Hi. I am interested in understanding how I can reduce the noise and hum in this amp. One thing I have found, after reading posts here about balancing the filament supply, is that the Lead Channel Foot-switch circuit draws current from one of the two filament taps on the PT. When the FS is engaged, the filament circuit becomes highly unbalanced, by a couple of volts. Testing is referenced to chassis, is that a good idea? Schematic is attached below. For a high gain amp, if I could change a few things with greatest effect, what would they be? DC filament supply? Low noise resistors on input and V1? Power tube current-balancing trimpots? All resistors that required replacement are 2W Carbon film for the signal path. No carbon comps anywhere. Fully Recapped with F&T and Nichicon, including Bias Supply. The amp runs a set of filament leads in a twisted manner up in the air, above the circuit. They have a single ground for most everything, but the speaker jacks are both grounded to a GND buss on the power tube circuit board, and to the chassis, as the jacks are metal. The input jacks are also metal and grounded to chassis. Signals are passed through shielded cable.
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An 'unbalanced' heater supply should not matter too much, provided it is correctly wired. If done properly, even a completely unbalanced heater supply (i.e. one side grounded) should not cause undue hum. In fact, that is what they should have done if they wanted to take a rectified DC supply from the AC heater circuit. How tightly twisted are the heater wires? If done properly, try removing the 100R resistors from the heater circuit and grounding the side which has the capacitor. That may increase hum under normal circumstances, but reduce buzz when the foot switch is used.
Incredibly stupid solution, amazing Mesa did that. In a high gain amp u must elevate the heater to > 10V, preferably between 20 - 80V. The hum from heaters is caused by the filaments swinging lower than the cathode potential, at which point electrons will flow from the hot heater filament to the cathode. This causes hum and buzz.
This is not a problem in HiFi amps b/c the gain is not even close to what found in guitar amps.
You need to figure out what the pedal uses the voltage for. Is it only to light the lamp, or is it for a relay in the pedal? And find another way of providing that voltage, if really needed.
This is not a problem in HiFi amps b/c the gain is not even close to what found in guitar amps.
You need to figure out what the pedal uses the voltage for. Is it only to light the lamp, or is it for a relay in the pedal? And find another way of providing that voltage, if really needed.
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