OT: JCM900SLX Filament Supply - Repair Question.

[lovekraft0]

We recently got a JCM900SLX in for repairs that had no juice at the preamp tube filaments, and after some checking, it became obvious that a previous "technician" had not only replaced the rectifier bridge for the DC filament circuit (preamp only - no idea why) with some huge 6 amp unit, but he'd also burned the necessary traces virtually off the board in the process, leading to the failure, and rendering the whole mess pretty much irreparable. 
At any rate, rather than attempting to "patch" the burnt board, we decided to simply run AC heaters to the preamp, and reference them to ground "Fender-style", with a pair of resistors to the star ground from each of the filament supply wires at the transformer. This has quieted the amp to the point that it's not easy to tell if it's powered up without a guitar plugged in, so it obviously works, but that begs the question - why would Marshall implement the complex, expensive DC heater setup to begin with, if the result of a simple two-resistor grounding setup is actually quieter? Am I missing something? Is there some compelling reason for the DC heaters that I'm unaware of, or some safety issue that needs addressing here? I may just be second-guessing myself, but I thought I'd ask the experts here for their take on it. TIA - any help, or just reassurance, will be appreciated! 

[geertjacobs]

A few possible reasons:
- using dc you can regulate the filament voltage with zener diode so your tubes will always have something very close to 6.3V no matter at what power setting your power line is (110/120/220/240V). Good for tube lifetime.
- putting in a dc circuit eliminates the need to have AC running across your pcb which might induce hum in other sections of the amp. Running the ac with wires close to the chassis and off the pc may not be an option for a factory line produced Marshall since that's handwiring and not just stuffing a pcb.

[JHS]

The DC-heater in the SLX is a cheap bridge rectifier and a 1000uF electrolytic filter cap, nothing more (both cost about 2-3$).

I don't think it's neccesary to have a DC heater for the 3 preamp tubes.  I would rewire the preamp tubes for 6,3V AC heating and connect the heater wires to the normal 6,3V AC heater suppy of the power tubes. Furthermore I would add a 220Ohm/5W humbalance pot to the heater lines to eliminate any hum. The rectifier and the 1000uF can be dropped, they are no longer needed.

JHS