JackOfBlades wrote:Midnight, I don't understand what you are suggesting, that's why I asked what it means on the first place. So, what does it mean? I can't find concrete information on the internet.
On the JCA circuit board the heater traces run from each 12AX7 to the next one. Here is a close up of the board. The heater traces connect to pin 9 on one side and pins 4+5 on the other side. As you can see the heater traces pass under and close to other components on the board. This can lead to induced hum from the heater traces into other parts of the preamp, especially sensitive high gain circuits.
Now compare the JCA to the pic in the post above which shows the more traditional way of wiring heaters. The yellow and orange heater wires are twisted together and elevated away from the tube sockets and the chassis. Twisting the wires reduces radiated hum and elevating the wires keeps them away from sensitive parts of the circuit like the tube grids.
To mod the circuit board to raise the heater leads off the board is a
lot of work. You have to cut the traces at each tube socket and rewire with twisted wires from one socket to the next. You can also use a shielded twisted pair to reduce the hum even further. This would not have to be done to the power tubes or the PI.
Even after modifying the PCB you may still have some residual hum from inside the tubes themselves. The best way IMO to reduce hum from the heaters is to convert the heater supply for the preamp tubes to DC. This much easier and can be done with a hand full of parts.
Many high end/ high gain amps have DC filament (heater) supplies at least for the first tube in the high gain channel. Another, even easier, low budget way to reduce heater hum is to elevate the heater winding center tap to about 70-80 VDC right from the B+. This takes two resistors and one capacitor. Any decent tech can do either of these mods for you if you are not inclined.
Of course if the hum is not from the heaters any mods to the heater circuit are not going to solve your problem. I think Aiken recommends to use a 6 volt lantern battery to power the filaments to see if there is an improvement before you do any modifications.
The JCA50 (and I think the 100 too) has a jumper that connects the heater supply circuit from the power tubes to the 9 pin tubes. This is a convenient point to disconnect the preamp heaters for testing and modification.
My personal JCA50 has been modified with a second filament transformer to power just the 9 pin preamp tubes. This new transformer has an elevated center tap to reduce hum. It also lets me run any power tube I want without possibly drawing too much current from the stock PT.
