It's rare for a new OT to exhibit this problem but I suppose anything is possible. Would you happen to have a spare OT laying around with similar specs that you could clip in temporarily for a sound check?

[bhyatt1]

I actually do... I had another for this build but used the deluxe reverb transformer due to the 138mA vs 90mA specs. Other thing to consider was only 3A for filament current for rectifier. Which maybe I could do with a 5u4 instead of the 5y3gt im using? I'm not confident enough to answer that question...

[bhyatt1]

I went through adjusting the filament wiring first thing... I tried elevating the wires, running them to the back of the chassis etc with no change. I may clip in the other transformer Sunday and see what I can find then. Maybe tomorrow night if I'm not too spent from work

[danman]

If your PT delivers 3 amps on the filament winding you will be safe to use a 5u4 if you like. If it's only a 2 amp winding on the PT, the the 5u4 would be pushing your luck.

[bhyatt1]

So... Clipped in the other PT today. Same issue. I didn't believe it was a transformer issue, but you have to check everything. I'm just going to pull the board out again and redo the whole thing... Resolder all tube sockets EVERYTHING. However, I do think I prefer the older stancor iron to the hammond, so it wasn't a loss to have to do that. The Stancor had a rounder more growl-y sound. It is only rated @ 90mA compared to the 138mA of the Hammond so I guess maybe there is a little more saturation happening which was the whole idea and vibe of doing a lower wattage JTM to begin with. I'm about to go into pulling everything apart here shortly and will update when I get some more done.

[danman]

I was suggesting a quick test with a different OT. Was it a different OT or PT that you tried?

[bhyatt1]

Oh... Sorry I completely misread your comment. I tried a different PT. I may grab another OT just for testing... If it ends up not being the OT I will use it for another build. I'm using an Allen Amps To26 that I ordered a few months ago. Do you think incorrect wiring of the transformer would cause an issues like this?

[danman]

Sorry about the confusion. I was wondering why you had asked about the rectifier and filament winding but I didn't catch the fact that you were trying to swap in a different PT. I doubt a wiring error with the OT would cause this issue. There is the chance that the polarity is flipped and the circuit is getting positive feedback instead of negative feedback. This usually shows up as a loud howl from the amp when you first power up. I did have one build where the polarity was reversed and instead of a loud howl, the amp just started making odd noises at higher volumes. Since it didn't howl I assumed that the nfb was good and I spent several days trying to find the problem. On a whim I decided to swap the OT primary wires at the sockets and the problem went away. Not sure if you have tried this yet or not. A quick and easy way to test is to simply disconnect the nfb wire from the output jack. If the problem disappears with the nfb disconnected, you now know there is a problem within the nfb circuit. Switching the OT wires will normally fix the issue but there is always a chance of a wrong resistor value in the nfb circuit that could cause a problem.

I was suggesting earlier that you may want to clip in a spare OT (anything with similar specs) just to rule out the possibility that the original OT was causing it. It's very rare for a new OT to have an issue it would be nice if you could rule that out before tearing the amp apart for a rebuild.

Is there anything in this build that is different from a stock Marshall such as a PPIMV or other mods?

[bhyatt1]

No ppimv at this point... Nothing outside of stock jtm45 besides the 6v6s and a higher 22k tail resistor on the phase inverter. Wanted to wait on the ppimv because I wasn't sure I'd even want to use one here... And, actually, upon first fire up I already had to reverse the transformer wires due to howling.

[bhyatt1]

Okay, so after rewiring the board, the noise I had is no longer there for the most part... HOWEVER there is a little fizz/buzz unless the presence pot is at 60% or higher.

[danman]

Glad you got most of the problem solved. Maybe some careful changes to the lead dress will get rid of that last bit of fuzziness. Some shielded wire may also be helpful if you haven't tried it yet.

[bhyatt1]

Still no dice...Man I'm seriously about ready to set this thing to sail out the bleeping window. ANY FURTHER ADVICE WOULD BE GREATLY APPRECIATED. I've got some images of the noise on oscilloscope, but I don't know what to do with this information from this point on, nor where to look next.

[glpg80]

You signal inject and start from the beginning until noise is found on the tone. If it’s everywhere then that tells you it’s in the supply. So then you scope the supply to see from the wall inward where it is coming from. If the wall is clean but your DC isn’t then try chop sticking to see if the noise changes based on wiring location or component.

I’m wondering if it is the rectifier itself or wiring.

With a scope you can definitely track it down further. They are very powerful troubleshooting tools.