Marshall 18W - Hum Master Pot

[blackwhaleamp]

Hi,
I have an issue with a Marshall 18W-like replica. The circuit is based on the TMB channel. I added a Reverb-Fender circuit, a Tube Town Active Effect Loop and Footswitch; and a London Power Scaling Kit SV1/RVX kit.
My issue is that there is a lot of hum, and this depends on the Master Pot. I attached a diagram with the layout. It is not very detail, but I hope you get the idea.
The technique that I used is quite straight forward. I have a bared cable attached to the chassis and close to the input jack that goes all the way around the circuit. Every ground that is part of the pre-amp is attached to this cable. The jacks are isolated.
I have a second bared cable that goes from the Impedance Selector to the Power Jack. Here I attached the Output Amp, the Output Transformer, Power Transformer, and Power Supply ground.
I read somewhere (I'll try to find the article) that this is a good practice to avoid ground loops.
I know that there are tons of threads that talk about this topic, but I would like any advise from the experts :-)
Thanks a lot in advance

[J M Fahey]

I have read this before "somewhere" ... did you also post this question in DIY Audio or The Gear Page?
In any case, we´ll try to help
* In your schematic I see one wire going from pot wiper to reverb input, if long and unshielded it´s a very good hum antenna, unsolder it from wiper and check whetyer hum situation changes for good.
If so, replace said wire with a shielded one, grounded only at one end to avoid adding unneeded ground loops.
* you might be getting hum from the added Loop board, disconnect wire leading to Master volume, from MV hot lug, to see if hume comes down that path.
* In fact, as a third test, disconnect both wires added to MV hot and wiper, leave only the one going bto the PI and check hum.
That wire may also be a hum antenna, poorly or not shielded, running where it shouldn´t, etc.
And so on, you must check all suspects to find who´s guilty ... there may be more than one.

Again chech connections to/from Reverb pot, also ground the PI pin where Reverb gets in.
In this case using a cap because it´s floating many DC volts above ground.

[blackwhaleamp]

True, I already posted the same question in diyAudio. My intention is not to spam the community, but to gather further audience :-)

Thank for the tips. Indeed, I have some cables which are relatively long. I will try to remove both the Reverb and FX Loop. I hope that the hum is coming from there. I'll keep you posted :-)

[loudthud]

Where is the input jack grounded? It should ground to the ground buss just behind it, your sketch doesn't show enough detail.

[pdf64]

Sorry it's not much help now, but for future reference I suggest to avoid layouts that put the output stage immediately adjacent to the input stage, as there will inevitably be a greater degree of parasitic coupling between them than a layout that put each sequential stage further in turn from the input.
Are you sure it isn't oscillating? Ultrasonic oscillation can manifest as hum.

[blackwhaleamp]

So, it is the FX loop that is causing the hum. I'll order some shield cables as I ran out of it. That should do the trick. Thanks for tip

[olddawg]

So, it is the FX loop that is causing the hum. I'll order some shield cables as I ran out of it. That should do the trick. Thanks for tip

I hope this works for you, but I haven seen a 18 watt clone with a tank and a loop that works well. If it's a home build be aware that there is a reason Marshall never offered one with Reverb. These amps tend to break up early in signal path and the signal level is high at to PI. Integrated Reverb doesn't sound right to most people ears. You might bec better off with a pedal in front or build a a digital unit into the input.