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Troubleshooting 50Hz noise in phase inverter

I've just finished troubleshooting a 1972 Marshall 2040 Artiste which intially didn't seem to need much attention apart from a complete recap and bias adjustment. While I was testing it I thought the hum coming from it was a bit too noticeable for my taste so I decided to get rid of it. It's taken quite a bit of detective work to fix something that seemed like a minor problem. I hope this will help others trying to fix the same problem in their amps as you're not getting much information trying to google for PI noise issues.

I knew I would end up rearranging ground connections entirely, but I decided to begin with elevating heaters. That immediately killed all hum coming from both preamps, but there was still some leftover 50Hz hum which sounded like a ground loop. I ripped all ground connections up and rewired everything back in correct order, but surprisingly that didn't do much to eliminate the hum, which wasn't already too distracting, but I wanted to find out what was causing it. 

I narrowed it down to the phase inverter - grounding the input capacitor would kill the hum. At this point I knew it wasn't a ground loop because I disconnected everything before the PI including the B+ to preamps and reverb and that didn't change a thing. I quickly discovered that the first grid wire was picking up a lot of AC noise as untwisted (!) heavy gauge heater wires from the transformer were laid right beside the PI tube which is located on the other side of power tubes. 

Installing a piece of shielded cable on the grid connection helped but only a little bit. I disconnected heater wires, twisted them together and soldered them back, moving them as far from V4 as possible as well. I hoped that would be the end of the battle, but there was still some 50Hz hum in the output! 

I realized that it must be the AC wires on the voltage selector which is very close to the PI circuit. I disconnected all wires, made a fixed connection on the 230V tap and moved everything away from the V4 - and that was it. The hum was now completely gone, but I found it difficult to reconnect the voltage switch. I tried putting the whole bunch of wires inside a braided shield, but that wasn't enough to keep the AC interference away from the PI. I guess installing a shielding plate below the switch would fix the problem nicely, but since it's unlikely that the amp owner will ever use other voltage than 230V, I left the voltage switch unconnected and zip tied all cables away from V4. Now the amp is dead silent. 

There was also a bit of hum inside the reverb circuit and it was entirely caused by RCA jack wires touching heater wires! Another jarring lead dress mistake in an old Marshall. Fortunately this was very easy to fix. 

Hello Tomislaw

You should be a detective Big Grin

The Marshall Artist is quite an old model, and I dare say it falls well into the period when that manufactory was still learning how to build production amplifiers. Some suggest they are still learning, or possibly still not learning?

Thank you, Tomi, for still learning and for sharing your experience with us.

Hi Tomi

Your story highlights the effectiveness of the DC-standoff and the fact that grid wiring should be well away from noisy circuits.

Have fun

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