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

 

[Gnobuddy]

...there is a lot of hum, and this depends on the Master Pot.

That tells you the hum is coming from something between the guitar and the master volume pot. Most likely either the FX loop, or the preamp itself.

When troubleshooting, the trick is to divide and conquer: keep narrowing down the area where the problem lies, until you can find and fix it.

You have to work on the amp with it live, so please take all appropriate safety precautions. Secure the chassis so it can't fall on you; wear electrician's protective Class 0 safety gloves; wear eye protection in case of sparks; wear an apron if you have one, to keep shocks and live wires from making contact with your chest, stomach, groin (ouch!) or legs. Wear shoes with insulating rubber or plastic soles. Sit on a wooden or plastic insulating chair or stool.

Safety precautions in place, the first thing I would do is disconnect the input of the FX loop from the wiper of the "Treble" tone pot. Do you still have hum? Then it's coming from the FX loop, and you have to track down the cause.

No hum? Good, the FX loop is okay. Now disconnect the FX loop output from the top of the master volume pot; instead, wire the master volume pot straight to the wiper of the treble tone control pot. Do you have hum now? Then it's coming from the preamp itself.

Let's assume you do have hum, and now know it's from the preamp. The next question is, is it from the circuitry around V1a, V1b, V2a, or the tone control?

To find out, solder a small cap (say 0.05uF or bigger, rated for the full B+ voltage of your preamp) to the "hot" end of the master volume. (FX loop still disconnected.)

Now jumper from the far end of the cap to the cathode of V2a. No hum? Then the hum was in the tone control itself; find and fix the fault.

Still humming at V2a cathode? Move the jumper to V1b anode. No hum? The fault is in V2a.

Still humming? Move to V1a anode. Gone? The problem is in the gain control, or V1B.

You get the idea - narrow down the part of the circuitry containing the fault, until you can identify and fix it. (My bet is you forgot to ground something.)

Let us know how it goes!


-Gnobuddy

 

[blackwhaleamp]

Hey,
Thank a lot for the tips. I'll give it a try over the weekend. It sounds tempting to test the amplifier live, but I rather not to risk my life and switch it on and off every time I have to touch it.
I'll let you know the outcome

 

[blackwhaleamp]

... the first thing I would do is disconnect the input of the FX loop from the wiper of the "Treble" tone pot. Do you still have hum? Then it's coming from the FX loop, and you have to track down the cause.

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