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09-24-2019, 11:45 AM
Hi Sinusoids
I assume you are having a hum, or that the amp is?
The leads from the IEC should be tucked back in the chassis held by friction or with a cable clamp. The photo shows it flying over all the tubes to get to the PT. Redressing this might reduce the hum.
It might have been wise to reverse the front panel layout and the card if you did not want to knock out a new IEC hole. However, most compromises of layout can be fixed in some way, or compensated for in the circuit.
You can test where the hum is coming from by turning the Volume and MV controls down and see which has an effect. The main filter could stand to be a higher value; even though there are three RC sections it is hard to beat what 100uF will do in the first position. Active hum filtering is the next big improvement to be made in the plate supply, which has to be clean with the UL connection - the screens are sensitive to supply ripple.
There are a few ways to add balancing for the output tubes, as well, and this would accommodate tube aging and different tube and OT samples. Note that the hum balance point for an OT is NOT the DC balance point for the tubes - these points are always off by a few milliamps.
I assume you are having a hum, or that the amp is?
The leads from the IEC should be tucked back in the chassis held by friction or with a cable clamp. The photo shows it flying over all the tubes to get to the PT. Redressing this might reduce the hum.
It might have been wise to reverse the front panel layout and the card if you did not want to knock out a new IEC hole. However, most compromises of layout can be fixed in some way, or compensated for in the circuit.
You can test where the hum is coming from by turning the Volume and MV controls down and see which has an effect. The main filter could stand to be a higher value; even though there are three RC sections it is hard to beat what 100uF will do in the first position. Active hum filtering is the next big improvement to be made in the plate supply, which has to be clean with the UL connection - the screens are sensitive to supply ripple.
There are a few ways to add balancing for the output tubes, as well, and this would accommodate tube aging and different tube and OT samples. Note that the hum balance point for an OT is NOT the DC balance point for the tubes - these points are always off by a few milliamps.
