6v6 JTM build noise/hair on notes

[bhyatt1]

I have recently built a JTM45 with 6v6 tubes and a deluxe reverb style (Hammond 290bx) transformer. All seems well except for this hair on notes that sounds almost like a buzzing speaker. I have tried other amps on the same cab and the same head on my other cabs and the noise is there each time this head is plugged into anything. I know the first thing would be bad solder joint. I have consistently resoldered and even completely redid the preamp tube wirings. I've reflowed solder joints to every component on the board. I have checked solder joints on Presence pot and NFB resistor. I have swapped preamp tubes as well. The noise is there regardless of channel and I believe that it is in the Phase inverter or later. Any help would be greatly appreciated as I am about to throw this thing out a window lol. It sounds great other than the slighy fizz/buzz noise... It is so light that it would not be noticed at gig volume but it is something that should not be there and I need to find the source and be enlightened as to why this is happening. Any information needed, please don't hesitate to ask. I will try to get some voltages posted soon.

[danman]

In cases like this, a careful adjustment of the lead dress to the sockets and pots can help to clear up that hash on the notes that you are hearing. Pay extra attention to the wire that leads from the board to the presence pot. The Metroamp instructions tell you to run it along the bottom edge of the board until it reaches the pot and then turn directly towards the pot. In many cases this hash that you are hearing is causes by oscillations which can be cured with careful lead dress.

Did you install grid stoppers on pin 5 of the power tube sockets during the build? Anything from 1.5k to 5k will help.

[bhyatt1]

I will get back with you when I get off work, but I may have added the grid resistors, I know it crossed my mind but can't recall if I did and especially not sure what value I used if so.Iwant sure what value to use with the 6v6 or how to know what value. I will also try to make some adjustments to wire routing to see if this fixes the issue.

[bhyatt1]

I did in fact have grid stoppers added, but I added the plexi values of 5.6k... I read from a few sources that these could be even higher without any affect on the high end and that some amps used higher to get rid of the harshness, so i went with a 47k to see what affect that would have. The noise is still there. I also attempted to rearrange the wires so that the grid wires and the secondary leads were not running in parallel. I don't have the most room in this chassis but I did what I could and no change. will continue to try and move things around but I may have to redo some wires to longer length just to be able to rout them further away from each other.

[bhyatt1]

Position of purple presence/nfb wires seems to make no difference. My wires from OT and PTare very close due to where they are positioned, maybe I should lengthen some wires and run around the perimeter of chassis to avoid proximity?
This whole thing was done in a JTM chassis from valvestorm, so I know this amp has been done before in this layout. However the way that the wired exit the Hammond power transformer is not ideal, to be honest.

[bhyatt1]

https://photos.app.goo.gl/ycYTqgyKNJrQzT51A

[danman]

What type of silver mica caps did you use or did you go with ceramics? I ask because several builders have reported issues with the generic black silver mica caps that so many places sell. They usually cause more of a static or crackly sound when they are defective though.

If rearranging the lead dress doesn't seem to help, it may be necessary to look at each stage on a scope to see where the noise may be creeping in.

[bhyatt1]

I used silver micas as from valvestorm., not sure if these are the same you speak of... I think a friend of mine has an oscilloscope but I have never used one myself

[bhyatt1]

I have used these same caps before on previous builds (jmp50 and jcm800 2204) but it could always be those, still... I may swap a few and see if that might fix it.

[bhyatt1]

I replaced all of the silver mica caps on board with cornell dubilier caps... No change. I resoldered everything last night... No change. Going to get an oscilloscope on it hopefully tonight to see what I can find. That said, I'm very much a newbie using an oscilloscope but I will hopefully be able to identify the issue if something is oscillating.
Oh, and I disconnected the NFB to see if they noise was still there and yes, it is.

[glpg80]

Be very, very, very careful with an oscilloscope. I recommend an isolation transformer for the amplifier if you’re using a scope to monitor an injected tone.

You have to watch out for ground loops which will ruin the scope. The occur from the clipped ground of the scope.

If you use a scope, get an iso transformer for your bench and isolate the chassis you are working on. Think of it as being analogous to a multi-meter then. Without the tech isolation transformer, you run the risk of damage to the scope.

I have a feeling the hair on top of the notes could be related to ripple on the caps for B+. If you have any snubber caps used and you’re using B+ as small signal ground, then a bad cap could allow feedback into another part of the circuit.

[bhyatt1]

I do not have a ground clip, and I'm still learning how to use the scope so I did not feel comfortable using it yet, anyways. If I do it will be under the careful direction of someone that can walk me through it. Mine wasn't very expensive, however it would be a loss to ruin it before I have the chance to even learn and utilize it as a tool.

[bhyatt1]

All cheap caps have been replaced with good CD mica caps and I wired the phase inverter filter cap up for more filtering (32uF vs 16uF) to see if that fixed the issue... Still have the same issue. Reflowed all solder joints, yet again. Now, I am stuck here with no idea what to try next. I have a feeling I'm going to keep resoldering and maybe it will eventually fix the problem. I pretty much assume that it's always a solder joint. I'm just getting tired of messing with it at this point lol

[glpg80]

No better time than now to learn how to use a scope!! We are here to help along the way

[bhyatt1]

I read this online... I was afraid of increasing filtering because of the 5y3gt rectifier im using (to keep b+ down). Id read that you can't increase too much with this rectifier tube.

Is this something practical that might be worth trying? Is there in reason that this is not feasible on my build?

Install a luf 600-1000V non-polar plastic capacitor across the last decoupling electrolytic in the line. This is the last filter, which filters the supply for the preamp. The capacitor will be wired to “by-pass” this filter, one lead to ground, the other to the capacitor lead.

Increase the filtering. On 50 watt models, it seems to take another 50mf across the main B+, and another 50mf for the screen supply.

For 100 watt Marshalls expect to add another 100-150mf across the main B+, and at least another 100mf for the screen supply. Now, the amp will be stiffer on bass notes, if not acceptable, you can add anywhere from a 100-200 ohm 209 watt resistor in series with the output transformer center tap, this will give back a looser feel.

Source: https://mercurymagnetics.com/pages/SSN/ ... tNotes.htm