6.3 v ac to dc filament schematic

Valie

I've built a pre amp based on one channel of a Fender bassman.(Alembic F-2B). The power transformer has a 5v tap normally used for a rectifier valve which I don't use and the normal 6.3v tap for the filaments. As I'm only using 1 ECC 83 valve I would like to run it on DC to eliminate the faint hum that I'm picking up. I've already put 2 100 ohm resistors to ground on the windings which helped a lot. Any help?

matta

Before we go down this path, are you SURE the hum you are getting is AC hum from the heaters?

While I'm an advocate in using DC heaters in some studio amps, MOST don't really need it if your wiring is doing well/neat. This icludes twisted pairs for the heaters, running them away from the B+ lines, prefer tucked under the lip of the rear of the chassis and neately mounted off/above the tube sockets to minimize hum.

Another source of hum is placement of the power transformer as well as output transformer. You want the PT to be as close to the rectifier as possible, likewise it is always best to have the OT and PT be at +- 45 degrees to each other so as to reduce the EMF radiating from the PT into the OT and a fair distance apart from each other. (you may not have an OT if you are building a preamp and not a full amp).

If you follow all these guidelines odds are you will end up with a near silent amp... if the hum STILL bothers you them you can use a bridge rectifier to convert the AC to DC for the filaments.

While you can MAKE one use 4 power diodes it might be easier to just buy a pre-made 3 or 5 amp bridge with an M4/M3 mounting hole to bolt it to the chassis. Connect the heater AC to the AC terminals and then place 4700uF/16V cap across the DC terminals (observe the POLARITY!!!) for added filtering and then take those to the heaters.

Remember that going through any diode is going to knock down your voltage a bit, but it should be sufficient for your needs.

Cheers

Matt

Valie

@Matta,
As this is not the first project that I'm building, I am well clued up on power transformer placing etc. There is no o/t as this is a pre amp. The rectified voltage is 8.5v which is well above 6.3v. The ripple is hardly audible but I want to do recordings so I need it to be perfectly smooth. Was thinking of using a 6v voltage controller (7806) or maybe resistors to bring it down to 6.3v. Power consumption is only 300ma (one valve)

matta

Vallie,

Thanks for clarification, as I shared I assumed that it was a standalone preamp, but often people talk about a preamp in relation to a complete amp, NOT an amplifier.

I'm not sure how you are getting 8.5V rectified off a single 6.3V winding? Surely you would LOOSE voltage as across a power diode (my memory is a little fuzzy, but me thinks it is .7V PER diode?) Somewhat perplexing.... and I'd advise against using 8.5VAC or VDC on ANY 6.3heaters unless you want to kill your valves very fast ???

If you want to use a voltage reg go ahead, any of the data sheets will give you the circuit you need and on a single preamp tube you should be fine as 7806 is a 1 amp reg, you just need to feed it 8V+ to regulate down (since you are not a newbie, you prob already know this but for those that don't you need a min of 2 volts source voltage above regulated voltage for ripple/noise free regulation).

The reason I specified a high current bridge diode is because the smaller ones don't have a mounting hole for chassis mounting and you can use a higher current diode/bridge, just not the other way around.

Also make SURE you just a mica insulator if you use a TO-220 reg between the reg and chassis, unless you like fireworks that can occur during shorts ???

Cheers

Matt

Valie wrote: @Matta,
As this is not the first project that I'm building, I am well clued up on power transformer placing etc. There is no o/t as this is a pre amp. The rectified voltage is 8.5v which is well above 6.3v. The ripple is hardly audible but I want to do recordings so I need it to be perfectly smooth. Was thinking of using a 6v voltage controller (7806) or maybe resistors to bring it down to 6.3v. Power consumption is only 300ma (one valve)

FenderBender

matta wrote: I'm not sure how you are getting 8.5V rectified off a single 6.3V winding?

Remember that when you use a filter cap, your DC voltage level will be at the peak of the AC wave. Vpeak = Vrms * 1.414

@Valie:
If you slap a bridge rectifier and cap on the 5VAC tapping of your transformer, you can expect:
(5V * 1.414) - (2 * 0.7V diode forward voltage) = 5.7VDC which should be sufficient for your heaters.
I calculate a cap value of 4245uF, so Matt is right on the button with a 4700uF cap.