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Step 7The Heater Power Supply(s)
Unfortunately, the filament secondary for my power transformer isn't a separate winding, and doesn't have a center tap. Maybe I could disassemble the trannie and see if the coils could be separated...but it's a "potted" transformer (dipped in resin), and I didn't want to ruin it.
The trannie also powered about 12 tubes, and the filament voltage is ~7v, and doesn't drop enough under load to get near 6.3V (load isn't big enough.) In fact, one 12AX7 "went nuclear" and burned out (~$25 "down the tubes".)
Placing two large diodes in parallel but opposite directions in an AC supply limits the voltage by the voltage-drop amount (.5 to .7V), just the same as one diode in a DC supply. That dropped the filament voltage right to 6.3V, and the tubes were happy.
The two-diode trick works only for AC--current flows through one diode at a time, dropping that half of the waveform by the diode's voltage drop amount. One diode would do the trick for DC.
Plan B
However, they weren't quiet. You really need separate windings to setup a false center tap, which can be used to quiet the heaters.
After trying various solutions, I decided to light just the power tubes with the main transformer, and use a cheap "wallwart" for the 12AX7 preamp. Now the preamp has it's own, "dedicated," DC supply This was very quiet, indeed. The wallwart was already on hand.
A ground reference (false center tap) was provided by bridging the 6V by a pair of 180 ohm resistors, tied to the chassis ground. It does make a difference.
For some power transformers, the required 2.7A @ 6.3V is a little much. Many are rated for 2.5A max. Of course, an extra 200 MA might be fine, depending on the transformer. But a separate DC supply for the preamp isn't a bad option.
OK, this is a little, ehem, unconventional, perhaps even ghetto. But it works well.
The trannie also powered about 12 tubes, and the filament voltage is ~7v, and doesn't drop enough under load to get near 6.3V (load isn't big enough.) In fact, one 12AX7 "went nuclear" and burned out (~$25 "down the tubes".)
Placing two large diodes in parallel but opposite directions in an AC supply limits the voltage by the voltage-drop amount (.5 to .7V), just the same as one diode in a DC supply. That dropped the filament voltage right to 6.3V, and the tubes were happy.
The two-diode trick works only for AC--current flows through one diode at a time, dropping that half of the waveform by the diode's voltage drop amount. One diode would do the trick for DC.
Plan B
However, they weren't quiet. You really need separate windings to setup a false center tap, which can be used to quiet the heaters.
After trying various solutions, I decided to light just the power tubes with the main transformer, and use a cheap "wallwart" for the 12AX7 preamp. Now the preamp has it's own, "dedicated," DC supply This was very quiet, indeed. The wallwart was already on hand.
A ground reference (false center tap) was provided by bridging the 6V by a pair of 180 ohm resistors, tied to the chassis ground. It does make a difference.
For some power transformers, the required 2.7A @ 6.3V is a little much. Many are rated for 2.5A max. Of course, an extra 200 MA might be fine, depending on the transformer. But a separate DC supply for the preamp isn't a bad option.
OK, this is a little, ehem, unconventional, perhaps even ghetto. But it works well.
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Jun 26, 2008. 2:27 PMWragie
says:
If you add a couple of filter caps in your heater circuit you'll find that will help out on the noise/clarity on the other end. With a half bridge like this you get lumpy dc (dc power with a ripple)that can inject a signal into the amp.
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Jun 26, 2008. 6:23 PMgmoon (author)
says:
The diodes in this case are just wired for the voltage drop, not as a rectifier--the heater supply is still AC. Had I used a bridge and gone with DC, I'd certainly add the filter caps and all... But the filament secondary drove quite a few heaters as AC, and the two power tubes aren't enough of a load to drop it below 7V (let alone as DC, which would be higher V.) I'd still need a way to drop the voltage to safe level. So, quite a few more components. It's actually quite quiet for pure Class A...(the guitars aren't...)
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