Ensure that none of the heater wires are wrapped around the valve sockets but instead go over the top of them. You could also try reducing the 1M on V4's grid to 470K to see if there is any reduction in noise. Shouldn't lose any signal by doing this as 470K will look like 10Meg to V4!
What did you change the 220uF Cap to? As you need to remember that the larger the cap, the longer it takes for the bias supply to get up to the voltage you've set it at. I think if I remember rightly 2 x 10uF Caps in a Marshall Bias Supply take 1 second. Any larger and you're output valves will be under stress for longer if you understand what I mean.
You could try reducing the coupling caps going to V5 and V6 from .1uF to .022uF, this might reduce the hum for you. Although it isn't resolving the source of the hum, more like covering it up . Can we have an audio clip of the hum or photos of the inside of the amp? It could be a grounding issue like the others said. Or lead dress which could be more likely the case as it is a high gain amp.
ok, i disconnected the signal wire feeding the PI, no hum at all.
V3 is not inserted, but when i touch the contact point with the wire i disconnected, it hums.
This is narrowing the problem down to the recovery stage (V3), or the path between the recovery and the PI.
But how can this hum exist, with no valve in that socket?
Another strange thing, when the signal wire is connected, and shorted to ground, the hum is not changing, only if i disconnect the signal wire from
it's eyelet, it's gone.
What are you actually touching? The grid of V4 where the 470K and 680K resistors meet or the .01uF cap coming off V3's anode? I wouldnt touch anything with your fingers in the amp in case you touch the wrong wire! Use a wooden splint or chopstick. Something that is insulated. Also if you touch a signal wire with your finger then you will cause hum as you basically become an antenna.
Now does the hum stop with V3 removed? If so then what if you put V3 back in and remove V2?
I'm not touching anything with my hands! I'm touching it with the cable i've disconnected to see what's happening. No connection = no hum, touched = hum on either sides of that cable.
The V3 is removed, so maybe my 50Hz is actually a 100Hz hum? Soundclip is in earlier post.
When the 0,01uF cap, coming from the recovery stage, is connected, the hum begins, even when there is no tube in the V3 position.
If i short this part to ground, the hum is not eliminated.
Right I've had a good look at your photos and listened to the clip. Its either a ground loop or its something near the amp. First of all try putting the amp in a different room to see if the hum stops or goes quieter. Also try this during the day so you can turn off all lights in the house in case they are causing noise.
If that doesnt work it could be how your grounds are connected. Are all if the big capacitors grounded in more than one place? I see all four capacitor cans are connected together, grounded at the back of the pots and then possibly grounded by the power switch. Best thing to do would be to remove the wire connecting the two sets of capacitors together, keep the pre amp filter capacitors grounded near the pre amp and the power amp filter capacitors grounded near the power amp. It could also be worth connecting the PT Centre Taps to the first filter capacitor in the amp but try seperating the two sets of filter capacitors first.
Well he still had noise with V3 removed but only with the .01uF coupling cap connected to V4. So thats why I think it could be something in the Power Supply doing it. Although can ground loops in early parts of a circuit, even if they are seperated from later parts of the circuit can they still be picked up? Also I've never seen a Standby Switch like that before I have to say.
Another thing, it could be the .01uf cap picking up noise itself. What is it close to?
. Can we have an audio clip of the hum or photos of the inside of the amp? It could be a grounding issue like the others said. Or lead dress which could be more likely the case as it is a high gain amp.
