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DC filament supply schematic


2016-02-15 6:38 pm
I've decided to build a custom Dynaco ST-70 style amp based on Bob Latino's VTA-70 schematic (12AX7 drivers). I'm going to build the amp into separate power supply and driver/output chassis. The ideal situation would be to keep all AC noise out of the driver/output chassis and run DC heaters on all tubes, with an independent supply on both channels' drivers and one for each EL34 pair (therefore 3 supplies in total due to current requirements). I've put together a quick schematic but have a couple questions:


1. Anything grossly wrong with this schematic?
2. I plan on using the Hammond 187E10 at 10V, 5A (for the EL34s) but this transformer is center tapped. Should I float the center tap or ground it?
3. Is running DC on the output stage a waste of time and money? At minimum I would prefer to run DC on the drivers, but would compromising with AC on the EL34s be acceptable? Keep in mind all leads will be cohabitating with B+ and grounds via umbilical cables.
4. I've seen some people suggest 0.47uF caps across individual filaments--what does this accomplish?

I have to agree. My amp is a Williamson based push-pull with AC heaters. No hum. All I did was take care with layout, tightly twist the supplies to the heaters, keeping the wires well away from signal wiring, and bias the heaters to ground using two equal resistors. I used a pair of 150 ohms in series across the 6.3vac with the centre point grounded. In the picture the heaters are the thin black twisted pairs. The resistors are the small green ones attached to the two tag strips.